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Formula: Ca(SO4).0.5H2O
Hydrated sulphate
Crystal System: Monoclinic
Specific gravity: 2.69 to 2.76 measured, 2.731 calculated
Streak: White
Colour: White
Environments
Evaporite deposits
Cave deposits
Fumeroles
Bassanite dehydrates to anhydrite at 130oC and rehydrates
gradually to gypsum in a normal slightly humid atmosphere
(Webmin, Dana).
It occurs in fumeroles, in dry lake beds and in caves, interlayered with gypsum.
Associated minerals include gypsum,
anhydrite, celestine,
calcite and gibbsite
(HOM).
Localities
At the type locality, Mount Vesuvius, Somma-Vesuvius Complex, Naples, Campania, Italy, bassanite occurs in
leucite-tephrite blocks from
the 1906 eruption, and with gibbsite in fumeroles, from the 1911 eruption
(Dana).
It occurs as white, opaque crystals to 1 cm, as pseudomorphs after
gypsum
(Mindat).
At Dry Lake, Panamint Valley, Inyo county, and at Danby Dry Lake, Salt Marsh, Ward Valley, San Bernardino county, both
in California, USA, bassanite is found with gypsum,
celestine and clay
(Dana).
The bassanite occurs in unconsolidated sediments as mm thick layers intercalated with layers of silty
clay. Gypsum is
associated with the bassanite (1) as crystals included in the clay
layers, (2) as light brownish granular layers adjacent to the bassanite, and (3) as a fine crystalline
constituent within bassanite laminae. The bassanite is snow white and long fibrous in habit
(AM 38.1266-1268).
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