Topaz

Topaz
A group of topaz crystals on matrix
General
Category	Silicate mineral
Formula (repeating unit)	Al2SiO4(F,OH)2
Strunz classification	9.AF.35
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pbnm
Unit cell	a = 4.65 Å, b = 8.8 Å, c = 8.4 Å; Z = 4
Identification
Color	Colorless (if no impurities), blue, brown, orange, gray, yellow, green, pink and reddish pink
Crystal habit	Prismatic crystals with faces striated parallel to long dimension; also columnar, compact, massive
Cleavage	[001] Perfect
Fracture	Subconchoidal to uneven
Mohs scale hardness	8 (defining mineral)
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Specific gravity	3.49–3.57
Optical properties	Biaxial (+)
Refractive index	nα = 1.606–1.629 nβ = 1.609–1.631 nγ = 1.616–1.638
Birefringence	δ = 0.010
Pleochroism	Weak in thick sections X = yellow; Y = yellow, violet, reddish; Z = violet, bluish, yellow, pink
Ultraviolet fluorescence	Short UV=golden yellow; Long UV=cream
References

Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al₂SiO₄(F,OH)₂. Topaz crystallizes in the orthorhombic system, and its crystals are mostly prismatic terminated by pyramidal and other faces.

Pure topaz is colorless and transparent but is usually tinted by impurities; typical topaz is wine red, yellow, pale gray, reddish-orange, or blue brown. It can also be white, pale green, blue, gold, pink (rare), reddish-yellow or opaque to transparent/translucent.

Orange topaz, also known as precious topaz, is the traditional November birthstone, the symbol of friendship, and the state gemstone of the US state of Utah.

Imperial topaz is yellow, pink (rare, if natural) or pink-orange. Brazilian Imperial Topaz can often have a bright yellow to deep golden brown hue, sometimes even violet. Many brown or pale topazes are treated to make them bright yellow, gold, pink or violet colored. Some imperial topaz stones can fade on exposure to sunlight for an extended period of time.

Blue topaz is the state gemstone of the US state of Texas. Naturally occurring blue topaz is quite rare. Typically, colorless, gray or pale yellow and blue material is heat treated and irradiated to produce a more desired darker blue.

Mystic topaz is colorless topaz which has been artificially coated giving it the desired rainbow effect.

Topaz is commonly associated with silicic igneous rocks of the granite and rhyolite type. It typically crystallizes in granitic pegmatites or in vapor cavities in rhyolite lava flows including those at Topaz Mountain in western Utah and Chivinar in South America. It can be found with fluorite and cassiterite in various areas including the Ural and Ilmen mountains of Russia, in Afghanistan, Sri Lanka, Czech Republic, Germany, Norway, Pakistan, Italy, Sweden, Japan, Brazil, Mexico; Flinders Island, Australia; Nigeria and the United States.