bararite

{{Infobox mineral | name = Bararite | category = Halide mineral | boxwidth = | boxbgcolor = | image = Bararite.jpg | imagesize = | caption = Pale pink yellowish crystals of bararite from Shamokin, Northumberland County, Pennsylvania, USA | formula = (NH4)2SiF6 |IMAsymbol=Brr | strunz = 3.CH.10 | system = Trigonal | class = Hexagonal scalenohedral (m) H-M symbol: ( 2/m) | symmetry = Pm1 | unit cell = a = 5.77 Å, c = 4.78 Å; Z = 1 | color = White to colorless | habit = Tabular, sometimes elongated on {0001},also appears in irregularly shaped or mammillary surfaces that comprise mainly cryptohalite | twinning = Interpenetration twins (paddlewheels/darts), axis parallel to {0001} | cleavage = [0001] perfect | fracture = | tenacity = | mohs = 2.5 | luster = Vitreous | polish = | refractive = nω = 1.406 ± 0.001,nε = 1.391 ± 0.003 | opticalprop = Uniaxial (-) | birefringence = 0.015 ± 0.003 | dispersion = | pleochroism = | fluorescence= | absorption = | streak = | gravity = 2.152 (synthetic) | density = | fusibility = | diagnostic = | solubility = Dissolves in water | diaphaneity = Transparent | other = salty taste | references = }} Bararite is a natural form of ammonium fluorosilicate (also known as hexafluorosilicate or fluosilicate). It has chemical formula (NH4)2SiF6 and trigonal crystal structure. This mineral was once classified as part of cryptohalite. Bararite is named after the place where it was first described, Barari in Jharia Coal Field, Dhanbad, India. It is found at the fumaroles of volcanoes (Vesuvius, Italy), over burning coal seams (Barari, India), and in burning piles of anthracite (Pennsylvania, U.S.). It is a sublimation product that forms with cryptohalite, sal ammoniac, and native sulfur.

==History== A. Scacchi first discovered cryptohalite in 1873. It appeared in a volcanic sublimate from the Vesuvian eruption of 1850. In 1926, W.A.K. Christie reported his own chemical study. A microscope was used to pick out enough material for analysis. Distilling with sodium hydroxide (NaOH) produced ammonia (NH3). The anions of hexafluorosilicic acid (H2SiF6) precipitated as potassium fluorosilicate (K2SiF6). Barium sulfate (BaSO4) was thrown into the filtrate, and then calcium fluoride (CaF2). Christie found 20.43% (NH4)+ and 78.87% (SiF6)2−.