By Alchemist-hp (www.pse-mendelejew.de)Alchemist-hp at de.wikipedia [CC-BY-SA-3.0-de or FAL], from Wikimedia Commons
This mineral is not abundant and is known from various localities. Cabochons are cut merely as curiosities, as they have no special features that would recommend them except rarity. There are reports of transparent grains or parts of crystals that have been cut by collectors, but these are merely curiosities and are seldom encountered.
| Data | Value |
|---|---|
| Name | Fergusonite |
| Colors | Black, brownish black; surface altered brown, gray, or yellow. |
| Hardness | 5.5 - 6.5 |
| Fracture | Subconchoidal |
| Cleavage | Traces |
| Stone Sizes | Cabochons are cut to several inches from massive material. Faceted stones are extremely tiny (less than 1 carat). |
| Formula | YNbO4 + Er, Ce, Fe, Ti. |
| Crystallography | Tetragonal Crystals prismatic, pyramidal; usually masses. |
| Crystallographic Forms | |
| Refractive Index | 2.05-2.19 (mean) |
| Birefringence | None. (Isotropic due to metamictization). |
| Pleochroism | Weak. |
| Optics | Isotropic due to metamictization. N = 2.05-2.19 (mean), variable. |
| Luster | Vitreous; submetallic; alters to dull surface. |
| Specific Gravity | 5.6-5.8. Formanite: 7.03 (calculated) |
| Transparency | Translucent to opaque, very rarely transparent. |
Series to Formanite YTaO4.
Streak: Greenish gray, yellowish brown, brown.
Optics: Isotropic due to metamictization. N= 2.05-2.19 (mean), variable.
Occurrence: Granite pegmatites rich in rare earths.
California; North Carolina; Virginia; Texas: Massachusetts; Norway; USSR; Ytterby, Sweden; East Africa; Zimbabwe; Madagascar
Formanite is from Western Australia.
Comments: This mineral is not abundant and is known from various localities. Cabochons are cut merely as curiosities, as they have no special features that would recommend them except rarity. There are reports of transparent grains or parts of crystals that have been cut by collectors, but these are merely curiosities and are seldom encountered.
Name: After Robert Ferguson, a Scottish physician.
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