Rocks, to begin with, are made of minerals. What is a mineral?
The definition may sound difficult--a mineral is a chemical element
or compound (combination of elements) occurring naturally as the
result of inorganic processes. But don't be discouraged. Things
will clear up soon.
The world contains more than 1,100 kinds of minerals. These can
be grouped in three general classes.
1. METALLIC MINERALS. These include things most of us would think
of if we were asked to name some minerals. Familiar examples are
copper, silver, mercury, iron, nickel and cobalt. Most of them are
found in combination with other things--as ores. We get lead from
galena, or lead sulfide. Tin comes from the ore cassiterite; zinc
from sphalerite and zincblende, or blackjack. Chromium that makes
the family car flashy comes from chromite. Many minerals yield
aluminum. Uranium occurs in about 50 minerals, nearly all rare.
Twenty-four carat gold is a metallic mineral. A 14 carat gold ring
contains 14/24 or 58% gold.
An average sample of earth contains 9% aluminum, 5.5% iron, .01% zinc,
.008% copper, .004% tin, .002% lead, .0005% uranium, and .0000006% gold
or platinum. It would be hopelessly expensive to 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.