Ettringite
Ettringite is an extremely rare gem and a member of the Ettingite Group of minerals that includes Charlesite, Ettringite, Sturmanite and Thaumasite. Ettringite crystals are usually very small so gems are very small. Ettringite is closely related to and difficult to distinguish from Sturmanite. Ettingite crystals are usually found as colorless but have been found as yellow to pale yellow from the source at the N’Chwaning mine, Kuruman district, Cape Province, South Africa. This location is currently the only source for gem quality crystals. Other locations of Ettringite only produce minute crystals too small for faceting.
| Category: | Sulfate minerals |
| Formula: | Ca6Al2(SO4)3(OH)12·26H2O |
| calcium tri-sulfoaluminate hydrate | |
| Crystallography: | Hexagonal – Dihexagonal Dipyramidal |
| Crystal Habit: | Crystals prismatic and striated, to 20 cm, commonly unterminated, rarely dipyramidal; may be acicular, fibrous or cottonlike. |
| Twinning: | n/a |
| Cleavage: | [1010] Perfect |
| Fracture: | Uneven |
| Tenacity: | Brittle |
| Hardness (Mohs): | 2.0 – 2.5 |
| Density: | 1.77 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioacitve |
| Other: | Partially soluble in water; partially dehydrates on exposure, becoming opaque |
| Color: | Yellow, pale Yellow, Colorless, milky White; turns White upon partial dehydration |
| Transparency: | Transparent to Opaque |
| Luster: | Vitreous |
| Refractive Index: | 1.470 – 1.491 Uniaxial ( – ); changes to Uniaxial ( + ) upon dehydration |
| Birefringence: | 0.0210 |
| Dispersion: | n/a |
| Pleochroism: | n/a |
