What is Vishnevite?
Vishnevite is a sulfate-bearing cancrinite-group feldspathoid found in unusual alkaline rocks and valued for its open framework and uncommon chemistry.
Mineralogy and structure
Reference formula: (Na,K)6Ca2Al6Si6O24(SO4)2. Vishnevite is hexagonal and compositionally variable in sodium, potassium, calcium and sulfate. COD 9004172 is a labelled natural representative model, not a claim that every sample has identical occupancy. Vishnevite is classified with silicates and has hexagonal crystal symmetry. Typical hardness is 5.5–6 on the Mohs scale and specific gravity is approximately 2.4–2.6.
Colour, lustre and habit
Typical colours include white, colourless, grey, yellowish. The usual massive, granular, prismatic habit and vitreous, greasy, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Vishnevite forms in or is associated with nepheline syenites, alkaline pegmatites and silica-undersaturated intrusive complexes. Important documented localities include the Kola Peninsula, the Urals, Greenland, Canada, Norway and Russia.
An open framework from an unusual alkaline magma
Vishnevite is a specialist collector mineral whose pale appearance hides a sophisticated cage-like framework. Its value grows when the alkaline locality and associated feldspathoids are documented.
Collecting and care
Protect from acids and abrasion; clean gently and keep the alkaline host-rock association with the specimen.
