Povondraite
Povondraite is an extremely rare member of the Tourmaline group of minerals that was discovered in 1987. The chemical formula of Povondraite is NaFe3+3(Mg2Fe3+4)(Si6O18)(BO3)3(OH)3O. It is a gem for collectors only since it is not particularly attractive as it is dark brown to black and usually only translucent to nearly opaque. Povondraite has a specific gravity of 3.26 (g/cm3) and the refractive index 1.751 – 1.820 Uniaxial ( – ). Until a recent find in Tanzania, the only known source of gem quality crystals was the San Francisco mine, near Villa Tunari, Alto Chapare, Cochabamba, Bolivia where it was discovered. Povondraite was originally called Ferridravite for its ferric iron content and similarity to dravite color, but was redefined and renamed upon its IMA approval in 1993.
| Chemical Formula: | NaFe3+3(Mg2Fe3+4)(Si6O18)(BO3)3(OH)3O |
| Crystallography: | Trigonal – Ditrigonal Pyramidal |
| Crystal Habit: | Crystals are equant but distorted prisms, terminated by trigonal pyramids, in subparallel aggregates, to several mm. |
| Twinning: | None |
| Cleavage: | None |
| Fracture: | Uneven to Conchoidal |
| Tenacity: | Brittle |
| Moh’s Hardness: | 7.0 ~ |
| Density: | 3.26 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Barely detectable; GRapi = 11.91 (Gamma Ray American Petroleum Institute Units) |
| Color: | Dark brown to brownish black, Black |
| Transparency: | Translucent to nearly opaque |
| Luster: | Resinous |
| Refractive Index: | 1.751 – 1.820 Uniaxial ( – ) |
| Birefringence: | 0.069 |
| Dispersion: | n/a |
| Pleochroism: | Strong; O = yellow-brown, dark brown, brown-black; E = dark red-brown, light brown. |
