Quartz is the second-most abundant mineral in the Earth’s continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz, several of which are semi-precious gemstones. Throughout the world, varieties of quartz have been since antiquity the most commonly used minerals in the making of jewellery and hardstone carvings.
The quartz group includes a number of different species/varieties. There are two categories crystalline quartz and cryptocrystalline quartz (chalcedony).
Source: en.wikipedia.org/wiki/Quartz
Chemical Composition :
Silica (Silicon Dioxide) SiO2
Crystal System / Forms :
Trigonal System
- Prismatic with pyramidal terminations (rhombohedron, trapezohedron, trigonal, pyramid)
- Horizontal striations on the prism face, rhomb shaped on the rhombohedral face, ‘V’ shaped markings in twinned crystals.
- Most quartz is commonly twinned.
- Types of twinning include Dauphine, Brazil and Japan law twinning.
- Exhibits both contact and interpenetrant twinning.
Cuts & Uses :
Facetted cuts, cabochon, carvings, beads, etc.
Dispersion : 0.013
Hardness : 7
Lustre : Vitreous.
Magnification :
Liquid and two-phase, colour zoning, negative crystals, zebra-stripe fingerprints (structural), crystal inclusions, green fuchsite mica flakes/platelets in aventurine quartz, rutile/tourmaline needles in sagenitic quartz. Brazil law twinning is seen in natural quartz.
Optic Character :
Anisotropic, D.R.; Uniaxial positive. May exhibit a bull’s eye optic figure (quartz rotates the plane of polarisation parallel to the c-axis).
Pleochroism: Dichroism - weak to moderate.
Refractive Index / Birefringence :
1.544 – 1.553 / 0.009. Range: 1.535 – 1.560
Sources :
Brazil, India, South Africa, Namibia, Madagascar, Zambia, Sri Lanka.
Specific Tests :
- Piezoelectric: develops an electric charge when pressure is applied.
- Pyroelectricity: develops an electrical charge when heated.
- Diasterism in star variety (star visible in reflected and transmitted light).
Spectrum :
Green aventurine: bands at 682nm, 649nm
Synthesis :
- Colours – colourless, pink, blue, green, etc.
- Identification: type of twinning, seed plate, breadcrumb inclusions, Raman / infra-red spectroscopy.
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