Schiavinatoite

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]

Schiavinatoite
Schiavinatoite
General
CategoryBorate, Oxide
Formula
(repeating unit)		(Nb,Ta)BO4
Strunz classification6.AC.15
Crystal systemTetragonal
Crystal classDitetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space groupI41/amd
Unit cella = 6.22, c = 5.49 [Å] (approximated); Z = 4
Identification
ColorColorless
Crystal habitzones of prismatic dipyramidal crystals (intergrown with béhierite)
Mohs scale hardness8
LusterVitreous
StreakWhite
Density6.55
Optical propertiesUniaxial (+)
Refractive indexn=2.30
BirefringenceYes
References[1][2][3]

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. "Schiavinatoite - Handbook of Mineralogy" (PDF). Handbookofmineralogy.org. Retrieved 2016-03-12.
  2. "Schiavinatoite: Schiavinatoite mineral information and data". Mindat.org. Retrieved 2016-03-02.
  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
  4. "Nioboholtite: Nioboholtite mineral information and data". Mindat.org. Retrieved 2016-03-03.


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