Sturmanite
Sturmanite is an extremely rare gem and a member of the Ettringite Group of minerals that includes Charlesite, Ettringite, Sturmanite, and Thaumasite. It is difficult to distinguish Ettringite from Sturmanite. Both are members of the Ettringite Group of minerals and have similar crystal habit, density, luster and often have the same bright yellow color. By its chemical composition, Sturmanite is about 46% H2O; it’s almost half water! Sturmanite was only recently discovered in 1981 at the Black Rock Mine in the famed Kalahari manganese fields of South Africa. This region is known for its many rare mineral occurrences.The only sources of Sturmanite are from the Kuruman district, with relatively large crystals in the Wessels and N’Chwaning II mines, Northern Cape Province, South Africa.
| Category: | Sulfate minerals |
| Formula: | Ca6Fe3+2(SO4)2.5[B(OH)4](OH)12 · 25 H2O |
| Crystallography: | Trigonal – Ditrigonal Pyramidal |
| Crystal Habit: | As dipyramidal tabular to elongated hexagonal crystals, to 40 cm, showing [1010], [1014], [1124], typically in groups, may be in stacked parallel aggregates. |
| Twinning: | Probably twinned (by analogy to ettringite). |
| Cleavage: | [1010] Perfect |
| Fracture: | n/a |
| Tenacity: | Brittle |
| Hardness (Mohs): | 2.5 |
| Density: | 1.847 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Bright to pale Yellow, Greenish Yellow, Brownish Orange (if altered) |
| Transparency: | Transparent to Translucent |
| Luster: | Vitreous, Greasy on fracture surfaces |
| Refractive Index: | 1.499 – 1.505 Uniaxial ( +/- ) |
| Birefringence: | 0.0020 |
| Dispersion: | None |
| Pleochroism: | Weak; O = pale green, E = pale yellowish green |
.
