Lets Collect Rocks and Shells | Page 5

recover such metals from an average ton of earth. That's why metallic
minerals are taken from concentrated deposits in mines.
Many valuable minerals are found in veins running through rock. Veins
can be formed when: (a) mineral-laden ground water seeps into cracks,
evaporates, and leaves mineral grains that build up into a vein; (b) hot
water from deep within the earth fills cracks, then cools and deposits
much of the material in solution as minerals in a vein--sometimes
including metals such as gold and silver; (c) molten gaseous material
squeezes into cracks near the earth's surface, then slowly hardens into a
vein.
2. NONMETALLIC MINERALS. These are of great importance to
certain industries. You will find them in insulation and filters. They are
used extensively in the ceramic and chemical industries. They include
sulfur, graphite (the "lead" in pencils), gypsum, halite (rock salt), borax,
talc, asbestos and quartz. Undoubtedly, you'll have some nonmetallic

minerals in your collection. Rocks containing asbestos are especially
handsome and varied.
3. ROCK-FORMING MINERALS. These are the building materials of
the earth. They make mountains and valleys. They furnish the
ingredients of soil and the salt of the sea. They are largely silicates--that
is, they contain silicon and oxygen. (Silicon is a nonmetallic element,
always found in combination with something else. It is second only to
oxygen as the chief elementary constituent of the earth's crust.)
Other rock-forming minerals are the large family of micas, with names
like muscovite and phlogopite. There are the feldspars, including albite
and orthoclase. Others are amphiboles, pyroxenes, zeolites, garnets and
many others you may never find or hear about unless you become a
true mineralogist.
A rock may be made almost entirely of one mineral or of more than one
mineral. Rocks containing different combinations of the same minerals
are different. Even two things made of the same single mineral can be
quite different. Carbon may turn up as a lump of coal or a diamond.
How Minerals Got Their Names
Names of most minerals end in "ite"--apatite, calcite, dolomite, fluorite.
But many do not: amphibole, copper (the most common pure metal in
rocks), feldspar, galena, gypsum, hornblende, mica, quartz.
Many minerals take their names from a Greek word referring to some
outstanding property of the mineral. For example, hematite, an oxide of
iron, was named about 325 B.C. from the Greek HAIMA, or blood,
because of the color of its powder.
Some minerals are named for the locality in which they were first
discovered. Coloradoite was first found in Colorado. Benitoite turned
up in San Benito County, California. And so with labradorite and
brazilite.
Other minerals got their names from famous people. Willemite was
named in honor of Willem I, King of the Netherlands. The great
German poet-philosopher, Goethe, could turn up in your collection as
goethite. And there's smithsonite, named for James Smithson, founder
of the Smithsonian Institution.
[figure captions]
Gold, jasper, uncut diamond, quartz (violet in color), halite (Carlsbad
N.M.), calcite (S. Dakota), copper, turquoise (brilliant color)

Out Of This World
Some minerals come from outer space. They're meteorites, which are
rock fragments. Every day, hundreds of millions of them enter the
earth's atmosphere. Most of them, however, are burned up by the heat
from air friction and never reach the ground. Meteors large enough to
reach the earth are called meteorites. Most minerals found in meteorites
are the same as those we have on earth. But, there are some rare
minerals known only in meteorites. Two of them are cohenite and
schreibersite.

MAIN KINDS OF ROCKS

Rocks are the building blocks of the earth's crust. They may be massive,
as in granite ledges, or tiny. Soil, gravel, sand and clay are rocks.
THERE ARE THREE MAIN TYPES OF ROCKS.
1. IGNEOUS rocks are those formed at very high temperatures or from
molten materials. They come from magmas--molten mixtures of
minerals, often containing gases. They come from deep below the
surface of the earth. If they cool off while below the surface, they form
intrusive rocks, which may later be revealed by erosion. When magmas
reach the surface red hot, they form extrusive rocks, such as volcanic
rocks. Thus, granite is an igneous, intrusive rock; lava is an igneous,
extrusive rock. (Notice how the type of rock tells its past history--if you
know what to look for.)
2. SEDIMENTARY rocks are formed by the action of wind, water, or
organisms. They cover about three quarters of the Earth's surface. Most
are laid down--as sediments--on the bottom of rivers, lakes and seas.
Many have been moved by water, wind, waves, currents, ice or gravity.
The most common sedimentary rocks are sandstones, limestones,
conglomerates and shales. Oil is found in sedimentary formations.
3. METAMORPHIC rocks are those that have been changed from what
they were at first into something