Avogadrite

Avogadrite
240px Ferruccite (white) on avogadrite (yellow-brownish), picture size: 5 mm
General
Category	Halide mineral
Formula (repeating unit)	(K,Cs)BF4
Strunz classification	03.CA.10
Crystal symmetry	Orthorhombic dipyramidal H-M symbol: (2/m 2/m 2/m) Space group: P nma
Unit cell	a = 8.6588 Å, b = 5.48 Å, c = 7.0299 Å; Z=4
Identification
Color	Colorless to white, yellowish to reddish
Crystal habit	Tabular to platy octagonal crystals
Crystal system	Orthorhombic
Luster	Vitreous, greasy
Diaphaneity	Translucent
Specific gravity	2.9
Optical properties	Biaxial (-)
Refractive index	nα = 1.3239, nβ = 1.3245, nγ = 1.3247
Birefringence	δ = 0.001
2V angle	75°(meas), 58° (calc)
References

Avogadrite ((K,Cs)BF₄) is a potassium-caesium tetrafluoroborate in the halide class.. Avogadrite crystallizes in the orthorhombic system (space group Pnma) with cell parameters a 8.66 Å, b 5.48 Å and c Å 7.03.

History

The mineral was discovered by the Italian mineralogist Ferruccio Zambonini in 1926. He analyzed several samples from the volcanic fumaroles close to Mount Vesuvius and from the Lipari islands. As a result, it can only found as a sublimation product around volcanic fumaroles. He named it after the Italian scientist Amedeo Avogadro (1776–1856).

See also

• Ferruccite, (NaBF₄), the sodium tetrafluoroborate.