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

· 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 ...

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 ...

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 ...

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 ...

. 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