What's That Rock or Mineral 9781465415929

Table of contents :
Contents 5
Introduction 6
Identifying Rocks and Minerals 8
ROCK AND MINERAL PROFILES 15
1COARSE-GRAINED ROCKS 16
2MEDIUM-GRAINED ROCKS 28
3FINE-GRAINED ROCKS 48
4MINERALS 74
ROCK GALLERY 108
MINERAL GALLERY 114
Glossary 120
Index 124
Acknowledgments 127

Citation preview

What’s that

rock

or MINErAL ? Tom Jackson

LONDON, NEW YORK, MUNICH, MELBOURNE, aND DELHI DK LONDON Senior Editor Peter Frances Editor Lili Bryant US Editor Jill Hamilton Project Art Editor Francis Wong Pre-production Producer Adam Stoneham Producer Linda Dare Jacket Designer Mark Cavanagh Jacket Design Development Manager Sophia MTT Managing Editor Angeles Gavira Guerrero Managing Art Editor Michelle Baxter Publisher Sarah Larter Art director Philip Ormerod Associate Publishing Director Liz Wheeler Publishing Director Jonathan Metcalf DK DELHI Senior Editor Anita Kakar Editors Susmita Dey, Himani Khatreja Art Editors Divya P R, Vaibhav Rastogi DTP Designers Sachin Gupta, Nityanand Kumar Managing Editor Rohan Sinha Deputy Managing Art Editor Sudakshina Basu Production Manager Pankaj Sharma Pre-production Manager Balwant Singh Picture Researcher Aditya Katyal First American Edition, 2014 Published in the United States by DK Publishing 4th floor, 345 Hudson Street New York, New York 10014 14 15 16 17 18 10 9 8 7 6 5 4 2 1 001–256805–Mar/2014 Copyright © 2014 Dorling Kindersley Limited Without limiting the rights under copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of the copyright owner and the above publisher of this book.

Published in Great Britain by Dorling Kindersley Limited. All rights reserved A catalog record for this book is available from the Library of Congress. ISBN 978-1-4654-1592-9 DK books are available at special discounts when purchased in bulk for sales promotions, premiums, fund-raising, or educational use. For details, contact: DK Publishing Special Markets, 345 Hudson Street, New York, New York 10014 or [email protected]. Printed and bound by South China Printing Co. Ltd, China

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ABOUT THE AUTHOR Tom Jackson is a science writer based in the UK. He began his career as a conservation worker and zoologist, surveying the jungles of Vietnam, capturing buffaloes in Zimbabwe, and working in UK zoos. He has written more than 100 books on science, technology, and nature, and contributed to many more. Tom lives in Bristol, England. His past books include DK Eyewitness Books: Science, Animal, Endangered Animal, Spot the Bug, and Help Your Kids with Science (with Carol Vordeman).

Contents Introduction ........................................................................ 6 Identifying Rocks and Minerals............................................ 8

ROCK AND MINERAL PROFILES

1 2 3 4

15

COARSE-GRAINED ROCKS 16 MEDIUM-GRAINED ROCKS 28 FINE-GRAINED ROCKS

48

MINERALS

74

ROCK GALLERY

108

MINERAL GALLERY

114

Glossary .......................................................................... 120 Index .............................................................................. 124 Acknowledgments .......................................................... 127

Introduction Rocks and minerals are everywhere. They form the ground we stand on and the solid floor of the oceans, reaching down into the planet for thousands of yards to the Earth’s crust. This book will help you identify the most common rocks and give you an insight into the minerals they contain. The book is arranged in four chapters. The first three deal with rocks and are divided on the basis of the sizes of mineral grains—the crystals you can see when you look closely at a specimen. Each chapter is subdivided by shade—pale, dark, or mixed. The book’s simple guidelines will open up the world of rocks for you and let you figure out what you are looking at—a rock layer in a cliff, a pebble on a beach, or perhaps the stone of a building. The final chapter contains the most important minerals. Some are the basis of common rocks, others are valuable ores, and yet more are cherished as gemstones. Rocks and minerals are diverse and can be tricky to pin down as one type or another. This book is a great place to start.

Tom Jackson

8

IDENTIFYING ROCKS AND MINERALS

What is a Rock? Rocks—of which about 300 types have been described to date—are defined in terms of how they are formed and their major mineral components. A few rocks are composed of a single mineral, but most contain a combination of different minerals. Gray quartz Pink feldspar Black biotite mica

Pink granite The differently colored grains in this granite sample are the minerals quartz, feldspar, and mica.

Rock formation Rocks are grouped into three families—igneous, sedimentary, and metamorphic—on the basis of the way they form. They occur in a range of sizes, from grains of sand or silt to pieces many feet across.

Basalt sand

Slate pathway

Black basalt sand is of igneous origin and, as with other igneous rocks, it forms when magma cools down and solidifies.

Existing rocks are squeezed and heated by immense forces underground to form metamorphic rocks, such as slate.

Sandstone landscape Small particles or grains, of solid material build up in layers on Earth’s surface to form the sandstone cliffs, such as the rock formation seen here.

WHAT IS A MINERAL?

9

What is a Mineral? Minerals are naturally occurring substances that have a specific chemical composition and are solid in normal conditions. Most of Earth’s rocks are composed of just a few minerals—mostly silicates, such as quartz, feldspars, micas, and pyroxenes. Quartz The simplest silicate, quartz, is made up of silicon dioxide—a compound of silicon and oxygen.

Prismatic quartz crystal

Translucent orthoclase crystal

Orthoclase The feldspars are a large group of silicate minerals. There are two main types: the alkali feldspars (including orthoclase) and the potassium feldspars.

Thin sheets of biotite mica

Nearly opaque augite crystal

Biotite This member of the complex mica group includes potassium, magnesium, and iron in addition to aluminum, silicon, and water.

Augite This mineral of the pyroxene group contains magnesium, iron, calcium, and aluminum as well as silicon and oxygen.

10

IdentIfyIng Rocks and mIneRals

Identifying Rocks An important part of identifying rocks is to figure out the conditions in which they were formed. Clues to this can be found in the surface features of a rock.

Texture In rock identification, the term texture refers to the way minerals combine to make up a rock, and also how the rock’s surface feels to the touch. An important aspect of a rock’s texture is the size of its grains. Rocks are usually described as being coarse-, medium-, or fine-grained. easy-todistinguish grains

CoaRse- gRaIned

grains visible with naked eye

MedIuM-gRaIned

large crystals in fine matrix

PoRPhyRITIC

microscopic grains

FIne-gRaIned

glassy texture

Hollow cavity

VolCanIC

VesICles minerals segregated into layers

Foliation When rocks are compressed and subjected to heat, their crystals align into sheets known as foliations. Many metamorphic rocks, including gneiss, are foliated. gneIss

Flow-banding Wavelike bands—evident in igneous rocks, such as rhyolite—reflect the flowing motion of the magma that formed them.

RhyolITe

layers formed by moving magma

IdentIfyIng Rocks

Bedding and folding The deposits that form in sedimentary rocks build up in layers, called beds, that are often visible in rock samples. Frequently, the beds are folded into curves by the forces acting on rocks underground. Fold formation Pressure and heat have created folds in these limestone formations on the island of Crete, Greece.

Bed of rocks Sandstone rocks in various shades of red are arranged in the form of beds at the Zhangye Danxia Landform Geological Park in Gansu Province, China.

Fossils Rocks containing the remnants or traces of organisms that once existed are known as fossils (pp.100–101). These fossils not only tell us what life on Earth was like in the distant past, but they also help us understand where and when the surrounding rock was formed.

Fossil in limestone In this fossil of the dinosaur Compsognathus, hard body parts, such as bone, have been turned to stone.

11

12

IDENTIFYING Rocks aND mINERals

Identifying Minerals Unlike rocks, minerals have a specific set of characteristics. Color, however, is not one of them. While a mineral has a typical color when pure, impurities can create a range of hues. More consistent characteristics, such as shape and luster, can be used to accurately identify minerals.

Mineral shapes

Prism face

PRIsMatIC

crystals emerge from center

RadIatIng

BotRyoIdal slender, pointed crystal

columnshaped mass

ColuMnaR cube-shaped crystal

needlelIke

CuBIC Flat, thin crystal

Fine, fragile tufts

FIBRous

MassIve Globular mass resembling a bunch of grapes

long, flattened crystal

Bladed

compact mass of indistinct crystals

Pyramid face

A mineral’s shape, or habit, is its outward, large-scale appearance. This is determined by the mineral’s molecular structure.

Platy

crystal with long, flat side

taBulaR

IDENTIFYING mINErals

Luster Not all minerals are sparkling crystals. The way light interacts with a mineral to create its surface appearance is called luster. All lusters can be easily distinguished by the naked eye.

GLassy

eartHy

metaLLic

increase in Hardness

Hardness While some minerals are very hard, others can be easily scratched or worn. Most mineralogists use a comparative system called the Mohs scale to measure a mineral’s hardness. In this system, the numbers 1 to 10 are assigned to ten marker minerals—1 being the softest. A mineral with a higher number will always scratch a mineral with a lower number, so a few simple scratch tests can indicate how hard a mineral is.

pearLy

Greasy

1 taLc 2 Gypsum 3 caLcite 4 fLuorite 5 apatite 6 ortHocLase 7 quartz 8 topaz

the mohs scale Scratch tests based on the Mohs scale can be used to help identify a mineral. For example, a mineral that scratches apatite but is scratched by quartz is rated 6 on the Mohs scale.

resinous

9 corundum 10 diamond

13

14

IDENTIFYING mINErals

Fracture

Crumbles into grains

long, bladed crystal

The way a mineral breaks, or fractures, is another identifying characteristic. For example, metals often show rough, jagged fractures.

Splintery arc-shaped fracture

sharp edges, jagged points

HaCkly

eartHy

ConCHoidal

Irregular fracture

Uneven

Cleavage Crystalline minerals have specific molecular structures—crystals are built from repeating units with a particular shape, such as cubes or tetrahedra. Cleavage refers to how cleanly and easily a mineral’s ultrafine crystal lattice allows it to split. If it splits along a smooth break, the mineral is said to have perfect cleavage. For example, the topaz sample shown here has perfect cleavage. Other minerals have imperfect or difficult cleavage, while some cannot be split at all.

Break parallel to base

perFeCt Cleavage

Breaks in different directions

Cleavage planeS

Streak The color of the powder left behind after a mineral is dragged across a rough tile is known as its streak. While impurities can cause a mineral’s color to be variable, the powder produced will always have the same color. For example, this iron oxide hematite has a red streak, while magnetite, another iron oxide, has a black streak. Orpiment has a distinctive golden yellow streak.

