Quartz

green-quartz-in-agate
Green quartz in agate

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 jewelry 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

Cause of Color :

Rose Quartz: Charge transfer reaction (Ti4+ and Fe2+) as a result of irradiation.

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 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, color zoning, negative crystals, zebra-stripe finger prints (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 :

Hydrothermal process:

  • Colors – colorless, pink, blue, green, etc.
  • Identification: type of twinning, seed plate, breadcrumb inclusions, Raman / infra-red spectroscopy.

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