Schiavinatoite
| Schiavinatoite | |
|---|---|
| General | |
| Category | Borate, Oxide |
| Formula (repeating unit) | (Nb,Ta)BO4 |
| Strunz classification | 6.AC.15 |
| Crystal system | Tetragonal |
| Unit cell | a = 6.22, c = 5.49 [Å] (approximated); Z = 4 |
| Identification | |
| Color | Colorless |
| Crystal habit | zones of prismatic dipyramidal crystals (intergrown with béhierite) |
| Crystal symmetry |
Tetragonal - ditetragonal dipyramidal H-M symbol (4/m 2/m 2/m) Space group: I41/amd |
| Mohs scale hardness | ca. 8 |
| Luster | Vitreous |
| Streak | White |
| Density | 6.55 (calc.) |
| Optical properties | Uniaxial (+) |
| Refractive index | n=2.30 |
| Birefringence | Yes |
| References | [1][2][3] |
Schiavinatoite is a very rare mineral,[2] a natural niobium borate of the formula (Nb,Ta)BO4.[3] Schiavinatoite is classified as monoborate. It contains tetrahedral borate anion instead of planar BO3 group, which is more common among minerals. Schiavinatoite is one of the most simple niobium minerals. It forms a solid solution with its tantalum-analogue, béhierite. Both minerals possess zircon-type structure (tetragonal, space group I41/amd) and occur in pegmatites.[2] Schiavinatoite and nioboholtite are minerals with essential niobium and boron.[4]
Occurrence and association
Schiavinatoite was detected in miaroles of a pegmatite at Antsongombato, Madagascar. It coexists with an apatite-group mineral, béhierite, danburite, elbaite–liddicoatite, feldspar, pollucite, quartz, rhodizite, and spodumene.[1]
Crystal structure
The main facts about schiavinatoite's structure:[3]
- isostructural with zircon
- niobium coordination number of 8 (coordination polyhedron is distorted triangular dodecahedron)
- tetrahedrally-coordinated boron
- chains of edge-sharing BO4 and NbO8 polyhedra, parallel to [001]
- edge-sharing dodecahedra link the chains
References
- 1 2 "Schiavinatoite - Handbook of Mineralogy" (PDF). Handbookofmineralogy.org. Retrieved 2016-03-12.
- 1 2 3 "Schiavinatoite: Schiavinatoite mineral information and data". Mindat.org. Retrieved 2016-03-02.
- 1 2 3 Demartin, F., Diella, V., Gramaccioli, C.M., and Pezzotta, F., 2001. Schiavinatoite, (Nb,Ta)BO4, the Nb analogue of behierite. European Journal of Mineralogy 13, 159-165
- ↑ "Nioboholtite: Nioboholtite mineral information and data". Mindat.org. Retrieved 2016-03-03.