What is Nosean?
Nosean is a blue to grey cubic feldspathoid of silica-poor volcanic rocks, valued for sodalite-family structure and classic alkaline associations.
Mineralogy and crystal structure
Reference formula: Na8Al6Si6O24(SO4)·H2O. Nosean has a cubic sodalite-family framework with sulfate and water in its cages. It can be difficult to separate visually from sodalite and haüyne without association or analysis. Nosean is classified with silicates and has a cubic crystal system. Typical hardness is 5.5–6 on the Mohs scale and specific gravity is approximately 2.3–2.4.
Colour, lustre and habit
Typical colours include blue, grey, white, black. The usual dodecahedral, granular, massive habit and vitreous, greasy lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Nosean forms in or is associated with alkaline volcanic rocks, phonolites, tephrites and nepheline-rich lavas. Important documented localities include Germany, France, Italy, Russia, Canada and Tanzania.
Blue feldspathoid crystals in alkaline lava
Nosean offers a compact lesson in cage structures: a blue crystal can hold sulfate and water inside an open framework created by unusual silica-poor magma.
Collecting and care
Protect from acids and abrasion; keep matrix and locality information intact.