ROCK AND MINERAL PROFILES In this book, the Earth’s most common rock types have been divided into three broad categories on the basis of the size of their mineral grains—coarse, medium, and fine. Also featured in this book are some of the most important minerals, which include rock-forming minerals, valuable ores, and gemstones. 16 Coarse-grained roCks 28 medium-grained roCks 48 fine-grained roCks 74 minerals

1

COARSEgRAinEd ROCKS The grains and crystals of coarse-grained rocks are easy to see with the naked eye. The grains are at least 1 ⁄16 in (2 mm) across and can, on rare occasions, be several inches long.

18

coarse-grained rocks

pale, coarse-grained rocks Graphic Granite Also known as textured granite, an igneous rock with a higher proportion of feldspars than most granites. Quartz, another component mineral, forms large crystals often with straight sides. This gives the surface a repeating, geometric look. Quartz crystal

Pink orthoclase feldspar

Pale

19

pink granite Pink igneous rock with dark, chunky flecks of biotite mica and grayish patches of quartz. It gets its pink color from alkali feldspars rich in potassium. It is commonly used as a decorative stone. Black biotite mica

Gray quartz crystal

pOlished stOne

Pink feldspar

granitic pegmatite Igneous rock with composition similar to granite, except with larger crystal grains. This gives the rock a rough, chunky appearance. The mica it contains may appear as layered sheets of crystal. Pegmatite sometimes contains large gemstones. Quartz

Feldspar

Needle-shaped tourmaline crystal

20

igneous rocks

Igneous Rocks igneous rocks are formed from the solidification of hot magma, either deep within earth or when it erupts onto the surface as volcanic lava. The mineral composition of an igneous rock depends on the chemical makeup of the magma from which it cooled. Rocks composed of quartz- and feldspar-rich magma are generally pale. Those made of silicate minerals containing metals such as iron and magnesium are usually darker.

Lava flow When magma—a hot soup of chemicals, mostly silicon compounds—reaches the surface, it is known as lava. As magma cools down, its compounds form crystals of various minerals.

igneous rocks

21

Intrusive rocks Igneous rocks that form deep underground are described as intrusive. They cool slowly, which gives minerals time to develop into large, easily seen crystals. As a result, intrusive rocks are usually coarse-grained.

Large, interlocking crystals

Half Dome at Yosemite, US

pEGMATITE

Made from granodiorite, the Half Dome formed as an underground magma chamber solidified and was then exposed as softer surrounding rocks wore away.

Extrusive rocks When exposed to air or water, lava cools rapidly and forms extrusive rocks. Unlike intrusive rocks, they typically have tiny crystals, and so appear fine-grained. noncrystalline surface produced by fast cooling

pUMIcE

Basalt columns Hanging basalt columns, like these near Svartifoss waterfall, Iceland, form when a pool of lava cools into rock. As it solidifies and shrinks, the rock cracks into hexagonal columns.

22

coarse-grained rocks

Dark, Coarse-grained rocks Kimberlite Igneous rock that is rough to the touch. It is always dark, although there may be a few crystals of calcite. A major source of diamonds, kimberlite is named after the diamond-industry centre of Kimberly, South Africa.

coarse texture

Light-colored crystal of ferromagnesian mineral

dark matrix

Pyroxenite Rough igneous rock made almost entirely of pyroxenes. These minerals give pyroxenite a greenish brown tinge. Other iron-rich minerals add to its dark color and also its weight. Pale crystals are rare and small. Plagioclase feldspar

PolisheD stone Pyroxene

DARK

23

SErpEntinitE Green metamorphic rock formed at shallow depths beneath Earth’s surface, mainly from peridotite. Its large, angular crystal grains may line up in bands. The dominant mineral is serpentine. Green olivine, dark hornblende, and reddish garnet may also be present.

Easily seen coarse grain

Mottled, patchy texture

EclogitE Rare and unusual metamorphic rock. It has a green-and-red color and visible crystals. The green color comes from a pyroxene called omphacite, while the red is from garnet crystals. The crystals are frequently foliated—lined up in crooked bands. Red pyrope garnet

Green omphacite

poliShED StonE

24

coarse-grained rocks

Coarse-grained rocks, mixed Shades granite The most common rock in Earth’s continental crust. This igneous rock is typically a mix of dirty pink, black, gray, and white crystals. Mica gives smooth, weathered specimens a waxy sheen, while rougher rocks get a sparkle from the quartz.

Potassium feldspar crystal

tourmaline pegmatite Igneous rock similar in color to granite (above). Compared with granite, it has much chunkier crystals of orthoclase—a type of pinkish alkali feldpsar—and quartz mixed with elongated, prismatic crystals of a much darker tourmaline. The tourmaline may have a striped appearance. dark prismatic tourmaline crystal

Pink orthoclase feldspar

poliShed Stone

Biotite

mixed shades

amphibolite Metamorphic rock largely made of amphibole minerals, most commonly the dark, green-black hornblende. The dark grains are normally aligned with each other. The occasional pale color is due to calcite and feldspar crystals, with a flash of red from garnet.

Coarse texture

Garnet crystal

amphibole crystal

diorite Igneous rock often seen as a black and white stone. It is made up of two-thirds alkali feldspars that form creamy crystals. Dark minerals such as hornblende and biotite make up the other third. It is sometimes called black granite. Unlike granite, however, it contains no quartz. hornblende

Potassium feldspar crystal

Plagioclase

25

26

Coarse-grained roCks

granodiorite Cross between granite (p.24) and diorite (p.25). Although it has the same rough texture and speckled appearance as granite, this igneous rock is a little darker because of its mix of blue-gray plagioclase feldspars and pink alkali feldspars. It can be pink or white. Plagioclase

Coarse texture

polished stone

dark, iron- and magnesiumbearing minerals

Mixed shades

27

conglomerate Distinctive sedimentary rock of varying colors, with smooth stones set in a cementlike matrix. The stones can be anything from pebbles to boulders, but an individual specimen generally contains stones of similar size.

Rounded pebble

Fine-grained matrix

breccia Sedimentary rock similar to conglomerate (above) but made up of angular, fragmented stones instead of smooth ones. These are held in a finegrained matrix. The fragments come in a wide range of sizes and colors. Gray, silica-rich fragment

Yellowish matrix

angular fragment

2

mediumgrained rOCKS Medium-grained rocks have grains from 1⁄256 in (0.1 mm) to 1 ⁄16 in (2 mm) in size—visible with the naked eye or under a simple magnifying lens. These rocks may contain a small number of larger crystals called phenocrysts.

30

medium-grained rocks

Pale, Medium-grained rocks quartzite Commonly grayish yellow but can also be pink, orange, or red in color. Quartzite is a metamorphosed sandstone, in which loosely connected sand has melted to form a denser, heavier, and harder rock made of tightly fused quartz crystals. Hard and resistant surface

interlocking grains

crystalline quartz

Pale

anorthosite Gray igneous rock that forms deep in Earth’s crust. It is not common at the surface, and when it does appear it is usually as an enormous mass. Anorthosite is mostly made from pale, calciumrich plagioclase, which occasionally appears as large, distinct crystals.

Mottled orthopyroxene

Plagioclase feldspar crystal

tuff Soft igneous rock formed from ash thrown out by a volcano. Tuff is generally uniform in color, ranging from pale yellows to browns and grays. It frequently has layers, formed where larger, heavier ash particles fall to the ground before the lighter grains.

Fine texture Sediments deposited in layers

31

32

meDium-graineD rocks

marble The most familiar metamorphic rock. Its chief minerals are calcite and dolomite, giving the rock its typical white color, but as a result of impurities, marble comes in just about any color and pattern. Marble can be cut and polished easily, making it a common building stone.

bUilDinG stone

White marble broken into fragments and recemented iron oxide cement between fragments

Hematite vein

Kyanite schist Yellowy gray metamorphic rock. It gets its name from the blue mineral kyanite, which can be seen as small blade-shaped flecks among twisted layers of quartz and feldspar.

Dark mica

Blade of kyanite

gray rock with foliated structure

pale

33

greensand Soft sedimentary rock with a dirty green tinge. This sandstone gets its color from glauconite—a mineral formed from organic debris chemically altering rock debris in shallow seas. Greensand has a soft surface and well-sorted grains, with similar-sized grains cemented together. Well-sorted sediment

Green color from glauconite

arkose Generally pink sandstone with high feldspar content. This sedimentary rock is made up of angular grains. In older specimens, the pink color, coming from iron oxide impurities, darkens to gray. Quartz grains

pinkish feldspar

34

medium-grained rocks

SandStone One of the most common sedimentary rocks seen on Earth’s surface. Sandstone—formed from grains of sand cemented together—often houses fossils. It can show ripples formed when the sands were blown by wind or washed up by water. Sandstone varies in color from cream to yellow and to orangey red. Folds due to slumping of original deposit

Building Stone

red color due to iron oxide

Red SandStone Red sedimentary rock that has rounded sand grains when formed in the desert and more angular grains when formed in shallow seas. In both cases, drying cracks, ripples, and cross-bedding—where the angle of deposition has changed—can be seen. red color due to iron oxide coating

Building Stone rounded grains

Pale

35

Gritstone Sedimentary rock formed from large and spiky sand grains, originally from ancient riverbeds. Together, the grains form a very hard type of sandstone, composed mostly of quartz. Different impurities make gritstones yellow, brown, pink, or gray.

BUilDinG stone

Reddish color from iron oxides

Well-sorted sediment

oolitic limestone Sedimentary rock that is pale yellow in its pure form. It consists of spherical or eggshaped calcite structures known as ooliths. The calcite is deposited from mineral-rich water. BUilDinG stone Oolitic grains

Calcite cement

36

sedimentary rocks

Sedimentary Rocks about 80 percent of the rocks found on earth’s surface are sedimentary. these are formed by the accumulation in layers of minerals and other rocks. Over millions of years, sediments are compressed by the weight of new deposits, which cements them together to form solid rock. Sedimentary rocks are softer than igneous ones because they are held together with a chemical cement instead of heat-fused crystals.

Chemical sediments Some rocks, such as rock salt, form from chemicals dissolved in water. As this water evaporates, the chemicals precipitate, sink to the bottom, and form a layer of sediment. RoCk Salt

Clastic sediments Most sedimentary rocks, including puddingstone, are formed from clasts—crystal grains and fragments broken from larger rocks. Clasts range in size from boulders to silts. rounded clast

Glaciers pick up and transport rock fragments

puddingStone

Water, wind, and ice deposit pieces of rocks in strata (layers), which over the years cement together to form rocks

rivers carry fragments of rocks, or sediments produced by weathering

sedimentary rocks

37

Weathering All rocks gradually weather, or wear away. Water trickles through rocks, reducing some minerals to chemical sediments. Water, ice, wind, and gravity also breaks rocks into smaller pieces—known as clasts when they become part of other rocks.

Weathered wonder The Grand Canyon in Arizona, is a product of erosion and weathering. The Colorado River cut through 1 mile (1.6 km) of solid sedimentary rock to create this rock formation.

