Genthelvite , Kupletskite-(Cs) Locality: Poudrette quarry (Demix quarry; Uni-Mix quarry; Desourdy quarry), Mont Saint-Hilaire , Rouville RCM, Montérégie , Québec , Canada ( Locality at mindat.org ) A superb example of this very rare species, from a single pocket collected about 2 years ago that features unusually large, unusually translucent, unusually pretty pastel green crystals. 1.4 x 0.9 x 0.9 cm — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Kupletskite
Kupletskite is an IMA-recognized mineral species in the silicate class, with the chemical formula K₂NaMn²⁺₇Ti₂(Si₄O₁₂)₂O₂(OH)₄F.
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 3.2–3.36; Brown streak; Vitreous luster; optically biaxial.
IMA symbol Kpt, grandfathered status — known before the IMA’s founding, named in 1956.
Found in the natrolite-rich central part and in the wall rocks of nepheline syenite pegmatites in a differentiated alkalic massif (Lovozero massif, Russia).
It is found associated with Manganoan pectolite, neptunite, microcline, eudialyte, nepheline, aegirine (Lovozero massif, Russia); aegirine, analcime, gonnardite, apophyllite, eggletonite.
The name refers to Boris Mikhailovich Kupletskii (1894–1965), Russian geologist, Institute of Geosciences, Moscow, and El'za Maximilianovna Bohnshtedt-Kupletskaya (1898–1974), Russian mineralogist.