| Alstonite | |
|---|---|
Alstonite crystals from Brownley Hill Mine, North Pennines, Cumbria, England (size: 5.1 x 4.1 x 2.7 cm) |
|
| General | |
| Category | Carbonate mineral |
| Chemical formula | BaCa(CO3)2 |
| Strunz classification | 5.AB.35 |
| Crystal symmetry | Triclinic Space group: P1 or P1 |
| Unit cell | a = 17.38 Å, b = 14.4 Å, c = 6.12 Å; α = 90.35°, β = 90.12°, γ = 120.08°; Z = 24 |
| Identification | |
| Color | Colorless to snow-white; also pale gray, pale cream, pink to pale rose-red |
| Crystal habit | Steep pseudohexagonal dipyramids bounded by pseudohexagonal prisms |
| Crystal system | Triclinic; pseudo-orthorhombic |
| Twinning | Common on pseudo-orthorhombic {110} and {310} |
| Cleavage | {110} imperfect |
| Fracture | Uneven |
| Mohs scale hardness | 4 - 4.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.67 - 3.711 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.526 nβ = 1.671 nγ = 1.672 |
| Birefringence | δ = 0.146 |
| 2V angle | Measured: 6° |
| Alters to | Color may fade on exposure to light |
| References | [1][2][3] |
Alstonite, also known as bromlite, is a member of the aragonite group of carbonate minerals. It is a calcium and barium carbonate mineral with formula, BaCa(CO3)2. It is triclinic in crystal structure and differs structurally from barytocalcite, which has a monoclinic structure. Simple crystals are not known. The crystals are invariably complex twins, and have the form of doubly terminated pseudo-hexagonal pyramids, like those of witherite but more acute; the faces are horizontally striated and are divided down their center by a twin suture.
The examination in polarized light of a transverse section shows that each compound crystal is built up of six differently oriented individuals arranged in twelve segments.
The crystals are translucent and white, sometimes with a shade of pink. Specific gravity is 3.706, and hardness is 4 - 4.1.
The mineral has been found in a variety of places, two of which are in the north of England. At the Fallowfield lead mine, near Hexham in Northumberland, it is associated with witherite; and at Bromley Hill, near Alston in Cumberland, it occurs in veins with galena. The species was named bromlite by T. Thomson in 1837, and alstonite by A. Breithaupt in 1841, both of which names, derived from the locality, have been in common use.
It occurs typically in low-temperature hydrothermal lead-zinc ore deposits and has been reported as a rare phase in carbonatites. It occurs associated with calcite, baryte, ankerite, siderite, benstonite, galena, sphalerite, pyrite and quartz.[1]