Cadmoselite is a rare cadmium selenide mineral with chemical formula CdSe. Cadmoselite crystallizes in the hexagonal system and occurs as black to pale grey opaque crystals and grains.http://rruff.geo.arizona.edu/doclib/hom/cadmoselite.pdf Handbook of Mineralogyhttp://webmineral.com/data/Cadmoselite.shtml Webmineral data
It was first described in 1957 for an occurrence in Tuva. The mineral occurs as interstitial grains in sandstone formed under reducing alkaline diagenetic conditions.
References
Emsley, John. Nature's Building Blocks. Oxford, 2001.