has
pyramidal terminations. Crystals are rhombohedral. They may be
right-handed or left-handed forms due to tetrahedral symmetry. Two
independent rhombohedra combine to give a hexagonal pyramid, but the
faces differ in size, three smaller faces alternating with three larger
faces; this is characteristic even in very small crystals. Twins are
also found, as also irregular puzzling growths, which could be oriented
and deciphered by the horizontal striations on the prism faces. The
crystals vary enormously in size.
Quartz
contains various inclusions: (a) solid inclusions of rutile, hematite,
epidote, etc.; (6) liquids, e.g. water and liquid C02; and (c) gaseous inclusions, mostly C02. It has conchoidal fracture. Hardness = 7. Specific gravity =
in pure quartz, varying from
in other varieties.
Lustre: Vitreous. Transparent to translucent or opaque. Uniaxial. Optically positive. Mean index of refraction =
, double refraction =
and dispersion =
(low).
It is optically active and rotates the plane of polarization to right
or left according as it is a right- or left-handed crystal.
Composition: Si02. Quartz may be very pure or contain
mineral inclusions and other foreign matter. Also forms pseudomorphs,
e.g. fossil wood. It is a fairly resistant mineral, but may be replaced
by other minerals.
Uses
Crystalline
and crypto-crystalline varieties of quartz are used for gem purposes.
The crystalline quartz is cut in facetted forms or cabochon. Rock
crystals are also cut into vases, crystal balls, idols, etc. When cut
it resembles beryl and topaz. On account of its piezoelectric
properties, properly oriented sections are used in radio apparatus as
resonators and oscillators in radio-telephony and radiotelegraphy and
for depth measurements in submarines.
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