Bassanite is a

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

sulfate mineral The sulfate minerals are a class of minerals that include the sulfate ion () within their structure. The sulfate minerals occur commonly in primary evaporite depositional environments, as gangue minerals in hydrothermal Vein (geology), veins and as ...

with formula CaSO₄· H₂O or 2CaSO₄·H₂O. In other words it has half a water molecule per CaSO₄ unit, hence its synonym ''calcium sulfate hemihydrate''. Bassanite was first described in 1910 for an occurrence on

Mount Vesuvius Mount Vesuvius ( ) is a Somma volcano, somma–stratovolcano located on the Gulf of Naples in Campania, Italy, about east of Naples and a short distance from the shore. It is one of several volcanoes forming the Campanian volcanic arc. Vesuv ...

. It was named for Italian paleontologist Francesco Bassani (1853–1916). At Vesuvius it occurs as alterations from

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

within

leucite Leucite (from the Greek word ''leukos'' meaning white) is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium tectosilicate KAlSi2O6. Crystals have the form of cubic icositetrahedra b ...

tephrite and as fumarole deposits. It occurs in dry lake beds in

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

and

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. It also occurs interlayered with gypsum in

cave Caves or caverns are natural voids under the Earth's Planetary surface, surface. Caves often form by the weathering of rock and often extend deep underground. Exogene caves are smaller openings that extend a relatively short distance undergrou ...

s. H. Schmidt and coinvestigators reported in 2011 that under dry conditions, the structure is

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

with space group ''C2'', but at 75% humidity, the structure is

trigonal In crystallography, the hexagonal crystal family is one of the six crystal family, crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the tr ...

with space group ''P3₂21''. This reflects the incorporation of additional water of hydration, such that the trigonal form has the formula CaSO₄·0.625H₂O.

References

Calcium minerals Sulfate minerals 0.5 Monoclinic minerals Minerals in space group 5 Minerals in space group 152 or 154 Mount Vesuvius Geology of Italy Minerals described in 1910 {{sulfate-mineral-stub