Hyalophane

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Hyalophane
Category	Tectosilicate
Chemical formula	(K,Ba)[Al(Si,Al)Si2O8]
Identification
Molecular Weight	302.06 gm
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
Twinning	commonly simple twins according to the Carlsbad, Manebach, or Baveno laws
Cleavage	{001} perfect, {010} imperfect
Fracture	brittle - generally displayed by glasses and most non-metallic minerals
Tenacity	brittle
Mohs Scale hardness	6 – 6½
Luster	vitreous (glassy)
Refractive index	nα = 1.542, nβ = 1.545, nγ = 1.547
Optical Properties	biaxial (-)
Birefringence	δ = 0.005
Dispersion	weak
Pleochroism	colorless
Streak	white
Density	2.81
References	[1] [2] [3] [4]

Hyalophane or jaloallofane is a crystalline mineral with chemical formula (K,Ba)[Al(Si,Al)Si₂O₈] and a hardness of 6 – 6½. It is part of the feldspar group of tectosilicates, and is considered a Barium-rich Potassium feldspar.^[5] Hyalophane comes from the Greek hyalos, meaning "glass", and phanos meaning "to appear".^[4]

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.^[4]

Hyalophane has a monolinic crystallography, with cell properties a = 8.52Å, b = 12.95Å, c = 7.14Å 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.^[1]

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