Parting planes are somewhat akin to cleavage and are known as false cleavage. Corundum has basal parting.
Fracture refers
to the nature of the surface obtained, when a mineral has no definite
cleavage or parting. Fracture is conchoidal when the fracture surface
is curved or shell-like, e.g. in glass or quartz. Fibrous minerals
break with a splintery fracture, e.g. jadeite.
Specific Gravity
The
specific gravity of a substance is the ratio of the mass of any
quantity of the substance to the mass of an equal volume of water at
4°C. The density of a substance is its mass per unit volume of the
substance; hence the specific gravity of a substance can also be
expressed as the ratio of its density to that of water at 4°C. In
C.G.S. units, the density of a substance is its mass in grams of one
cubic centimetre of the substance and since the mass of one cubic
centimetre of water at 4°C. is one gram, the specific gravity is
numerically equal to the density.
The
specific gravity of a substance varies with temperature, the variation
being considerable where a wide range of temperature is involved. In
determining the specific gravity accurately, it is necessary,
therefore, to make corrections for temperature and also for air
buoyancy. "When working at ordinary room temperatures, however, the
theoretical corrections for temperature and air buoyancy are less than
the experimental error involved in the determination of specific
gravity. Hence they may be ignored. Moreover, these corrections never
affect the determination to the second decimal place.
The
specific gravity of a mineral is of great importance as it has a
constant value for each mineral species and could be determined
accurately without injury to the gem-stones. It forms a. valuable aid
in determining gem-stones, e.g. quartz 2-65 and diamond 3-52.
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