Alstonite
Alstonite, also known as bromlite, is a low temperature hydrothermal mineral that is a rare double carbonate of calcium and barium with the formula BaCa(CO
3)
2, sometimes with some strontium. Barytocalcite and paralstonite have the same formula but different structures, so these three minerals are said to be trimorphous. Alstonite is triclinic but barytocalcite is monoclinic and paralstonite is trigonal. The species was named Bromlite by Thomas Thomson in 1837 after the Bromley-Hill mine, and alstonite by August Breithaupt of the Freiberg Mining Academy in 1841, after Alston, Cumbria, the base of operations of the mineral dealer from whom the first samples were obtained by Thomson in 1834. Both of these names have been in common use.
| Alstonite | |
|---|---|
| General | |
| Category | Carbonate mineral |
| Formula (repeating unit) | BaCa(CO3)2 |
| IMA symbol | Asn |
| Strunz classification | 5.AB.35 |
| Dana classification | 14.02.05.01 |
| Crystal system | Triclinic Unknown space group |
| Unit cell | a = 17.38, b = 14.40 c = 6.123 [Å]; α = 90.35° β = 90.12°, γ = 120.08°; Z = 24 |
| Identification | |
| Formula mass | 297.42 g/mol |
| Colour | Colourless to snow-white; also pale gray, pale cream, pink to pale rose-red |
| Crystal habit | Steep pseudohexagonal dipyramids, pseudo-orthorhombic |
| Twinning | Common on pseudo-orthorhombic {110} and {310} |
| Cleavage | Imperfect on pseudo-orthorhombic {110} |
| Fracture | Uneven |
| Mohs scale hardness | 4 to 4.5 |
| Lustre | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.70 |
| Optical properties | Biaxial (−) |
| Refractive index | nα = 1.526 nβ = 1.671 nγ = 1.672 |
| Birefringence | δ = 0.146 |
| Pleochroism | None |
| 2V angle | Measured 6°, calculated 8° |
| Dispersion | Weak, r > v |
| Ultraviolet fluorescence | Weak yellow under LW and SW |
| Solubility | Soluble in dilute HCl |
| Alters to | Colour may fade on exposure to light |
| Other characteristics | Not radioactive |
| References | |
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