Childrens Illustrated Encyclopedia

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Rocks and mineRals

442

Crystals

Minerals often form crystals—

solids that grow in regular

shapes with flat sides. light

sparkles from crystals because they are

often transparent and have smooth, shiny

surfaces. Each mineral forms crystals with

particular shapes, such as columns and

cubes. Crystals grow from

molten minerals or

minerals that

are dissolved in

liquids, such

as water.

MinErals

an impressive rock collection will feature rocks that

contain beautiful mineral crystals. Minerals are the

different substances of which rocks are made.

For example, limestone and marble contain the

white mineral calcite. Minerals include precious

stones, such as diamonds, and ores—minerals

that contain metals, such as iron and

aluminum. almost all metals are

produced by mining and quarrying

ores and then treating the ores to

extract their metals.

HalitE

table salt

comes from the

mineral halite.

Halite forms where

seawater dries at the

shore. Underground

deposits of halite are the

remains of ancient salt

lakes. Pure salt has no

color, but impurities in

halite give it a pink color.

tUrqUoisE

Jewelers cut beautiful

gemstones and

ornaments from

turquoise, a

blue-green

mineral that

often runs in

a thin vein

through other

rocks.

DEsErt rosE

the mineral gypsum

forms petal-shaped

crystals in deserts and dry

regions. this happens as

water dries up, leaving

mineral deposits behind.

the crystals often look like

flowers, so they are called

desert roses or gypsum flowers.

GalEna

Glistening gray crystals

of galena stick out

from a piece of white

limestone. Galena

forms cubic crystals.

it is the main ore in

which lead is found,

and it often appears as

a vein in limestone. lead

is combined with sulfur in

galena. smelting the ore by

heating it in a furnace removes

the sulfur and leaves lead metal.

sUlFUr

yellow crystals form

when molten sulfur cools.

large underground

deposits in places such as

the United states provide

sulfur for making rubber

and chemicals.

qUartz

quartz is

one of

the most

common

minerals.

Electronic

clocks and

watches

contain small

cut pieces of

quartz that control time-

keeping with great accuracy.

Uses of rock

rocks in one form or another

surround us in towns, cities, and the

countryside. Hard rocks such as

granite, sandstone, and limestone

provide good building materials for

houses and walls, and roads contain

fragments of crushed rock. soft rocks

have uses, too. Heating clay or shale

with crushed limestone produces

cement for making concrete and laying

bricks. Bricks themselves are made by

baking clay in molds.

The first tools were made

of stone. Early people broke

pieces of rocks and stone to

make sharp cutting

implements such as axes.

Sculptors

work rocks,

stones, and

pure minerals to make

statues and ornaments.

Some minerals,

such as solecite,

form needle-

shaped crystals.

Hexagonal

crystals form

in six-sided

columns.

Find out more

atoms and molecules

Clocks and watches

Comets and meteors

Fossils

science

Volcanoes

Crystals form in columns,

such as in this piece of

the mineral beryl.

Cubic crystals

form in four-

sided columns.

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