Nioboholtite

Nioboholtite is an extremely rare mineral with the formula (Nb_0.6[]_0.4)Al₆BSi₃O₁₈. It is the niobium-rich member of the dumortierite supergroup, and the 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.^[2] Nioboholtite and schiavinatoite are both minerals with essential niobium and boron.^[4]

Nioboholtite
General
Category	Silicate mineral
Formula	(Nb0.6[]0.4)Al6BSi3O18
IMA symbol	Nhlt[1]
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Unit cell	a = 47.00, b = 11.83 c = 20.24 [Å] (approximated)
Identification
Color	Creamy-white to brownish yellow, grey-yellow
Streak	White
Diaphaneity	Translucent
Optical properties	Biaxal (-)
Refractive index	nα=1.74-1.75, nβ~1.76, nγ~1.76 (approximated)
References	[2][3]

Association

Nioboholtite is mainly associated with holtite and a number of other minerals (see szklaryite).^[2]

Notes on chemistry

Main impurities in nioboholtite are antimony and arsenic, with trace or minor aluminium iron, tantalum, titanium, phosphorus and hydroxyl groups.^[2]