What is "hyalophane"

Hyalophane \Hy*al"o*phane\, n. [Gr. "y`alos glass + ? to appear.] (Min.) A species of the feldspar group containing barium. See Feldspar.

n. (context mineral English) A monoclinic crystalline mineral, a barium-rich potassium feldspar.

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)SiO], 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 = 1.542, n = 1.545, and n = 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.