Erosion Pieces of rocks can be removed by wind, by moving ice (such as glaciers) or by running water, especially rivers. The process by which rock fragments are loosened and carried away is called erosion. Formation of sedimentary rocks These rocks—found on or near Earth’s surface—are formed in layers from pieces of weathered and eroded rocks.

River transport Large, heavy clasts are not carried far by river water. However, lighter silt particles move great distances with the current, before settling to the bottom of a river mouth or seabed.

Diagenesis Sediments form rocks through the process of diagenesis. In this process, deposited grains are compressed and any water squeezed out, leaving behind chemicals that cement the grains to form a rock.

Fragments and sediments may also be deposited on the seabed

38

medium-grained rocks

dark, Medium-grained rocks Gabbro Igneous rock with greenish tinge from augite and other pyroxene minerals. Gabbro has a highly variable mineral content, but it lacks quartz. Some large crystal grains of pale minerals, such as calcium plagioclase, may be seen among the dark pyroxene minerals.

Polished stone

Plagioclase feldspar

dark pyroxene

Peridotite Olivine-rich, heavy igneous rock that is usually dark green. The dark color is due to the presence of pyroxene minerals. Specimens that are especially rich in olivine (and have fewer pyroxenes) will be a paler green.

dark pyroxene crystals

coarse texture

dark

39

DoleRite Dark gray igneous rock with composition similar to that of gabbro (opposite), but the minerals occur in different proportions. The dominant mineral is calcium-rich plagioclase, which occurs as inclusions—tiny crystals. These are surrounded by pyroxenes that give the rock its dark color.

Plagioclase feldspar

Brown weathered edge

Medium texture

laMpRophyRe Dark brown rock, sometimes black. It has a rough matrix of small grains, sometimes studded with larger crystals of pale orthoclase. The matrix of this igneous rock is rich in metals, especially magnesium, iron, and potassium minerals.

Brown, weathered surface

Embedded crystalline particles

40

MEDIUM-graInED rocks

muscovite schist Metamorphic rock with a large amount of a type of mica called muscovite, which appears as folded layers of fragile white crystal. This rock shares the same general mineral composition and structure as other schists. Wavy texture

folded schist Metamorphic rock squeezed into ripples and folds by great pressure and heat. These folds may crumble as sheets of silvery mica break away from the rock. The rock specimen itself may also split easily.

Pale muscovite mica

silvery muscovite mica

Wavy folds picked out by mineral bands

Dark biotite mica

DARK

41

orthoquartzite A form of sandstone made from small, rounded grains of quartz sand cemented together. The color of this sedimentary rock depends on other minerals present with the quartz. It is generally gray, but it can also be pink, white, or yellow.

Quartz grains

Crystalline appearance

ironstone Sandstone or limestone that has a very high iron content and typically formed in the distant past. This sedimentary rock is often formed from ooliths—rounded structures coated in iron minerals—that make it red or brown. Red color from iron

Rounded oolith

42

MEDIUM-graInED rocks

Medium-grained Rocks, Mixed Shades LAYERED GAbbRo Type of gabbro (p.38) in which dark, metal-rich minerals form as layers between paler plagioclase feldspars. The main metal in this igneous rock is iron, but the bands can also include chromium, titanium, and platinum.

Dark ferromagnesian minerals

Light plagioclase feldspar

LEUCoGAbbRo Black and cream igneous rock. Sometimes coarse-grained, it is a mix of plagioclases and pyroxenes, and very little olivine. This composition makes it less dense and lighter than other gabbros. White plagioclase feldspar

Dark pyroxene

MIXED ShaDES

43

Carbonatite Consists of carbonate minerals, which is unusual for an igneous rock. Specimens are generally a creamy yellow but have wavy bands of greens, browns, and oranges. The grain size is highly variable. Carbonate minerals

Medium-grained texture

PorPHYrY General name for a range of igneous rocks with varying compositions. All porphyries have a medium or fine-grained background matrix filled with conspicuous larger crystals of other minerals, often quartz or feldspars. Fine grains

Feldspar crystal

PolisHed stone

44

medium-grained rocks

MigMatite Mixture of rock types, typically metamorphic schist or gneiss contains bands of hard, quartz-rich igneous rock, such as granite. The quartz and feldspar from the granite appear as pale streaks and veins in the dark, mica-rich host rock.

dark gneissic component

Folded layer of granite

Light quartz and feldspar

MIXED ShaDES

Granulite Metamorphic rock named for its uniform grain size. It is similar to gneiss in composition, but unlike gneiss it is not banded. Granulite is generally dark, with paler flecks of plagioclase feldspars. Garnets may add a pinkish hue.

Fine matrix

Plagioclase feldspar

Skarn Metamorphic rock formed from limestones and other sedimentary rocks made from calcite. The calcite ranges from white to pink to brown and is split by darker silicate minerals, especially by jagged veins of greenish dolomite.

Diopside and tremolite

Brown calcite

45

46

medium-grained rocks

syenite Mix of pink, black, and creamy gray. This igneous rock may be confused with granite (p.24), because it is found in the same upland areas. However, syenite generally has smaller grains and lacks clear, twinkling quartz crystals.

Feldspar

amphibole

banded gneiss Common metamorphic rock characterized by a broken banding of light and dark minerals. The crystal grains are flattened, or foliated, as a result of the immense pressures that created the rock.

dark and light foliated bands

Pale feldspar

dark biotite

MIXED ShaDES

folded gneiss Similar to banded gneiss (opposite), except with more distinct bands twisted into curved and rippled layers. Sometimes, a large distinct mass of pale feldspar crystal known as an augen or eye is seen.

alternating mineral bands

Pale quartz and feldspar

Dark hornblende and biotite mica

47

3

FinE-grainEd rOCKS Most fine-grained rocks have microscopic grains, which are smaller than 1⁄256 in (0.1 mm). In some cases, these rocks do not have grains at all. Instead, they form a seamless aggregate of crystals, or are amorphous or noncrystalline, like glass.

50

FINE-graINEd rocks

pale, Fine-grained rocks Scoria Igneous rock often found cracked and weathered into rubble. This hard, prickly volcanic rock is reddish when new but darkens with age. No crystals can be seen, and it has hollow, rounded cavities, or vesicles, where gas was trapped as the rock cooled.

rounded cavity

Brown color

pumice Volcanic rock that looks like a stone sponge. Like scoria (above), pumice is an igneous rock filled with gas vesicles. However, it contains silica-rich minerals, which make it quite soft and light. Some pumice specimens can float in water.

Light color

rounded hollow

Frothy texture

PALE

51

trachyte Igneous rock made chiefly of alkali feldspars, which form a pale gray matrix. The matrix can also be yellow or pink. It has dark phenocrysts (large crystals) of metal-rich minerals. Fine texture

poliShed Stone

Dark phenocryst

SoapStone Soft, greasy metamorphic rock made largely of talc. Most soapstones are white, green, or yellow-brown, but they can be darker. This flaky rock is often built up of thin sheets, which can be scratched away using a fingernail.

Massive habit

Greasy luster

52

FINE-graINEd rocks

limestone Sedimentary rock, usually dirty white but can be pink or gray. It can be made from microcrystalline calcite—too small to differentiate into grains—or shells and other remains of living things. It is very often a mixture of both. Fine texture

BuilDinG stone Fossil of shell

tufa Sedimentary rock often stained red by the presence of iron oxides. Tufa is made up of minerals laid down by water from hot springs or desert lakes. The minerals precipitate as the water evaporates, forming gnarled tubes and towers.

Highly porous surface

soft, powdery texture

Irregular shape

PALE

53

dolomite Gray, yellow, pink, or pale brown sedimentary rock. It forms from limestones in which the calcite has been replaced with dolomite, a carbonate mineral containing magnesium as well as calcium. The mineral is in a microcrystalline form that makes the rock very hard and gives it a uniform texture.

Compact carbonate rock

Fine to medium texture

chalk Soft white sedimentary rock made almost exclusively from shells of microscopic sea creatures. Chalk has a powdery feel and is often damp to the touch due to the presence of clay minerals. Chalk leaves a layer of powder when rubbed on harder surfaces, although the “chalk” used for writing is generally gypsum. Soft, powdery texture

Fine white grain

54

FINE-graINEd rocks

green marl Smooth, hard sedimentary rock, composed of clay and carbonate minerals. Its grains are only visible under a microscope. The green color is derived from organic matter mixed into the rock-forming sediments.

Fine grains

curved fracture

rock salt Sedimentary rock form of common salt, or sodium chloride. There is generally a reddish tinge to the white crystals, due to the presence of iron oxides, but pink and even blue orange color forms exist as well. Rock salt specimens are very delicate, from iron-rich not least because they dissolve in water. clays

crystalline salt

PALE

rock gypsum Sedimentary rock that may appear as curving fibers or plates, and also as lumps of tiny crystals. Composed almost entirely of gypsum, this rock is white, yellow, or pale brown. It has other minerals—such as salt and iron oxides—or different rocks running through it. Iron oxide impurities

Massive habit

55

56

FINE-graINEd rocks

diatomite Soft, dirty white sedimentary rock. Rough to the touch, it often has large pore spaces—enough to allow some specimens to float. It is made of the silica-rich remains of microscopic algae called diatoms. Diatomite can be crushed easily. Swedish scientist Alfred Nobel used this powder as the stabilizing element in dynamite.

rough texture

Loose, porous structure

Loess Smooth, crumbly, sedimentary stone rich in clay. Loess is pale in color and may appear as a tightly packed earth rather than a rock. It is found in deep, dry beds, and were originally formed in deserts. sandy yellow color

spongy texture

DARK

57

Dark, Fine-grained rocks basalt Dark, compact igneous rock that is generally rough and dull, but some glassy specimens are smooth and shiny. Its crystals are so tiny that they cannot be seen using most microscopes. The most common rock on Earth’s surface, basalt forms from rapidly cooled lava.

builDiNG stoNe

Fine grains

Dull, dark gray surface

vesicular basalt Essentially the same as basalt (above) but filled with vesicles, or cavities, made by gas bubbles trapped inside the rock as it solidified. The presence of these spaces means that, when weathered, this igneous rock has a very rough surface. Empty cavity

Dark, weathered surface

58

FINE-graINEd rocks

andesite Dark volcanic rock, composed mainly of plagioclase feldspars. An igneous rock, andesite contains the same minerals as diorite (p.25), but it cools more rapidly so its crystals are smaller and only visible with a lens. Some specimens have a fine matrix of dark andesine with larger crystals of paler oligoclase. oligoclase crystal dark, fine-grained matrix

Obsidian A striking igneous rock composed of naturally occurring volcanic glass. It is often jet black due to iron impurities, although red and brown varieties exist as well. The lustrous glass cracks to form edges that can be sharper than surgical scalpels. curved fracture glassy texture

DARK

rhyolite Gray-brown microcrystalline igneous rock. It looks similar to flint (p.68) because some of its minerals form natural glass. Rhyolite may have bands that show the flow of the magma from which it originated; other forms are pale gray, even pink, with many dark phenocrysts.

