Hyalophane

Hyalophane
240px hyalophane on matrix (Busovaca, Bosnia and Herzegovina)
General
Category	Tectosilicate
Formula (repeating unit)	(K,Ba)[Al(Si,Al)Si2O8]
Crystal symmetry	Monoclinic 2/m
Unit cell	a = 8.52 Å, b = 12.95 Å, c = 7.14 Å, β = 116° Z=4
Identification
Formula mass	302.06 g
Color	Colorless, yellow, white, red
Crystal habit	Crystalline – fine – occurs as well-formed fine sized crystals; massive – uniformly indistinguishable crystals forming large masses
Crystal system	Monoclinic – Prismatic
Twinning	Commonly simple twins according to the Carlsbad, Manebach, or Baveno laws
Cleavage	{001} perfect, {010} imperfect
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	6 – 6½
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.81
Optical properties	Biaxial (-)
Refractive index	nα = 1.542, nβ = 1.545, nγ = 1.547
Birefringence	δ = 0.005
2V angle	48 – 79°
Dispersion	Weak
References

Hyalophane or jaloallofane is a crystalline mineral, part of the feldspar group of tectosilicates. It is considered a barium-rich potassium feldspar. Its chemical formula is (K,Ba)[Al(Si,Al)Si₂O₈], and it has a hardness of 6 – 6½. The name hyalophane comes from the Greek hyalos, meaning "glass", and phanos meaning "to appear".

An occurrence of hyalophane was discovered in 1855 in Lengenbach Quarry, Imfield, in the municipality of Binn, Switzerland. The mineral is found predominantly in Europe, with occurrences in Switzerland, Australia, Bosnia, Germany, Japan, New Jersey, and the west coast of North America.

Hyalophane has a monoclinic crystallography, with cell properties a = 8.52 Å, b = 12.95 Å, c = 7.14 Å, and β = 116°. Optically, the material exhibits biaxial birefringence, with refractive index values of n_α = 1.542, n_β = 1.545, and n_γ = 1.547 and a maximum birefringence of δ = 0.005. It has weak dispersion and low surface relief.

Hyalophane has sometimes been used as a gemstone.