Grandidierite
Grandidierite is an extremely unusual mineral and gem by having a breathtaking color that is blue-green. Although it is very rarely transparent, faceted, translucent gems have actually a color that is beautiful. The type locality and many wells understood location is Andrahomana, southwest of Fort Dauphin, southern Madagascar. Other sources that are minor Kachenbere Hills, Malawi; Béjaia, Algeria; Ontario, Canada; Warren County, NY, USA; Tuscany, Italy; Masataka, Surinam; Cuvier Island, brand new Zealand and Norway.
| Category: | Nesosilicate |
| Chemical Formula: | (Mg,Fe2+)Al3(BO3)(SiO4)O2 |
| Magnesium Iron Aluminum Boro-Silicate | |
| Molecular Weight: | 296.03 gm |
| Composition: | Magnesium | 6.16 % | Mg | 10.21 % | MgO |
| Aluminum | 27.34 % | Al | 51.67 % | Al2O3 | |
| Iron | 4.72 % | Fe | 6.07 % | FeO | |
| Silicon | 9.49 % | Si | 20.30 % | SiO2 | |
| Boron | 3.65 % | B | 11.76 % | B2O3 | |
| Oxygen | 48.64 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | As anhedral elongated individuals, to 8 cm, typically strongly corroded. |
| Twinning: | None |
| Cleavage: | Perfect; more perfect on {100}, and {010} |
| Fracture: | Uneven |
| Tenacity: | Brittle |
| Moh’s Hardness: | 7.5 |
| Density: | 2.98 – 2.99 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Bluish green, Greenish blue; increasingly greener with greater Fe content. |
| Transparency: | Transparent, Translucent, Opaque |
| Luster: | Vitreous, Pearly on cleavages |
| Refractive Index: | 1.590 – 1.639 Biaxial ( – ) |
| Birefringence: | 0.0370 |
| Dispersion: | Very strong; r < v |
| Pleochroism: | Visible; X = deep blue to blue-green; Y = colorless; Z = light greenish blue to deep green |
