FOV 3.8x 2.5 mm. Thanks to Ralph Thomas. Analyzed find. MOB coll. Golden to reddish brown niobokulpetskite with black aegirine prisms and very thin, transparent, platy albite. Associated with (verified) franconite. From: Poudrette quarry, Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada. — Photo: Modris Baum · CC0
Niobokupletskite
Niobokupletskite is an IMA-recognized mineral species in the silicate class, with the chemical formula K₂NaMn₇(Nb,Ti)₂(Si₄O₁₂)₂O₂(OH)₄(O,F).
It crystallizes in the triclinic crystal system (space group P-1). It is a member of the Astrophyllite-Kupletskite structural group.
Key properties: Mohs hardness 3–4; specific gravity 3.325; Vitreous luster; optically biaxial.
IMA symbol Nbk, IMA status: approved, named in 1999.
A late-stage mineral in nepheline syenite pegmatite in an alkaline complex.
It is found associated with Kupletskite, aegirine, albite, analcime, calicoancylite-(Ce), calcite, catapleiite, epididymite, fluorite, genthelvite, microcline, natrolite, pyrochlore.
The prefix, niobo, indicates the niobium analog of kupletskite.