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-(Cs)
Kupletskite-(Cs) is an IMA-recognized mineral species in the silicate class, with the chemical formula Cs₂NaMn²⁺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 4; specific gravity 3.68; Dull luster; optically biaxial.
IMA symbol Kpt-Cs, IMA status: approved, named in 1970.
At the border of aegirine-microcline-quartz pegmatites, and in polylithionite-quartz replacement complexes in pegmatites.
It is found associated with Pyrochlore, stillwellite, tienshanite, sogdianite, thorite.
The name refers to the cesium in the composition and similarity to kupletskite.