Ettringite crystal, 5.0 x 3.5 x 2.0 cm, N'Chwaning Mines, Kuruman, Kalahari manganese fields, Northern Cape Province, South Africa. © Rob Lavinsky, www.iRocks.com. Used with permission.
Ettringite is not generally facetable. Any cut stone would be considered an extreme rarity.
| Data | Value |
|---|---|
| Name | Ettringite |
| Colors | Colorless to light yellow, milky. |
| Hardness | 2-2.5 |
| Cleavage | Perfect, rhombohedral |
| Formula | Ca6AI2(SO4)3(OH)1224H2O |
| Crystallography | Hexagonal; usually flattened hexagonal dipyramids; sometimes prismatic, fibrous. |
| Refractive Index | 1.470-1.491 |
| Birefringence | 0.021 |
| Luminescence | None |
| Luminescence Present | No |
| Absorption Spectrum | Not diagnostic |
| Pleochroism | None; colorless in transmitted light; dehydrates and turns white. |
| Optics | o = 1.491; e = 1.470. Uniaxial (-). |
| Optic Sign | Uniaxial - |
| Luster | Vitreous |
| Specific Gravity | 1.77 |
| Transparency | Transparent to translucent |
| Etymology | After the locality in Germany. |
| Occurrence | At type locality (Ettringen, Rhine Province, Germany) in cavities of metamorphosed limestone inclusions in alkaline igneous rocks. Also in metamorphosed limestones. |
South African material has yielded minute stones, some of which may have been labeled sturmanite.
Series to Sturmanite.
Ettringen, Germany: In tiny crystals.
Franklin, New Jersey: Minute crystals.
South Africa: Associated with sturmanite at the Jwaneng mine, near Hotazel. The sturmanite forms yellowish coatings, with ettringite cores.
Material from Ettringen and Franklin is microscopic. A 2.5 carat pale yellow ettringite was faceted from material found in the center of a mass of sturmanite from South Africa. This may be the largest known for the species.
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