Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0], via Wikimedia Commons
Ettringite is not generally facetable; any cut stone would be considered an extreme rarity. South African material has yielded minute stones, some of which may have been labeled sturmanite.
| Data | Value |
|---|---|
| Name | Ettringite |
| Colors | Colorless to light yellow, milky. |
| Crystallography | Hexagonal; usually flattened hexagonal dipyramids; sometimes prismatic, fibrous. |
| Refractive Index | 1.470-1.491 |
| Luster | Vitreous. |
| Hardness | 2 - 2.5 |
| Specific Gravity | 1.77 |
| Birefringence | 0.021 |
| Cleavage | Perfect, rhombohedral |
| Stone Sizes | 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. |
| Luminescence | None |
| Luminescence Present | No |
| Transparency | Transparent to translucent. |
| Absorption Spectrum | Not diagnostic. |
| Formula |
Ca6AI2(SO4)3(OH)1224H2O |
| Pleochroism |
None; colorless in transmitted light; dehydrates and turns white. |
| Optics |
o = 1.491; e = 1.470. Uniaxial (-). |
| Optic Sign | Uniaxial - |
Series to Sturmanite.
Optics: o = 1.491; e = 1.470.
Uniaxial (-).
Occurrence: At type locality (Ettringen, Rhine Province, Germany) in cavities of metamorphosed limestone inclusions in alkaline igneous rocks. Also in metamorphosed limestones.
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.
Comments: Ettringite is not generally facetable; any cut stone would be considered an extreme rarity. South African material has yielded minute stones, some of which may have been labeled sturmanite.
Name: After the locality in Germany.
IGS Newsletter
