The mineral is a naturally occurring
homogeneous solid that is an inorganic
substance, having an orderly atomic structure
and a definite chemical composition. The basic
source of all minerals is the hot magma in the
interior of the earth.
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When magma cools,
crystals of minerals appear. These first
crystals may sink in the magma so that the
composition of the magma changes with
depth. Thus, a sequence of minerals is formed
in the rocks as the magma cools. Besides,
certain minerals such as coal, petroleum and
natural gas are organic substances found in
solid, liquid and gaseous forms respectively.
Physical Properties of Minerals
Minerals can be identified by their hardness,
colour, the way they reflect light (lustre), the
way they break (cleavage and fracture), and
Mineral crystals fall into six crystal systems,
each of which is defined in terms of its
(i) Isometric System :
Three axes of crystal are
of equal length and lie at right angle to each
other, e.g. Halite mineral.
(ii) Tetragonal System :
Three axes make right
angles with each other. The two horizontal
axes, a and b, are of equal length but the
third, c axis, is of different length, e.g
(iii) Hexagonal System :
Four axes are present.
Three of the axes are horizontal and of
equal length, intersecting in angles of 600
and 1200. The fourth axes is perpendicular
to other axes and is of different length, e.g.
(iv) Orthorhombic System :
Three axes intersect
at right angles to each other, but they
are of unequal lengths, e.g. Staurolite