Geology Reference
In-Depth Information
AppendixB
Geochemistry and Main Uses of Minerals
B.1 Main groups of minerals found in Nature
Some of the main groups found in Nature are briefly discussed next, indicating
which principal minerals are included in each group. The information has been
primarily extracted from Mason (1966).
B.1.1 The silica minerals
Silica (SiO
2
) occurs in nature as five distinct minerals: quartz (including chal-
cedony), tridymite, cristobalite, opal and lechatelierite or silica glass. Of these,
quartz is very common, tridymite and cristobalite are widely distributed in vol-
canic rocks and can hardly be called rare; opal is not uncommon and lechatelierite
is very rare.
B.1.2 The feldspar group
Feldspars are the most common of all minerals. Their general formula is
XAl(Al;Si)Si
2
O
8
, with X representing Na, K, Ca, and Ba. They are closely
related in form and physical properties and fall into two groups: the potassium
(orthoclase, sanidine and microcline) and barium feldspars, which are monoclinic
or very nearly monoclinic in symmetry and the sodium and calcium feldspars (pla-
gioclases), which are triclinic. The barium-containing feldspars are very rare and
are thus not important rock-forming minerals. In the plagioclase subgroup, albite,
oligoclase, andesine, labradorite, bytownite and anorthite are the most important
minerals.
B.1.3 The pyroxene group
The pyroxenes are a group of minerals closely related in crystallographic and other
principal properties, as well as in chemical composition. They crystallise in two
different systems, orthorhombic and monoclinic. Pyroxenes have the general for-
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