Avogadrite

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.

Avogadrite
Ferruccite (white) on avogadrite (yellow-brownish), picture size: 5 mm
General
Category	Halide mineral
Formula	(K,Cs)BF4
IMA symbol	Avg[1]
Strunz classification	3.CA.10
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Unit cell	a = 8.6588, b = 5.48 c = 7.0299 [Å]; Z = 4
Identification
Color	Colorless to white, yellowish to reddish
Crystal habit	Tabular to platy octagonal crystals, tiny crystals, elongate
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)
Other characteristics	Bitter taste
References	[2][3][4]

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. In nature, it can only be found as a sublimation product around volcanic fumaroles.^[2] He named it after the Italian scientist Amedeo Avogadro (1776–1856).^[5]

Bibliography

• Palache, P.; Berman H.; Frondel, C. (1960). "Dana's System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, Etc. (Seventh Edition)" John Wiley and Sons, Inc., New York, pp. 97-98.