Poldervaartite
Poldervaartite is an extremely rare mineral that was discovered in 1992 at the Wessels Mine, Hotazel, Kalahari manganese fields, Northern Cape Province, South Africa. The chemical formula of Poldervaartite is (CaMn2+)2(SiO3OH)(OH) (Calcium Manganese Silicate Hydroxide) and its molecular weight is 197.68 gm. Wessels Mine and N’Chwaning Mine, both at the Kalahari manganese fields, are still the only known sources of Poldervaartite. Poldervaartite has a specific gravity of 2.91 (g/cm3) and the refractive index 1.634 – 1.656 Biaxial ( + ). It is a very 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 existance! 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: | 197.68 gm |
| Composition: | Calcium | 30.41 % | Ca | 42.55 % | CaO |
| Manganese | 13.90 % | Mn | 17.94 % | MnO | |
| Silicon | 14.21 % | Si | 30.39 % | SiO2 | |
| Hydrogen | 1.02 % | H | 9.11 % | H2O | |
| Oxygen | 40.47 % | O |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | Crystals are prismatic, with {110}, {100}, {010}, and {001}, to 7 mm; aggregates in sheaves. |
| Twinning: | None |
| Cleavage: | None observed |
| Fracture: | Irregular/Uneven |
| Tenacity: | Very Brittle |
| Moh’s Hardness: | 5.0 |
| Density: | 2.91 (g/cm3) |
| Luminescence: | Fluoresces deep red under SW UV. |
| Radioactivity: | Not Radioactive |
| Color: | Colorless, milky white, pink-white |
| Transparency: | Transparent, Translucent |
| Luster: | Vitreous on fractures to Sub-Vitreous on crystal faces |
| Refractive Index: | 1.634 – 1.656 Biaxial ( + ) |
| Birefringence: | 0.0220 |
| Dispersion: | Weak; r < v |
| Pleochroism: | Weak; X = colorless; Y = light gray; Z = bluish gray |
