Bararite
| Bararite | |
|---|---|
| General | |
| Category | Halide mineral |
|
Formula (repeating unit) |
(NH4)2SiF6 |
| Strunz classification | 3.CH.10 |
| Crystal system | Trigonal |
| Crystal class | Hexagonal scalenohedral (3m) H-M symbol: (3 2/m) |
| Space group | P3m1 |
| Unit cell | a = 5.77 Å, c = 4.78 Å; Z = 1 |
| Identification | |
| Color | White to colorless |
| Crystal 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 |
| Mohs scale hardness | 2.5 |
| Luster | Vitreous |
| Diaphaneity | Transparent |
| Specific gravity | 2.152 (synthetic) |
| Optical properties | Uniaxial (-) |
| Refractive index | nω = 1.406 ± 0.001, nε = 1.391 ± 0.003 |
| Birefringence | 0.015 ± 0.003 |
| Solubility | Dissolves in water |
| Other characteristics | 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, 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.
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−.
Bararite is named after Barari, a locality in India. This was where the species was first completely described. Earlier, bararite was recognized as part of mixtures with cryptohalite. However, it did not receive its own name until 1951. The East Indian Coal Company provided the sample that Christie used to evaluate bararite.
...
Wikipedia