1911 Encyclopædia Britannica/Autunite

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AUTUNITE, or Calco-uranite, a mineral which is one of the "uranium micas," differing from the more commonly occurring torbernite (q.v.) or cupro-uranite in containing calcium in place of copper. It is a hydrous uranium and calcium phosphate, Ca(UO2)2(P04)2,+8(or 12)H2O. Though closely resembling the tetragonal torbenite in form, it crystallizes in the orthorhombic system and is optically biaxial. The crystals have the shape of thin plates with very nearly square outline (89° 17’ instead of 90°). An important character is the perfect micaceous cleavage parallel to the basal plane, on which plane the lustre is pearly. The colour is sulphur-yellow, and this enables the mineral to be distinguished at a glance from the emerald-green torbenite. Hardness 2-2½; specific gravity 3·05-3·19. Autunite is usually found with pitchblende and other uranium minerals, or with ores of silver, tin and iron; it sometimes coats joint-planes in gneiss and pegmatite. Falkenstein in Saxony, St Symphorien near Autun (hence the name of the species), and St Day in Cornwall are well-known localities for this mineral.