Hard, flinty appearance

Flow banding

volcanic bomb Lump of rock formed in midair from lava ejected by a volcano. These igneous rocks fall from the sky and cool in flight into an elongated shape. Brown or red, weathering to a yellow-brown color, they can be 21⁄2 in (6.5 cm) to 3 ft (1 m) long.

Spindle-shaped

Weathered surface

Fine texture

59

60

FINE-graINEd rocks

hornfels Dark metamorphic rock that forms when a range of parent rocks is exposed to great heat. Hornfels is a hard rock with a uniform surface, and its mineral composition is varied. It can appear glassy or matte, and it fractures into flakes, leaving rounded ridges on the surface.

dark pyroxene crystals

Evenly colored rock

blueschist Metamorphic rock with a bluish tinge to its dark gray, often banded, surface. The blue comes from the mineral glaucophane. A similar rock with more epidote and chlorite is known as greenschist. Blueschist is found in mountainous regions, where it forms from the heat of magma acting against older rocks.

aligned glaucophane crystals

Indistinguishable grains

Fine-grained texture

dark

61

slate Lead-gray, rarely black, metamorphic rock known for the way it splits into flat sheets. It forms from shales (p.65) and mudstones (p.66). Slate has few surface features because its tiny crystals are aligned into sheets.

BUilDiNG stoNe Foliated (layered) structure rock splits easily into flat sheets

phyllite Metamorphic rock with a waxy sheen due to high mica content and a green tinge because of chlorite. Although similar to slate (above) in the way it can be split, phyllite has larger crystals, so it breaks into thicker, rougher sheets. Waxy sheen

Wavy bands (foliation)

polisheD stoNe

62

MetaMorphic rocks

Metamorphic Rocks the mineral composition of rocks can be changed without re-melting when they are subjected to extreme temperatures or pressures, or both. such altered rocks are known as metamorphic rocks. Any rock—whether igneous, sedimentary, or metamorphic— can undergo metamorphosis. Usually, however, the rock is so transformed by the process that it is hard to identify what it originally was.

When magma intrudes into rock, it creates high temperatures and low pressures. the surrounding rock is altered in a process called contact metamorphism Magma chamber

Types of metamorphism

heat and pressure deep in earth’s crust can effect widespread changes in the rocks there. this is called regional metamorphism

The different types of metamorphism include contact, dynamic, regional, and shock. In shock metamorphism, the great heat and pressure generated by a meteorite impact results in the formation of new types of rocks.

The term metamorphic grade describes the intensity of metamorphism. It can refer to the amount of increase in temperature or pressure, or both. The three broad metamorphic grades are low, medium, and high.

Metamorphosis of shale This sequence shows the various types of metamorphic rock into which the sedimentary rock shale can be transformed by the increasing application of heat (from left to right) and pressure (from top to bottom).

InCREASE In pRESSURE

Metamorphic grades

SHALE

SLATE

BLUESCHIST

MetaMorphic rocks

Movement of rocks occurs along a line called a fault

63

rocks can change as a result of the pressure of movement in earth’s crust. this process is called dynamic metamorphism

Layers of rock in earth’s crust

INCREASE IN TEmpERATuRE

HORNFELS

SCHIST BANDED GNEISS mIGmATITE

64

FINE-graINEd rocks

red marl Hard, red-tinged sedimentary rock that breaks into chunks instead of layers. A close relative of green marl (p.54), its uniform red color is due to the presence of iron oxide in the mud from which it formed.

red iron oxide

Fine texture

shale The most common sedimentary rock, made of tiny clay grains. Shale is generally gray—darker varieties indicate the presence of organic matter mixed with the original clay. Shale splits easily into sheets along bedding planes. It often contains fossils.

Fossilized ammonite

Visible layers Fossilized mollusk shell

dark

oil shale Oily, dark brown or black sedimentary rock that produces a strong petroleum odor when freshly broken. The color and sheen are due to kerogen, a tarlike substance derived from plant and animal matter in the rock-forming clay. The presence of kerogen makes it a commercially important oil-producing rock.

darker layer rich in kerogen

Oily luster

Fine texture

65

66

FINE-graINEd rocks

mudstone Sedimentary rock made from minute grains of waterborne clay mixed with the remains of algae and other life forms; the latter make the rock dark in color. Unlike siltstone (opposite), this gray or black rock can break into smaller flakes and layering is rare. Mud-sized grains

curved fracture

DARK

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siltstone Sedimentary rock, made from silt with a grain size bigger than that of clay but smaller than sand. Siltstones often show bedding planes and even ripples, where successive layers of silt have been deposited. Most siltstones are dark due to organic matter in the sediment. Fossils are common.

Dark color from carbon

graywacke Hard sedimentary rock, mostly fine-grained but with a mixture of larger sand particles also present. It is gray in color, but brown and black varieties are also found. The rock is also called dirty sandstone.

Finegrained matrix

Dirty gray color

Silt-sized grains

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FINE-graINEd rocks

TILLITE

Large clast

Sedimentary rock with dark gray clay matrix filled with a wide range of other rock fragments. It is formed from the debris dumped by glaciers. Also known as boulder clay, tillite forms from whatever rocks a glacier erodes as it descends a mountainside.

gray clay matrix

FLINT Nodular form of chert (opposite) that occurs in areas of marl, chalk (p.53), and limestone (p.52)— all softer rocks that erode to expose flint. This gray sedimentary rock fractures to form sharp rounded edges, which can be used as cutting tools. sharp edge

BUILDING sToNE

chalk

DARK

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CHERT Sedimentary rock made entirely of microcrystalline quartz, or chalcedony. Chert has a smooth, glassy surface, especially when fractured. Generally gray or brown, chert can form as layers but is often found as nodules—a nodule is a rounded lump surrounded by another rock.

Rounded fracture

Fine-grained texture

BOG IRON Sedimentary rock formed in swampy areas from spring water rich in iron minerals. It consists of misshapen nodules of iron ores, such as red hematite and rusty brown goethite, typically mixed with mudstone (p.66). Mudstone

Oxides and hydroxides of iron

70

FINE-graINEd rocks

COAL Dark brown or black sedimentary rock, mined for fuel. Coal is a carbon-rich rock formed from the remains of woody plants. It is generally quite soft and a little greasy to the touch due to hydrocarbon tars and oils. Hard coal, such as anthracite, has a glassy surface.

Near-metallic luster

Hard, black surface

TEKTITE Once described as meteorites, these rounded, glassy nodules are now thought to be formed by the heat of a meteor strike melting surrounding rocks. Tektites are rare and generally less than a couple of inches across. They are rich in silica and other oxides.

disk-shaped tektite

smooth surface

DARK AnD MIXED

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Fine-grained rocks, mixed shades amygdaloidal basalt Striking form of the common igneous rock basalt (p.57) in which gas vesicles become filled with large crystals of white minerals, especially quartz and zeolites. These minerals get into the rock in water trickling through cracks. The water can also turn dark iron minerals in the rock rusty red.

Rusting iron minerals

Gas bubble filled with zeolite

laterite A bed of loose pink, yellow, and red earths, as well as hardened nodules. Formed when iron and aluminum oxides mix with sand grains, laterites are found in warm, damp climates. They are soft enough to cut into bricks or crush into gravel.

Sand grains

Iron oxide minerals

72

FINE-graINEd rocks

travertine Porous, white sedimentary rock. It is formed by calcite precipitating out of spring water, usually in caves or around waterfalls. Although similar to tufa (p.52) in composition, travertine forms in layers and often comprises rounded structures cemented together.

pOlished stOne

Layer of sediment calcite crystals

dacite Igneous rock formed near volcanoes, containing nearly equal proportions of quartz and plagioclase feldspars. Dacite is generally dark gray but may contain patches of pink where certain plagioclase feldspars have solidified more slowly than the surrounding fine-grained matrix. Feldspar crystal

Hornblende

mixed shades

banded ironstone Sedimentary rock made from alternating layers of iron minerals and gray shales (p.65) or cherts (p.69). The iron layer may be rich in a red oxide (hematite), a black oxide (magnetite), or a brown mixture of the two. Banded ironstones are ancient rocks believed to have formed in the oceans about 3.5 billion years ago, when oxygen was first released into the waters.

hematite-rich layer

magnetite and chert layer

mylonite Metamorphic rock with no specific mineral composition, but characterized by rippled and twisted bands of light and dark running through it. These bands are made by the flattening and stretching of mineral grains. alternating light and dark bands

Wavy texture

73

4Minerals

While all rocks are made of minerals, not all minerals are common constituents of rocks. Minerals are organized into groups based on their atomic structures and chemical makeup. They occur in a range of colors and external shapes, or habits.

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MINERALS

silicates Most of Earth’s crustal surface is made up of silicates. The minerals in this group contain varying combinations of oxygen and silicon, along with other substances.

quartz One of the most common rock-forming minerals. The glassy, prism-shaped crystals are colorless when pure. Milky white specimens are more common. Impurities produce dozens of other colors. In rocks, quartz generally occurs as visible white or gray crystals and sand grains.

Rhombohedral end

Prismatic crystal

Grooves on prism face

amethyst Pink or purple gemstone variety of quartz (above). Large crystals may have a pyramid-shaped peak. The striking colors are due to iron replacing some of the silicon inside the crystals.

Purple crystal about 4 in (10 cm) long

Glassy luster

silicates

smoky quartz

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Milky quartz

Dark brown or black variety of quartz (opposite), also known as cairngorm. The color comes from defects in the crystal structure produced by natural radiation. Like all varieties of quartz that produce large crystals, smoky quartz forms prisms with pointed tips. smoky quartz

rose quartz Ranging from a cool pink to rosy red, this collectible form of quartz contains trace amounts of titanium, manganese, and iron. Rose quartz is often translucent, due to the presence of tiny bubble spaces in the crystal lattice. Massive rose quartz

chalcedony A variety of quartz in which individual microscopic crystals combine to form rounded nodules, fibers, or tubes. Chalcedony is white when pure, but impurities give it a wealth of alternative hues; some are highly prized. Unlike shiny crystalline quartz (opposite), chalcedony has a waxy appearance. crystals form globular aggregates

Waxy luster

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gems

Gems A gem is any mineral or rock that is used in jewelry or as decoration. Flawless and symmetrical gemstones are rare and, therefore, highly prized and valuable. There are dozens of gems available, with most minerals having at least one gem variety that is attractive enough to be used in jewelry. Most gems are brightly colored, tough, and durable. They shine when polished and sparkle when cut.

suspended natural pearl

gold setting

Finely polished emerald

Large central ruby

Diamond inlay

Turban ornament This gold turban ornament from India has a variety of colorful gems—emerald, diamond, ruby, sapphire, and pearl—all set in a leaflike motif.

gems

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Precious stones The most popular gems are the four so-called precious stones: ruby, diamond, emerald, and sapphire. Pearls are also classified as precious stones, although they are formed by shellfish and are not strictly minerals. diamond

emerald

SaPPHire

rUBY

Pearl

Gem cutting Gems are highly prized for the way they sparkle. This is due to the optical property of refraction, whereby light bends when passing from one medium to another. A crystal can be cut in such a way that the light always reemerges from the upper surface. This gives the gem a dazzling sparkle. Incident light

Light reflects off internal surfaces before exiting

Partial reflection from surface

Light enters gem and changes direction

internal reflection When light shines into a highly refractive crystal, such as a diamond, it will reflect internally several times before coming back out.

diamond cutting The hardest mineral in the world, diamond is difficult to cut—only a diamond can cut another diamond. It must be cut carefully because a wrong incision could be a costly mistake.

