The three axes are of
unequal length. Two axes intersect at right
angles whereas the third axis intersects
obliquely forming an acute angle, e.g.
Gypsum mineral.
(more content follows the advertisement below) A D V E R T I S E M E N T
(vi) Triclinic System :
The three axes are of
unequal length and intersect at oblique
angles, e.g. Albite mineral.
Cleavage and Fracture
The smooth planar surfaces of weakness along
which a mineral has pronounced tendency to
split is known as cleavage planes. It bears a
close relationship to internal atomic structure
and external crystal form of mineral. Minerals
lacking cleavage break along various types of
fracture surfaces. For example, the curved
fracture surfaces of a glass constitute
conchoidal fracture, that is seen in mineral
quartz.
Specific Gravity
Each mineral has a certain specific gravity which
is the ratio of its density to the density of water
at 40 Celsius. Specific gravity of minerals is a
property of great importance because it
determines the density of a given rock and rock
density in turn determines the gross layered
structure of the earth.
Hardness
The degree to which a mineral surface resists
being scratched is known as its mineral
hardness. Hardness is geologically important
because it determines how easily a mineral is
worn away by the abrasive action of streams,
waves, wind and glaciers in the processes of
erosion and transportation. Ten standard
minerals constitute the Mohs Scale of
hardness ranging from the softest to the
hardest.
Lustre
The appearance of a mineral surface under
reflected light is referred to as its mineral
lustre. It is described by several descriptive
adjectives, such as metallic (metal like),
adamantine (diamond like), vitreous (glass
like), resinous (oil like), pearly or silky (pearl
or silk like).
Colour
Certain minerals possess a distinctive mineral
colour that facilitates recognition. The
impurities present in the mineral provides
shades of colour to a mineral.
Streak
When a mineral is rubbed across the white
porceline plate, known as streak plate, it may
leave a streak of mineral powder of distinctive
colour. The colour of powder of mineral may
sometimes be different than the colour of the
mineral.
Besides physical properties, minerals are
also identified on the basis of their optical
properties. These optical properties are
evaluated by means of a microscope using
polarised light rays and are of great value in
mineral identification.
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AKHAND PRATAP SINGH
