Minerals are natural chemical
substances of inorganic origin. Most rocks are composed of minerals, but some
are of organic origin, and others consist of natural glass.
A mineral is typically a
crystalline substance, having regular atomic pattern. When conditions are
favorable, minerals develop definite geometric forms called crystals. These are
classified in six crystal systems according to the number, relative lengths,
and regular relationships of imaginary lines called axes.
Most rocks are composed of silicate
minerals, consisting of a silicon atom surrounded by four oxygen atoms in a
tetrahedral pattern. Attached to the oxygen atoms are atoms of other elements, of
which six—aluminum, iron, magnesium, calcium, sodium, potassium – are the most
common; oxygen, silicon, and these six other elements make up most of the crust
of the earth.
Many minerals are commercially or
scientifically important even though not essential to the forming of the common
rocks. Some are ores, others are so-called industrial (nonmetallic) minerals,
and some have considerable scientific
interest.
Rocks formed by the
solidification of molten matter are
termed igneous. These are regarded as the primary rocks –not being derived from
still older rock—although none of the original crust of the earth is believed
to exist, for it has presumably been reworked more than once.
Rocks formed by the breakdown of
other rock are called sedimentary. They may accumulate as fragments, or they
may deposit from chemical solutions.
Either igneous or sedimentary
rock may be slowly changed into metamorphic rock by heat, pressure, and
solutions. Most metamorphism occurs as a result of mountain building, which
involves high temperatures and pressures. Contact metamorphism takes place at
margin of an intrusive body of magma.
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