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MINERALS

AGATE Semiprecious form of chalcedony, forming in nodules and characterized by concentric rings when cut in cross section. The colors of agate’s rings vary depending on impurities but are often golden, brown, white, blue, or red. Concentric layers of agate

Bands follow cavity outline

OPAL Semiprecious mineral that occurs as smooth, glassy stone rather than as crystals. Pure opal is colorless and transparent. However, most stones contain impurities and appear opaque with a shimmering, pearly coloring, mixing blues, yellows, and reds.

Glassy luster Ironstone matrix Common opal

Curved fracture

SILICATES

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ORTHOCLASE One of the most common rock-forming minerals, orthoclase belongs to the alkali feldspar group. It has a subtle pink color—as seen in granite (p.24) and other igneous rocks. Orthoclase’s glassy crystals often form prismatic grains, but they can also appear as sheets or in larger blocks.

Glassy luster

Prismatic orthoclase crystal

MICROCLINE An alkali feldspar that can be yellow, brown, pink, or red. It often forms prismatic or tabular crystals. Microcline has a glassy luster, and it is hard enough to be used in jewelry, especially the green variety known as amazonite. Smoky quartz

Prismatic crystal block

Pegmatite

82

MINERALS

anorthoclase

Glassy luster

An alkali feldspar with glassy prismatic or tabular crystals that can be colorless, white, gray, pink, yellow, or green. Anorthoclase is often seen as twinned crystals. In rocks, it usually appears as fine grains.

Single prismatic crystal

albite Feldspar mineral intermediate in composition between the alkali and plagioclase feldspars, appearing as tabular or platy crystals, often twinned. Albite gets its name from the Latin for white, but some specimens can be colorless, pink, or green. Glassy to pearly luster

Twinned, tabular crystal

oligoclase The most common of the plagioclase feldspars. It occurs in granular form in rocks, while larger specimens are generally glassy and massive. Often pale gray, oligoclase can also be red, green, brown, or yellow in color. Glassy luster

Perfect cleavage

SilicateS

anorthite Common plagioclase feldspar, occurring as small grains in iron-rich rocks. It appears as brittle prismatic crystals of rich pink, red, blue, white, or gray. It is very common in Moon rocks.

83

Glassy luster

anorthite crystal augite

labradorite Plagioclase feldspar that exhibits an iridescent surface. Labradorite is mainly blue, but its surface reflects a shimmering rainbow of colors. The mineral is seldom found as single crystals. Instead, it occurs as a compact mass of microscopic crystals. Perfect cleavage

Glassy luster

andesine Plagioclase feldspar named after the Andes mountains, where it is found in andesite lavas. Andesine is glassy gray-white when pure but can also be a darker yellow-brown. It occurs in massive form or as flat tabular crystals.

Blue porphyry

andesine crystal

84

MINERALS

serpentine Group of distinctive green minerals, which can also have yellow or gray hues. Serpentine has a greasy or waxy luster. Pure specimens are generally massive or form as plates or columns.

No visible cleavage

Greasy luster

Translucent serpentine

chrysotile Yellow-white fibrous mineral, and the main constituent of asbestos. Chrysotile is very soft and crumbles in the fingers. Its thin fibers, which form in the veins between rocks, give the mineral a greasy luster. Greasy luster

Bent and broken fiber

talc The softest mineral of all. It is the main ingredient in the dry astringent talcum powder used to soothe inflamed skin. Talc can be white, green, gray, or brown. It can occur in a crumbly massive form, or as fragile fibers or flattened sheets. Lines from stress during metamorphism

Translucent surface

Greasy luster

SilicateS

Mica The name given to a group of minerals, the most abundant member of which is muscovite or common mica. Micas typically occur as flat, platy specimens that flake easily. Micas are usually colorless—large sheets were traditionally used as window panes—but can also be brown, gray, green, or red.

tabular crystals

Glassy luster

Silver-brown muscovite

biotite Dark mica mineral found as large, flat, tabular specimens or stout prismatic crystals. Biotite is black when rich in iron impurities but fades to brown and yellow when magnesium dominates. Specimens are fragile and split into flat sheets. Brilliant luster

Rock matrix tabular crystal

thin, flexible sheets of biotite

85

86

MINERALS

Bentonite Sandy gray mineral, with a uniform color and dull surface. One of the most common clays, bentonite is the basic material for earthenware and pottery. It forms soft, earthy masses with crystals that are too small to see.

Dull, earthy surface

Massive habit

kaolinite Clay mineral used to add sheen to paper, furniture polishes, and paint. Kaolinite occurs as pale gray, earthy masses that are powdery to the touch or in harder massive forms.

Earthy luster

Granite matrix

Powdery texture

SilicateS

diopside Variable pyroxene mineral generally occurring as a green prismatic crystal. It can also be colorless, blue, or brown. Crystalline specimens are often square in cross section. Diopside can also occur in columnar form, as thin sheets, and as massive, glassy aggregates.

87

Prismatic diopside crystal

Quartz

augite The most common pyroxene, almost always seen as a dark mineral ranging from green to black. Augite specimens are opaque with a rather dull luster. Large crystals are prismatic and square or octagonal in cross section.

Dark, nearly opaque crystal

Volcanic tuff matrix

hornblende The name given to a small group of dark minerals rich in metal impurities. Hornblendes generally have a green hue but can also be brown or black. Large crystals have a glassy luster and grow in a bladed habit—elongated with a flat top.

Vertical grooves

Prismatic hornblende crystal Glassy luster

88

MINERALS

tourmaline

Cross section of watermelon tourmaline

A group of 11 minerals with blue, red, green, and colorless varieties. The most common tourmaline is the dark, opaque prismatic crystal found in pegmatites. Its crystalline form is highly prized.

Pink center Green crystal rim

beryl Silicate mineral containing the metal beryllium. Pure beryl is a yellowish transparent crystal, but its impure varieties are better known. It is known as the green gem emerald when chromium is present. Its other varieties are the semiprecious blue-green aquamarine and the pink morganite. Glassy luster

epidote Easily recognizable by its shades of green, ranging from near black to olive green. Epidote grows as prismatic crystals, often with striations, or grooves, along their length. Specimens can also appear needlelike or as massive aggregations.

Parallel grooves

Glassy luster

Long, prismatic crystal

Prismatic crystal

SilicateS

olivine

Rounded, transparent crystal

One of the most abundant minerals on Earth, forming most of the rocks deep down in the upper mantle. Usually pale green, olivine crystals are wedge-shaped or tabular. Granular masses are more common than crystals. It is better known for its gem variety, peridot.

Secondary clay minerals

kyanite This attractively colored mineral gets its name from the Greek for “deep blue.” It can also occur as orange, green, or colorless variants. Kyanite crystals can appear bladelike or occur in columnar or radiating forms.

long, bladed crystal

Rock matrix Quartz

garnet The name given to a group of minerals, all generally a striking dark red. The most common garnet, called almandine, can be red, pink, or violet. Garnet usually occurs as well-developed crystals, often embedded in rocks. Schist matrix

89

Well-formed crystal

90

MINERALS

Sulfides These minerals contain the non-metal element sulfur, often combined with one or two metals. This makes sulfides important ores—sources of valuable metals.

Galena Heavy, dark gray mineral often with well-defined cubic crystals and a metallic sheen. Galena is the main ore of lead and is frequently found with calcite (p.96) and quartz (p.76). It is soft enough to be scratched by a coin.

Cubic crystal

Metallic luster

Cinnabar Distinctive red mineral usually seen as a granular mass or powdery coating on a rock matrix, often with calcite (p.96). Cinnabar is the main ore of mercury and is highly toxic. Crystals of cinnabar are very rare.

Rock matrix Diamondlike luster

SulfideS

orpiment Golden yellow mineral containing arsenic, a highly toxic element. It is typically found as powdery crusts on volcanic rocks and around hot springs. However, it can also form striking massive specimens with a shiny, resinous surface similar to gold (p.103).

uneven fracture

Wavy texture

Resinous luster

ChalCopyrite A metallic yellow mineral frequently found surrounded by quartz crystals (p.76). Chalcopyrite is an important ore of copper (p.102). Often mistaken for pyrite (p.92), it has pyramidal crystals and a more brassy hue. Yellow chalcopyrite

Metallic luster

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92

MINERALS

PYRITE Golden yellow, iron-rich mineral. Pyrite often occurs as well-formed cubic crystals embedded in a rock matrix. It also has an eight-sided, or octahedral, habit. When it occurs as granular nodules, it may be mistaken for nuggets of gold (p.103), which is why it is famously known as “fool’s gold.”

Brassy yellow color

Cubic shape

Metallic luster

Marl matrix

STIBNITE Ore of the heavy metal antimony. Stibnite forms as a tangle of long, sometimes crooked, crystals, which are dark gray with a metallic luster. It frequently has a radiating habit with slender, needle-shaped crystals. It can also be massive or granular, or occur in sheets. Prismatic crystal

Quartz and barite

oxides

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Oxides oxygen is the most common element in minerals and rocks. oxides are minerals formed when oxygen bonds with other substances—usually metals and semimetals.

RUTILE Titanium ore found as needle-shaped crystals. Rutile is generally red but can also be yellow, brown, or black. When embedded in quartz (p.76), it appears golden. The mineral is also found in massive and radiating habits.

Vertical grooves along length of crystal

diamondlike sheen

Typical prismatic crystal shape

HEMATITE Iron ore found in a number of forms, from rhombohedral crystals to kidney-shaped (botryoidal), platy, and granular masses. Crystalline hematite is dark brown or black and has a metallic luster. Botryoidal hematite is dull red.

Metallic luster

Colorful tarnish on surface

Modified rhombohedral crystal

94

MINERALS

magnetite Black magnetic mineral and iron ore with a soft metallic luster. Magnetite can form octahedral (8-sided) or dodecahedral (12-sided) crystals, but is commonly seen in massive or granular forms and as crystal clusters. It also forms most black sands. Magnetic surface

Metallic luster

Corundum A colorless mineral better known for its gem varieties of sapphire and ruby. Second only to diamond in hardness, the red ruby variety contains chromium impurities, while the blue sapphire contains titanium and iron. Corundum crystals are hexagonal, tapering to a point, and sometimes have two pyramid-shaped ends.

