Olmiite
Olmiite is a rare mineral that was discovered in 2006 at the N’Chwaning II Mine, Kalahari manganese fields, Northern Cape Province, South Africa. The chemical formula of Olmiite is (CaMn2+)2(SiO3OH)(OH) (Calcium Manganese Silicate Hydroxide) and its molecular weight is 203.44 gm. The Wessels Mine and N’Chwaning Mines, both in the Kalahari manganese fields, are still the only known sources of Olmiite. A Moh’s Hardness is 5.0 – 5.5 and the specific gravity 3.05 (g/cm3). It is a rare gem because most crystals or not suitable for faceting. It ranges in color from colorless to light orangish-pink. (photograph above courtesy of Jay Medici, gem in picture also faceted by Jay Medici).
It has recently been determined that most Poldervaartite mineral specimens and gems are actually the mineral Olmiite. The two are very closely related, being differentiated only by the percentage of Calcium (Ca) versus Manganese (Mn) contained. Specimens with Mn dominance are Olmiite and those with Ca dominance are Poldervaartite. This would mean that most, if not all, Poldervaartite specimens and gems currently on the market are, in fact, Olmiite and that there may not even be any Poldervaartite gems in existence. It is impossible to determine the difference between the two visually and must be tested by electron microprobe analysis.
| Chemical Formula: | (CaMn2+)2(SiO3OH)(OH) |
| Calcium Manganese Silicate Hydroxide | |
| Molecular Weight: | 203.44 gm |
| Composition: | Calcium | 22.65 % | Ca | 31.70 % | CaO |
| Manganese | 22.95 % | Mn | 29.64 % | MnO | |
| Iron | 0.27 % | Fe | 0.35 % | FeO | |
| Silicon | 13.81 % | Si | 29.53 % | SiO2 | |
| Hydrogen | 0.99 % | H | 8.86 % | H2O | |
| Oxygen | 39.32 % | O | |||
| 100.00 % | 100.08 % | = TOTAL OXIDE |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | Crystals are prismatic, to 7 mm; aggregates in sheaves. |
| Twinning: | None |
| Cleavage: | None observed |
| Fracture: | Irregular/Uneven |
| Tenacity: | Brittle |
| Moh’s Hardness: | 5.0 – 5.5 |
| Density: | 3.05 (g/cm3) |
| Luminescence: | Fluoresces deep red under SW UV |
| Radioactivity: | Not Radioactive |
| Color: | Colorless, white, pink, orangish pink |
| Transparency: | Transparent to Translucent |
| Luster: | Vitreous |
| Refractive Index: | 1.663 – 1.694 Biaxial ( + ) |
| Birefringence: | 0.0310 |
| Dispersion: | Distinct; r > v |
| Pleochroism: | None |
