“Bismutotantalite,” Madagascar, 0.5 cts. Photo © Joel E. Arem, PhD, FGA. Used with permission.
Extremely rare as a cut gem, even in very complete collections. Many 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.
| Data | Value |
|---|---|
| Name | Bismutotantalite |
| Crystallography | Orthorhombic. Crystals sometimes large; massive; stream pebbles. |
| Refractive Index | 2.388-2.428 |
| Colors | Light brown to black |
| Luster | Adamantine to submetallic. |
| Hardness | 5 |
| Fracture | Subconchoidal |
| Specific Gravity | 8.84 (Brazil) |
| Birefringence | 0.040 |
| Cleavage | Perfect 1 direction |
| Dispersion | No data |
| Stone Sizes | Cut gems always small, less than 5 carats |
| Luminescence | None |
| Luminescence Present | No |
| Transparency | Opaque, transparent in very thin fragments. |
| Absorption Spectrum | Not diagnostic. |
| Formula | (Bi, Sb) (Ta, Nb)O4 |
| Pleochroism | No data |
| Optics | α = 2.388; β = 2.403; γ = 2.428. Biaxial (+), 2V = 80°. |
| Optic Sign | Biaxial + |
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. Many 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.
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