“Bismutotantalite,” Madagascar, 0.5 cts
Extremely rare as a cut gem, even in very complete collections. Manu of the minerals in the tantalite group have been faceted; bismutotantalite is perhaps the rarest of them all. The color is attractive, but low hardness and good cleavage make use in jewelry unadvisable.
Bismutotantalite Value
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Bismutotantalite Information
| Data | Value |
| Name | Bismutotantalite |
| Crystallography | Orthorhombic. Crystals sometimes large; massive; stream pebbles. |
| Colors | Light brown to black |
| Luster | Adamantine to submetallic |
| Hardness | 5 |
| Density | 8.84 (Brazil) |
| Birefringence | 0.040 |
| Cleavage | Perfect 1 direction. Fracture subconchoidal. Brittle |
| Dispersion | No data |
| Stone Sizes | Cut gems always small, less than 5 carats |
| Luminescence | None |
| Spectral | Not diagnostic. |
| Formula | (Bi, Sb) (Ta, Nb)O4 |
| Pleochroism | No data |
STREAK: Yellow-brown to black
OPTICS: α= 2.388; β = 2.403; γ = 2.428
Biaxial (+), 2V = 80°
OCCURRENCE: Pegmatites.
Gamba Hill, Uganda; Acari, Brazil.
COMMENTS: Extremely rare as a cut gem, even in very complete collections. Manu of the minerals in the tantalite group have been faceted; bismutotantalite is perhaps the rarest of them all. The color is attractive, but low hardness and good cleavage make use in jewelry unadvisable.
NAME: On account of the composition.