Glassy luster

Tapering, barrelshaped ruby crystal

OXIDES

95

Spinel Hard mineral with well-formed, eight-sided crystals and a long history as a gemstone. Spinel occurs in many shades, including blue and green, but it is best known in its red form, which was mistaken for ruby until the 19th century. Spinel is found as large crystals in igneous rocks. It can also exist in granular forms.

Eight-sided spinel crystal

Quartz matrix

ice All naturally occurring nonorganic solids are classed as minerals, making ice the most common mineral on Earth’s surface. It is mostly found in massive aggregates, although snowflakes are crystalline. Pure ice is blue, but when air bubbles become trapped inside ice they give it a white, frosted color.

Broken edge

Small iceberg broken off from glacier

Layered structure

96

MINERALS

Carbonates Carbonate minerals contain carbon and oxygen in a ratio of 1:3. These minerals react readily with some acids, are generally soft, and wear away easily.

CalCite The most common calcium carbonate mineral. It is white or colorless when pure, and pinkish red and brown when it contains impurities. Spiked crystal specimens are called dogtooth spar, while those with flat-topped, columnar shapes are known as nailhead spar. Calcite is found in limestones, chalks, and marbles.

Crisscrossed lines show where specimen could split

Glassy luster

Group of dogtooth crystals

Dolomite Carbonate mineral ranging from colorless and white to brown, gray, and red. Dolomite contains magnesium as well as calcium. It is generally found in darker limestones. Although typically granular or massive, dolomite can also occur as large, rhombohedral crystals. Curved crystal face

Quartz matrix

Twinned crystals

Carbonates

aragonite Carbonate with crystal habits ranging from tabular to needlelike, prismatic, and radiating. It is the main mineral in speleothems—rocky cave formations such as stalactites. Aragonite has the same chemical formula as calcite (opposite), but a different molecular structure. Specimens can be white, yellow, green, red, blue, or brown. semitransparent crystal

radiating habit

Prismatic crystal

malachite Distinctive green copper ore. This mineral is found in botryoidal form—globular, like a cluster of grapes—or as a prickly, fibrous crust on a rock. Polished malachite frequently has delicate green banding. Single crystals are prismatic. rock Chrysocolla

matrix

Globular, clustered habit

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98

MINERALS

Organics Organic substances—anything produced by or made of living things—can form rocks and are collected as gems, just like inorganic minerals.

amber Soft, fossilized tree resin that fractures to form a sharp edge. Often found in areas rich in coal, amber is noncrystalline and occurs in nodules. It is generally orange, with a resinous surface.

Translucent mass of amber

Resinous luster

pearl Ball of aragonite (p.97)—a form of calcium carbonate—that forms inside the shells of oysters and other bivalve mollusks. The shell’s inner surface is coated with a mix of calcium carbonate and a small amount of conchiolin—an organic substance—to form a layer known as mother-of-pearl. This material collects around a sand grain trapped in the shell to form a pearl.

Black pearl

Iridescent mother-of-pearl

Conchiolin-rich outer shell

organics

shell Made from calcium carbonate, a mixture of calcite and aragonite. In mineralogical terms, shell refers to the protective armor of mollusks, such as scallops. A sediment of shells from dead mollusks, crushed up by turbulent waters, forms some limestones.

Dull to glassy luster

spider conch inner surface of shell

coral Material formed from the shells of generations of jellyfish-like coral polyps. Corals occur in many forms, from rounded, brainlike structures to horn-, tube-, and fan-shaped varieties. For conservation reasons, it is illegal to trade most types of coral specimens.

Wood-grain pattern

intensely red coral

coral branch

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Fossils

Fossils Millions of years old, fossils show how life in the past differed from the plants and animals that inhabit Earth today. Geologists also use fossils to date rocks. Fossils are almost exclusively found embedded in sedimentary rocks. The most common fossils are those of aquatic plants and animals. The fossilization process (opposite) always goes through the same stages, although the time taken may vary greatly.

Fossilized turtle This turtle fossil dates back to the time of the dinosaurs. After the creature was buried, its soft parts decomposed, while the shell and bones were replaced with new minerals and cemented into stone.

What is a fossil? Any evidence of a living thing left in rock is a fossil. This could be the impression of a body, a footprint, an egg, a nest, or even feces. Hard body parts—bones or shells—of animals buried in rock eventually turn to stone. Plant imprint

ImpressIon

Reptile footprint

trace

Cast of ammonite shell

hard body preservatIon

fossils

101

How fossils form After an organism dies, its body decays. However, if it is buried in mud, its hard parts usually remain intact while the sediment turns to stone. Sometimes fossils form when the original minerals are replaced by others carried in the water that saturates the sediment. Often, the body parts are gradually washed away, leaving a mold of the original, which may be filled with new minerals to make a cast fossil. The fossil is only revealed millions of years later, when the softer rocks around it erode.

deatH

burial

decay

rock ForMation

replaceMent

Making of a fossil The body of this dead coelacanth—a primitive, bony fish—may take only several decades to mineralize. The consolidation of sediments around the fossil into rock can take millions of years.

Fossil record

Strata at New sedimentary rocks location a form as layers (strata) on top of older rocks. So, the deeper a fossil is in the layers of sedimentary rock, the older it must be. This fact can be used to construct a fossil record that traces the evolution of life on Earth. Some fossils are found only in rocks of a certain age. Called index fossils, they are useful in determining the age of a rock.

Strata dating By comparing fossils found in rocks at different locations (as shown here), it is possible to work out the relative ages of rock layers.

Strata at location b

102

MINERALS

Native elements While most elements occur in a combined state within minerals, native elements are found in a pure form. This means they cannot be divided into simpler ingredients.

Sulfur Distinctive yellow mineral that melts to become red. Pure sulfur is found in volcanically active areas. It can form resinous crystals shaped like needles or rectangular prisms but is generally found as powdery blocks. Resinous luster Rock matrix

copper Pinkish red metal rarely found in a pure state. Metallic copper is found in veins or mixed with iron- and copperrich minerals. It forms flat masses or is branching, spreading out in a continually dividing form. It can also be found as cubic crystals.

Massive copper

Quartz

NAtive elemeNts

103

GolD Deep yellow metal, almost always uncombined. Gold occurs as nuggets and plates, but more often as grains embedded in rock. It is nonreactive and keeps its luster even after millions of years of having been buried underground.

scaly gold mass of soft, pure gold

thin plate of gold

massive quartz

SIlver

Wiry silver

Metallic white element found in rocks where hot waters once trickled. Deposits of pure silver are wiry or branching and seldom shiny. Silver’s surface turns black or brown when it comes into contact with other chemicals.

Brown, tarnished surface

Crystal growth visible as ridges

Quartz

104

MINERALS

GRAPHITE Metallic-looking form of pure carbon, which takes shape as shiny lumps with a slippery feel. In contrast to diamond (below), graphite is very soft and its surface scrapes off easily. It is commonly used as pencil lead.

Perfect cleavage

Massive habit

Metallic luster

DIAMOND Colorless form of carbon and hardest of all minerals. It is prized as a gem. Natural diamonds occur as crystals, sometimes tinged with pinks, blues, and yellows due to impurities. Flecks of carbon can often be seen in crystals. Low-quality, gray-black diamonds are used in industrial cutting tools.

Rock matrix

Yellowish crystal with a brilliant luster

other minerals

105

other Minerals these include specimens from smaller and rarer mineral groups, such as halides (containing halogens), borates (containing boron), and sulfates and phosphates.

borax

Prismatic crystal

Borate mineral associated with deserts and salt flats. Most specimens are anhydrous, meaning that their water content has evaporated, leaving a yellow-gray earthy mass. However, it is colorless when its crystals contain trapped water molecules. Rarely, crystals are tabular.

Chalky white coating, due to water loss

rock matrix

halite Mineral form of table salt, generally found in massive aggregates of rock salt (p.54) or granules left by evaporated saltwater. It is colorless when pure, and reddish or orange in most rock salts. Halite forms cubic crystals and is unusual in that it dissolves in water.

Glassy luster

Cubic crystal rock matrix

106

MINERALS

Gypsum Colorless or white, with a tinge of color when impure. In its dried form it is a component of plaster of Paris and solidifies upon the addition of water. Its desert rose form can be made of slanted blades, prismatic columns, or delicate fibers. Transparent, prismatic crystal

Pearly luster

Bauxite

Dull to earthy luster

A mixture of several aluminum ores that occurs as clay-rich, earthy deposits, nodules, and pisoliths—spongelike rocks filled with pea-sized grains. It is generally yellow or brown but can also be pink or red.

Aluminum oxide matrix

Goethite Mineral form of rust, typically red-brown but can be darker depending on crystal size—smaller crystals are light, larger ones are dark. Goethite forms velvet crusts, radiating fibers, flat tabular crystals, and globules.

Radiating crystals

Quartz matrix

other minerals

ApAtite Mineral similar to the stiffening material found in bones and teeth. Apatite forms glassy, prismatic crystals or waxy, compact masses and nodules. Its hydrated forms are yellow, while other types are brown, pink, or rosy red.

arsenopyrite

turquoise Named for its major trading center, Turkey, this microcrystalline mineral is often carved into figurines and cabochons. Turquoise is found as veins running through rocks or amorphous nodules. It has a waxy luster but can be polished to a shine.

iron oxide matrix

turquoise

Prismatic crystal

107

108

rock gallery

rock gallery This gallery displays the rocks profiled in this book, grouped by type: igneous, sedimentary, or metamorphic. Within each group, the rocks are arranged by grain size, starting with coarse-grained and ending with fine-grained. you can use this gallery to see how rocks that vary in appearance, and feature in different parts of the book can belong to the same type.

igneous rocks graphic granite p.18

Pink granite p.19

granitic pegmatite p.19

kimberlite p.22

Pyroxenite p.22

granite p.24

Tourmaline pegmatite p.24

granodiorite p.26

anorthosite p.31

Diorite p.25

Tuff p.31

rock gallery

gabbro p.38

Peridotite p.38

Dolerite p.39

lamprophyre p.39

layered gabbro p.42

leucogabbro p.42

carbonatite p.43

Porphyry p.43

Syenite p.46

Pumice p.50

Trachyte p.51

Scoria p.50

Basalt p.57

Vesicular basalt p.57

andesite p.58

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110

rock gallery

obsidian p.58

rhyolite p.59

amygdaloidal basalt p.71

Dacite p.72

Volcanic bomb p.59

sedimentary rocks conglomerate p.27

Breccia p.27

greensand p.33

arkose p.33

gritstone p.35

Sandstone p.34

red sandstone p.34

oolitic limestone p.35

rock gallery

orthoquartzite p.41

Ironstone p.41

limestone p.52

Tufa p.52

Dolomite p.53

chalk p.53

green marl p.54

rock salt p.54

rock gypsum p.55

Diatomite p.56

loess p.56

red marl p.64

Shale p.64

oil shale p.65

Mudstone p.66

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112

rock gallery

Siltstone p.67

graywacke p.67

Tillite p.68

Flint p.68

chert p.69

Bog iron p.69

coal p.70

laterite p.71

Travertine p.72

metamorphic rocks Serpentinite p.23

Banded ironstone p.73

eclogite p.23

amphibolite p.25

Quartzite p.30

rock gallery

Marble p.32

113

kyanite schist p.32

Muscovite schist p.40

Folded schist p.40

Migmatite p.44

granulite p.45

Skarn p.45

Banded gneiss p.46

Folded gneiss p.47

Soapstone p.51

Hornfels p.60

Blueschist p.60

Slate p.61

Phyllite p.61

Mylonite p.73

114

mineral gallery

mineral gallery This gallery presents the minerals profiled in this book. They are arranged according to their mineral families, which are defined by their chemical constituents. it is not possible to include all minerals—there are thousands of them, arranged in dozens of families and groups—but this section allows you to see the great variety of mineral colors and shapes.

