Hyalophane

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]

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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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