Nioboholtite

Nioboholtite is an extremely rare mineral with the formula (Nb0.6[]0.4)Al6BSi3O18. It is niobium-rich member of dumortierite supergroup, and niobium-analogue of holtite of the holtite group. It is one of three quite recently found minerals of this group, the other two being titanoholtite and szklaryite, all coming from the Szklary village near Ząbkowice Śląskie in Poland. They occur in a unique pegmatite.[1] Nioboholtite and schiavinatoite are both minerals with essential niobium and boron.[3]

Nioboholtite
General
CategorySilicate mineral
Formula
(repeating unit)		(Nb0.6[]0.4)Al6BSi3O18
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPnma
Unit cella = 47.00, b = 11.83
c = 20.24 [Å] (approximated)
Identification
ColorCreamy-white to brownish yellow, grey-yellow
StreakWhite
DiaphaneityTranslucent
Optical propertiesBiaxal (-)
Refractive indexnα=1.74-1.75, nβ~1.76, nγ~1.76 (approximated)
References[1][2]

Association

Nioboholtite is mainly associated with holtite and a number of other minerals (see szklaryite).[1]

Notes on chemistry

Main impurities in nioboholtite are antimony and arsenic, with trace or minor aluminium iron, tantalum, titanium, phosphorus and hydroxyl groups.[1]

References
  1. Pieczka, A.; Evans, R. J.; Grew, E. S.; Groat, L. A.; Ma, C.; Rossman, G. R. (2013). "The dumortierite supergroup. II. Three new minerals from the Szklary pegmatite, SW Poland: Nioboholtite, (Nb0.6〈0.4)Al6BSi3O18, titanoholtite, (Ti0.75〈0.25)Al6BSi3O18, and szklaryite, 〈Al6BAs3+3O15". Mineralogical Magazine. 77 (6): 2841. doi:10.1180/minmag.2013.077.6.10.
  2. "Nioboholtite: Nioboholtite mineral information and data". Mindat.org. Retrieved 2016-03-03.
  3. "Schiavinatoite: Schiavinatoite mineral information and data". Mindat.org. Retrieved 2016-03-10.


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