SILICATES Quartz p.76

amethyst p.76

Smoky quartz p.77

rose quartz p.77

Chalcedony p.77

agate p.80

Opal p.80

microcline p.81

anorthoclase p.82

Orthoclase p.81

albite p.82

mineral gallery

Oligoclase p.82

anorthite p.83

labradorite p.83

andesine p.83

Serpentine p.84

Chrysotile p.84

Talc p.84

mica p.85

Biotite p.85

Bentonite p.86

Kaolinite p.86

Diopside p.87

augite p.87

Hornblende p.87

Tourmaline p.88

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116

mineral gallery

Beryl p.88

epidote p.88

Kyanite p.89

garnet p.89

Olivine p.89

SulfIDES

Orpiment p.91

Stibnite p.92

galena p.90

Cinnabar p.90

Chalcopyrite p.91

Pyrite p.92

mineral gallery

OXIDES

magnetite p.94

rutile p.93

Hematite p.93

Corundum p.94

Spinel p.95

Calcite p.96

Dolomite p.96

ice p.95

CARBONATES

aragonite p.97

malachite p.97

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118

mineral gallery

OrgaNics

amber p.98

Shell p.99

Pearl p.98

Coral p.99

Native elemeNts

gold p.103

Diamond p.104

Sulfur p.102

Copper p.102

Silver p.103

graphite p.104

mineral gallery

other minerals Borax p.105

Halite p.105

gypsum p.106

Bauxite p.106

goethite p.106

apatite p.107

Turquoise p.107

119

120

GLOSSARY

Glossary The study of rocks and minerals has its own jargon. A few terms can help you understand rocks and minerals better and identify them with greater precision. Aggregate A large cluster of similar mineral crystals and rock fragments. Alkali feldspar A type of silicate mineral that contains a high percentage of potassium and sodium. Bed The smallest division of a sedimentary rock formation. Each bed indicates a change in the original material being deposited. Bedding plane The surface that separates adjacent beds. All beds are originally horizontal but are often rotated, twisted, or buckled over the years. Bladed A crystal habit in which the crystals resemble a blade. Botryoidal A habit in which the mineral occurs in globular masses—and resembles a bunch of grapes. Breccia A sedimentary rock consisting of angular fragments of older rocks. Cabochon A gem that has been cut and polished to give it a domed top and a flat bottom. Carbonate A compound that contains carbon and oxygen atoms in a 1:3 proportion. Clast A fragment of rock or mineral that becomes part of a new sedimentary rock. Clay A member of a large group of silicate minerals that occur as very fine grains. Cleavage The way a mineral breaks into sections with flat surfaces. Some minerals can do this, others cannot. Columnar A habit in which the mineral forms into thick, elongated columns. Conchoidal fracture A property of certain rocks and minerals that have a curved or shell-like fracture. Flints break in this way. Cross-bedding Layering in which neighboring beds are orientated in different directions—one may be horizontal, the other slanted at an angle. Crystal A mineral in which the inner symmetry of its atomic arrangement is reflected in flat outer faces in a geometric pattern. Cut The particular shape and geometric form given to a gem. Dodecahedral A term used to describe a crystal having 12 sides. Dull luster A type of luster in which little light is reflected. Earthy luster A type of luster in which the mineral’s surface looks powdery or granular. Erosion A process in which fragments of rock and mineral are loosened and carried away by wind, water, or ice. Eruption A natural phenomenon in which volcanic material—such as magma, ash, and gas—bursts out of Earth’s surface through a volcano or vent. Extrusive rock An igneous rock that forms on the surface of Earth.

glossary

121

Face A flat surface of a crystal. Feldspar A group of common silicate minerals that are found in many intrusive igneous rocks. Feldspar means “field crystal” in German. Fissile texture A rock texture which allows the rock to be split into sheets. Flow banding Marks in an igneous rock that show that the molten lava or magma was flowing until it cooled and became a solid rock. Foliation A characteristic of metamorphic rocks whereby the component crystals have been arranged into sheets or waves. Fossil Evidence of life left in a rock. Fracture The way that rocks and minerals break. Garnet A member of a group of silicate minerals that often contain magnesium, calcium, and iron. Gem/gemstone A precious crystal or stone used in decoration, especially in jewelry. Gem cutting The process of shaping a gem to maximize its sparkle and color. Glass A noncrystalline solid in which the molecules are jumbled in a random order, rather than arranged in a repeating pattern, as in crystals. Glassy texture The smooth and shiny surface appearance of a rock or mineral, similar to that of a piece of glass. Grain The basic unit that makes up a rock. Granular texture A mineral texture in which the grains are visible to the naked eye. Groundmass see matrix Habit The large-scale appearance of a mineral. Some minerals occur in a range of habits. Hackly fracture The fracture of a mineral, often a metal, that breaks into jagged sections. Heavy metal A metal that is usually dense and heavy. Examples include mercury and antimony. Igneous rock A rock that forms when melted rock called magma cools down and solidifies. Inclusion A crystal within another crystal. The two crystals are usually of different minerals. Intrusive rock An igneous rock that cools from magma under the surface of Earth. Iridescence The reflection of light from internal elements of a stone, yielding a rainbowlike play of colors. Lava Magma that has reached the surface. Luster A characteristic of a mineral describing how light interacts with the surface, giving it a particular appearance. Magma Molten or liquid rock that forms deep beneath Earth’s surface, where it is hot enough to melt minerals.

122

GLOSSARY

Massive A mineral habit in which the material has no definite form and individual crystals cannot be seen. Matrix The fine-grained background material of a rock into which or on top of which larger crystals are set. Metal A substance characterized by high electrical and thermal conductivity as well as by malleability, ductility, and high reflectivity of light. Metallic luster The shiny, reflective surface appearance of certain minerals, resembling that of a metal. Metamorphic rock Rock that forms when another rock is subjected to heat and pressure underground, but without remelting. Mica A member of a series of silicate minerals, frequently occurring in thin sheets, which are common components of rocks. Microcrystalline A mineral with crystal grains too small to see with the naked eye. Mineral group A family of minerals that share a certain chemical composition or structure. Native element A naturally occurring pure chemical element that cannot be separated into simpler components. Examples include gold and sulfur. Nodule A generally rounded accumulation of sedimentary material differing from its enclosing sedimentary rock. Nonmetal A material that does not have metallic properties. Octahedral Having eight sides arranged as two base-to-base pyramids. Ooliths Individual, spherical sedimentary grains, usually of calcite, from which oolitic rocks are formed. Ore A mineral that is a valuable source of useful materials, such as metals. Oxidation The process by which a substance reacts with oxygen in air and water. Pegmatite An intrusive igneous rock in the form of a vein, and composed of large crystals. Phenocryst A large crystal set into an igneous rock composed of a matrix of smaller grains. Platy A mineral habit in which the crystals occur in plates. Porphyritic texture The texture of a rock studded with large crystals. Porphyroblast A large single crystal that forms within a metamorphic rock. Precious A term referring to the most highly prized minerals and metals. Prismatic Having the shape of a prism. Pyramidal Having the shape of a pyramid. Pyroclastic rock A rock formed from the ash and other material forced out of a volcano in an explosive eruption. Pyroxene Part of a group of 21 important rock-forming silicate minerals containing silicon, oxygen, and metals such as calcium, aluminum, and magnesium among many others.

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123

Radiating habit A type of habit in which the crystals have grown outward from a common center in radiating form. Refractive index A figure that shows how much a beam of light is bent as it shines through a particular transparent solid. Resinous luster The shine of resin—a soft sheen combined with a greasy appearance. Rhombohedral Having the shape of a rhombus, or parallelogram. Rock dating The process of determining the age of a rock. Semimetal A material that exhibits characteristics of both a metal and a nonmetal. Silicon is the most common semimetal. Semiprecious A term describing a gem that is highly prized but less valuable than the so-called precious stones. Sedimentary rock A rock formed on Earth’s surface from layers of sediment deposited on each other. Silicate A mineral compound containing silicon and oxygen and usually a number of other elements. Most rocks are made of silicate minerals. Stalactite A rock formation that hangs down from the roof of a cave. Stalagmite A rock formation rising up from the floor of a cave. Striations Parallel horizontal or vertical grooves or lines appearing on a crystal. Tabular A mineral habit in which the crystals occur in flat blocks. Tarnish The process in which a rock’s metallic luster is diminished by the oxidation of the surface, which reduces its shine and reflectivity. Termination Faces that make up the ends of the crystal. Texture A description of the look and feel of the surface of a rock. Trace element An element that occurs in very small amounts in a mineral. Twinned crystals Two crystals of the same mineral that share a face and grow away from each other in opposite directions. Variety A form of a mineral with a different appearance but the same chemical composition. Vein A geological feature where a thin, sheetlike mineral or rock fills a crack running through another rock. Vesicle A spherical or oval cavity produced by a gas bubble in magma that remains after the magma has solidified to form a rock. Weathering A process by which wind, water, ice, natural acids, or the action of animals and plants break rocks into fragments.

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INDEX

Index Page numbers in bold indicate main entry.

