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Sellaite in Fluorite - Sold

Sellaite and fluorite are simple fluorides of magnesium and calcium, respectively. Differences in the atomic radii of magnesium and calcium lead to very different crystal structures for these two species. Fluorite has eight fluorine atoms coordinating each calcium atom, whereas sellaite has six fluorine atoms coordinating each magnesium atom. Sellaite has the same crystal structure as rutile, while fluorite has the same crystal structure as uraninite. Strüver (1869) named sellaite after Quentino Sella (1827-1884), an Italian mining engineer who went into politics. Sella was an avid mountaineer, who also founded the Club Alpino Italiano.

Sellaite forms a white radiating aggregate, 5 cm across, in massive fluorite. The sellaite is pale pink while the fluorite is very pale green. From the collection of E.R. Laskowski (1949-2020), a mining engineer who retired to Tucson, Arizona.