Quartz is a member of the silica group of minerals, which have tekto-silicate structure and the chemical composition is SiO2. The atomic structure as is found in the crystalline varieties does not apply to silica-glass (i.e., Lechatelierite). The crystalline variety occurs in the following distinct forms:
(i) Quartz (ii) Tridymite
(iii) Cristobalite (iv) Coesite
(v) Stishovite (vi) Keatite.
There are the three crystalline polymorphs of silica. While quartz is a low-temperature polymorph formed below 870°C, trydi-mite is formed between 870° to 1470°C and cristobalite is formed at a temperature above 1470°C.
The non-crystalline variety of silica occurs as
(i) Lechatelierite (ii) Opal, and
White quartz is a member of the Hexagonal system, Tridymite belongs to orthorhombic system and Cristobalite belongs to Isometric system.
2. Streak. White.
3. Lusture. Vitreous to sub-vitreous.
4. Hardness. 7 (seven).
5. Cleavage. No cleavage (an important characteristic).
6. Sp. gravity. Low, i.e., 2.65.
7. Twinning (i) Common twins are Dauphine type, a penetration twin with the ‘c’-axis as the twin axis.
(ii) Brazil type:
A penetration twin with (1120) as the twin Plane.
(ii) Japanese law:
Contact twins with (1122) as twin plane.
Quartz is ‘piezo’ as well as ‘pyro’- electric.
Quartz is colourless but the non-crystalline varieties are coloured.
(i) Non-pleochroic colourless.
(ii) Ref. index. Low +ve, Birefringence=0 009.
(iii) Polarisation colour. 1st order gray or yellow.
(iv) Extinction. Wavy (Glassy varieties are isotropic). In Brazil-twinned plates-‘Aery spiral’ is seen.
Polymorphs of quartz:
Besides tridymite, cristobalite, ccesite, stishovite, there are two modifications of quartz, viz., a-quartz and P-quartz.
Low-temperature quartz, formed below 573°C.
High-temperature quartz, formed between 573° and 870°C (found in volcanic rocks as phenocrysts).
Varieties of quartz basing on colour:
(i) Rock-crystal. Clear and transparent.
(ii) Amethyst. Violet.
(iii) Citrin. Pale yellow.
(iv) Morion. Black quartz.
(v) Cat’s eye. Fibrous variety, showing opalescence due to the presence of titanium.
(vi) Rose quartz. Pink.
(vii) Milky quartz. White.
Non crystalline variety:
Excepting silica glass ‘Lechatelierite’, the other two, i.e., opal and chalcedony contains some water Si02.nH20.
Opal is of some varieties like Fire opal, siliceous sinter or geyserites, diatomite, etc.
Chalcedony is having varieties like agate, jasper, sard, chert, flint, chrysoprase, blood stone (i.e., carnelian), horn stone etc.
It is present in silica rich igneous rock. It is the basic material of sandstone and is found in metamorphic rocks like gneisses, schists, charnockites and khondalites.