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Bassanite is a
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
sulfate mineral The sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), ...
with formula
CaSO4
CaSO<sub>4</sub>
· H2O or 2CaSO4·H2O. In other words it has half a water per CaSO4 unit hence its synonym ''calcium sulfate hemihydrate''. Bassanite was first described in 1910 for an occurrence on
Mount Vesuvius Mount Vesuvius ( ; it, Vesuvio ; nap, 'O Vesuvio , also or ; la, Vesuvius , also , or ) is a somma A somma volcano (also known as a sommian) is a volcano, volcanic caldera that has been partially filled by a new central volcanic cone, ...
. It was named for Italian paleontologist Francesco Bassani (1853–1916). At Vesuvius it occurs as alterations from gypsum within
leucite Leucite is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium Aluminium (aluminum in American English, American and Canadian English) is a chemical element with the Symbol (che ...

leucite
tephrite Leucite tephrite from Mayen, Eifel, Germany Tephrite is an igneous rock, igneous, volcanic (extrusive) rock (geology), rock, with aphanitic to porphyritic Texture (crystalline), texture. Mineral content is usually abundant feldspathoids (leucite ...

tephrite
and as
fumarole __NOTOC__ Sampling gases at a fumarole on Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** Federal government of the United States (metonym) ** Washington metr ...

fumarole
deposits. It occurs in dry lake beds in
California California is a U.S. state, state in the Western United States. With over 39.3million residents across a total area of approximately , it is the List of states and territories of the United States by population, most populous and the List of ...

California
and
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

Australia
. It also occurs interlayered with gypsum in
cave A cave or cavern is a natural void in the Earth#Surface, ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can also refer to much small ...

cave
s. H. Schmidt and coinvestigators reported in 2011 that under dry conditions, the structure is
monoclinic In crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure). The word "crystallography" is derived from the Greek language, Greek words ''crystallon'' "c ...
with space group ''C2'', but at 75% humidity, the structure is
trigonal A triangle is a polygon with three Edge (geometry), edges and three Vertex (geometry), vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC. In Euclidean geometry, any ...
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

Sulfate minerals Monoclinic minerals Minerals in space group 12 Minerals in space group 154 Mount Vesuvius Geology of Italy {{sulfate-mineral-stub