A agate 80, 114 albite 82, 114 alkali feldspar 9, 19, 24, 25, 26, 51 almandine 89 amazonite 81 amber 98, 118 amethyst 76, 114 amphibole 25, 46 amphibolite 25, 112 amygdaloidal basalt 71, 110 andesine 83, 115 andesite 58, 109 anorthite 83, 115 anorthoclase 82, 114 anorthosite 31, 108 apatite 13, 107, 119 aragonite 97, 98, 117 arkose 33, 110 augen 47 augite 9, 38, 87, 115

B banded gneiss 46, 113 banded ironstone 73, 112 basalt 8, 57, 109 amygdaloidal 71, 110 column 21 vesicular 57, 109 bauxite 106, 119 bedding 11 bentonite 86, 115 beryl 88, 116 biotite 9, 85, 115 black granite see diorite blueschist 60, 113 bog iron 69, 112

borax 105, 119 boulder clay see tillite breccia 27, 110

C cairngorm see smoky quartz calcite 13, 22, 25, 32, 35, 45, 71 calcite 96, 117 calcium plagioclase 38, 39 carbon diamond 104 graphite 104 carbonates 96–97 carbonatite 43, 109 chalcedony 69, 77, 114 chalcopyrite 91, 116 chalk 53, 111 chemical sediments 36 chert 69, 112 chrysotile 84, 115 cinnabar 90, 116 clast 36 clastic sediments 36 cleavage 14 coal 70, 112 conchiolin 98 conglomerate 27, 110 contact metamorphism 62 copper 102, 118 coral 99, 118 corundum 13, 94, 117 cross-bedding 34

D dacite 72, 110 diagenesis 37 diamond 13, 79, 104, 118 diatomite 56, 111 diatoms 56 diopside 87, 115 diorite 25, 26, 108

dirty sandstone see graywacke dogtooth spar 96 dolerite 39, 109 dolomite (mineral) 96, 117 dolomite (rock) 53, 111 dynamic metamorphism 63

E eclogite 23, 112 epidote 88, 116 erosion 37 extrusive rock 21

F flint 68, 112 flow-banding 10 fluorite 13 folded gneiss 47, 113 folded schist 40, 113 folding 11 foliation 10 fossils 11, 100–101 formation 101 record 101 fracture 14

G gabbro 38, 109 layered 42, 109 leucogabbro 42, 109 galena 90, 116 garnet 23, 25, 89, 116 pyrope 23 gem 78–79 gem cutting 79 glauconite 33 glaucophane 60 gneiss 44 banded 46, 113 folded 47, 113 goethite 69, 106, 119 gold 103, 118

INDEX granite 24, 26, 44, 108 graphic 18, 108 pink 8, 19, 108 granitic pegmatite 19, 108 granodiorite 26, 108 graphic granite 18, 108 granulite 45, 113 graphite 104, 118 graywacke 67, 112 green marl 54, 111 greensand 33, 110 greenschist 60 gritstone 35, 110 gypsum 13, 106, 119

H halite 105, 119 hardness 13 hematite 32, 69, 93, 117 hornblende 23, 25, 47, 72, 87, 115 hornfels 60, 113

I ice 95, 117 identifying minerals 12–13 identifying rocks 10–11 igneous rock 8, 20–21 extrusive 21 intrusive 21 index fossils 101 internal reflection 79 intrusive rock 21 ironstone 41, 111 ironstone, banded 73, 112

K kaolinite 86, 115 kimberlite 22, 108 Kimberly 22

kyanite 32, 89, 116 kyanite schist 32, 113

L labradorite 83, 115 lamprophyre 39, 109 laterite 71, 112 lava 20 layered gabbro 42, 109 leucogabbro 42, 109 limestone 52, 111 limestone, oolitic 35, 110 loess 56, 111 luster 13

M magnetite 94, 117 malachite 97, 117 marble 32, 113 medium grained rocks 28–47 dark 38–41 mixed shades 42–47 pale 30–35 metamorphic grade 62 metamorphic rock 8, 62–63 metamorphism 62 contact 62 regional 62 shock 62 dynamic 63 mica 24, 85, 115 biotite 9, 19, 24, 25, 40, 46, 47, 85 muscovite 40 microcline 81, 114 migmatite 44, 113 mineral 9, 74–107 carbonates 96–97 cleavage 14 fracture 14

125

mineral (continued) habit 12 hardness 13 identification 12–13 luster 13 native elements 102–03 organics 98–99 oxides 93–97 silicates 76–89 streak 14 sulfides 90–92 Mohs scale 13 mother-of-pearl 98 mudstone 66, 111 muscovite 40 muscovite schist 40, 113 mylonite 73, 113

N nailhead spar 96 native elements 102–103

O obsidian 58, 110 oil shale 65, 111 oligoclase 58, 82, 115 olivine 23, 38, 42, 89, 116 omphacite 23 ooliths 35, 41 oolitic limestone 35, 110 opal 80, 114 organics 98–99 orpiment 91, 116 orthoclase 9, 13, 18, 24, 39, 81, 114 orthopyroxene 31 orthoquartzite 41, 111 oxides 93–97

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INDEX

P pearl 98, 118 pegmatite, granitic 19 tourmaline 24 peridotite 23, 38, 109 phyllite 61, 113 pink granite 8, 19, 108 plagioclase 25, 26, 31 plagioclase feldspar 22, 26, 38, 39, 42, 45, porphyry 43, 109 potassium feldspar 9, 24, 25, 31 pumice 50, 109 pyrite 92, 116 pyrope garnet 23 pyroxene 22, 23, 38, 42, 60 pyroxenite 22, 108

Q quartz 9, 13, 18, 24, 30, 32, 33, 35, 41, 47, 76, 114 rose 77, 114 smoky 77, 114 quartzite 30, 112

R red marl 64, 111 red sandstone 34, 110 refraction 79 regional metamorphism 62 rhyolite 59, 110 rock gypsum 55, 111 rocks igneous 8, 20–21 metamorphic 8, 62–63 sedimentary 8, 36–37

rock salt 54, 111 rose quartz 77, 114 rutile 93, 117

S sandstone 8, 33, 34, 35, 110 orthoquartzite 41 red 34, 110 schist folded 40, 113 kyanite 32, 113 muscovite 40, 113 scoria 50, 109 scratch test 13 sedimentary rock 8, 36–37 serpentine 23, 84, 115 serpentinite 23, 112 shale 62, 64, 111 oil 65, 111 shell 99, 118 shock metamorphism 62 silicates 76–89 siltstone 67, 112 silver 103, 118 skarn 45, 113 slate 8, 61, 113 smoky quartz 77, 114 soapstone 51, 113 speleothem 97 spinel 95, 117 stibnite 92, 116 strata dating 101 streak 14 sulfides 90–92 sulfur 102, 118 syenite 46, 109

T talc 13, 51, 84, 115 tektite 70 texture 10 textured granite see graphic granite

tillite 68, 112 topaz 13 tourmaline 19, 24, 88, 115 tourmaline pegmatite 24, 108 trachyte 51, 109 travertine 72, 112 tufa 52, 111 tuff 31, 108 turquoise 107, 119

VW vesicular basalt 57, 109 volcanic bomb 59, 110 weathering 37

Acknowledgments 127

Acknowledgments Dorling Kindersley would like to thank the following people for their assistance in the preparation of this book: Priyaneet Singh for editorial assistance, Sanjay Chauhan for design assistance, and Steve Setford for proofreading. The publisher would like to thank the following for their kind permission to reproduce their photographs: (Key: a-above; b-below/bottom; c-center; f-far; l-left; r-right; t-top) 6-7 Dorling Kindersley: Rough Guides. 10 Dorling Kindersley: Oxford University Museum of Natural History (c). 11 Dorling Kindersley: Natural History Museum, London (br). Dreamstime.com: Matauw (tr). Fotolia: suronin (c). 12 Dorling Kindersley: Oxford University Museum of Natural History (cb); Natural History Museum, London (ca, cra, bl). 13 Dorling Kindersley: Natural History Museum, London (tl, tr, br). 14 Dorling Kindersley: Natural History Museum, London (cra, cr, crb). 15 Dorling Kindersley: Rough Guides. 16-17 Dreamstime.com: Shargaljut. 19 Dreamstime.com: Jolin (tr). 20 Dreamstime.com: Gardendreamer. 21 Dreamstime.com: Paul Van Den Berg (b). 22 Alamy Images: Francis Vachon (cr). 23 123RF.com: Vladimir Gurov (tr). 24 Dreamstime.com: Lunamarina (tr). 26 Dreamstime.com: Siim Sepp (tr). 28-29 Dreamstime.com: Csongor Tari. 33 Fotolia: siimsepp (b). 34 Dreamstime.com: Pancaketom (cr); Singkamc (tr). 35 Alamy Images: Nic Cleave (cr). Dreamstime.com: Falcon stock (tr). 37 Dorling Kindersley: Rough Guides (t). Fotolia: Dario Lo Presti (cr). 38 Alamy Images: Nic Cleave (tr). 43 Alamy Images: RF Company (t). Getty Images: De Agostini (cr). 48-49 Dreamstime.com: Alon Othnay. 50 Alamy Images: Phil Degginger (t). Dorling Kindersley: Natural History Museum, London (b). 51 Corbis: Visuals Unlimited / / Scientifica (b). Getty Images: De Agostini (tr). 52 Dreamstime.com: Dziewul (tr). 53 Fotolia: Tyler Boyes (t). 57 Fotolia: Dudarev Mikhail (tr). 58 Dorling Kindersley: Oxford University Museum of Natural History (b); Natural History Museum, London (t). 61 123RF.com: Waraporn Sang-Arwut (tr). Science Photo Library: Joyce Photographics (cr). 68 Fotolia: Claudio Divizia (cr). 69 Fotolia: siimsepp (b). 70 Dorling Kindersley: Natural History Museum, London (b). 71 Dorling Kindersley: Neil Fletcher (b). 72 Dorling Kindersley: Oxford University Museum of Natural History (t). Dreamstime.com: Gualtiero Boffi (tr). 74-75 Dreamstime.com: Ukrphoto. 79 Dorling Kindersley: Natural History Museum, London (tc, tr). Getty Images: (br). 84 Dorling Kindersley: Natural History Museum, London (t). 85 Dorling Kindersley: Natural History Museum, London (t). 87 Dorling Kindersley: Natural History Museum, London (b). 88 Dorling Kindersley: Natural History Museum, London (t, c). 90 Dorling Kindersley: Natural History Museum, London (t). 94 Dorling Kindersley: Natural History Museum, London (t). 95 Corbis: All Canada Photos / Klaus Lang (b). 100 National Geographic Stock: O. Louis Mazzatenta (c). 104

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ACKNOWLEDGMENTS

Dorling Kindersley: Natural History Museum, London (b). 109 Alamy Images: RF Company (cl); Phil Degginger (clb). Dorling Kindersley: Natural History Museum, London (cb, br). 110 Dorling Kindersley: Oxford University Museum of Natural History (tl). Fotolia: siimsepp (cb). 111 Fotolia: Tyler Boyes (ca). 112 Dorling Kindersley: Neil Fletcher (c); Oxford University Museum of Natural History (cr). Fotolia: siimsepp (cra). 113 Corbis: Visuals Unlimited / / Scientifica (clb). 115 Dorling Kindersley: Natural History Museum, London (ca, c, bc, br). 116 Dorling Kindersley: Natural History Museum, London (tl, c). 117 Corbis: All Canada Photos / Klaus Lang (cl). Dorling Kindersley: Natural History Museum, London (cla). 118 Dorling Kindersley: Natural History Museum, London (bl) Jacket images: Front: Dreamstime.com: Pod666 crb, Radkol cra All other images © Dorling Kindersley For further information see: www.dkimages.com