Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0], via Wikimedia Commons
Tantalite is too dark to be of use as a faceted gem but is sometimes cut as a collector curiosity, either faceted or in cabochons. These could be of any desired size.
| Data | Value |
|---|---|
| Name | Tantalite |
| Varieties | Manganotantalite |
| Stone Sizes | Very large crystals weighing many pounds have been found. The material is usually dark colored, opaque, and of interest only for cabochons. |
| Formula | (Fe,Mn)(Nb,Ta)2O6. |
| Colors | Black, brownish black, reddish brown; may tarnish iridescent. |
| Fracture | Uneven |
| Hardness | 6-6.5 |
| Cleavage | Distinct 1 direction |
| Crystallography | Orthorhombic. Crystals tabular prismatic, in aggregates, massive, compact. |
| Refractive Index | 2.26-2.43 |
| Birefringence | 0.160 |
| Luminescence | None. |
| Luminescence Present | No |
| Absorption Spectrum | Not diagnostic. |
| Pleochroism | Strong; brown/red-brown. |
| Optics | a = 2.26; β = 2.30-2.40; γ = 2.43. Biaxial (+), 2V Large. Usually opaque, indices measured on powders or thin splinters |
| Optic Sign | Biaxial + |
| Luster | Submetallic to vitreous. |
| Specific Gravity | 8.2; decreases with Ta content (columbite, 5.2). |
| Transparency | Transparent. |
TANTALITE Series to Columbite: (Fe,Mn)(Nb,Ta)2O6.
Streak: Black, brownish black, reddish brown.
Optics: a= 2.26; β= 2.30-2.40; γ=2.43.
Biaxial (+), 2VLarge. Usually opaque, indices measured on powders or thin splinters
Spectral: Not diagnostic.
Occurrence: In granite pegmatites.
Colorado; Wyoming; New England.
Canada; Brazil; Madagascar; France; Sweden: Finland; USSR; Zimbabwe; Western Australia.
South Dakota: various localities.
California: various localities.
Comments: Tantalite is too dark to be of use as a faceted gem but is sometimes cut as a collector curiosity, either faceted or in cabochons. These could be of any desired size.
Name: After the mythical character Tantalus, because it is difficult to dissolve the mineral in acids prior to analysis.
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