weddellite

{{Infobox mineral | name = Weddellite | category = Oxalate minerals | image = File:Weddellite-92799.jpg | caption = White weddellite crust from the Cerchiara mine, Borghetto di Vara, Liguria, Italy | formula = CaC2O4·2H2O | IMAsymbol = Wed | strunz = 10.AB.40 | system = Tetragonal | class = Dipyramidal (4/m) H-M symbol: (4/m) | symmetry = I4/m | unit cell = a = 12.371, c = 7.357 [Å]; Z = 8 | color = Colorless to white, may be yellowish brown to brown from impurities | habit = Isolated crystals, may be corroded | twinning = Single or multiple | cleavage = Good on {010} | fracture = Conchoidal | mohs = 4 | luster = Vitreous | refractive = nω = 1.523 nε = 1.544 | opticalprop = Uniaxial (+) | birefringence = δ = 0.021 | pleochroism = | streak = White | gravity = 1.94 | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Transparent | other = Dehydrates on air exposure | references = }}

Weddellite (CaC2O4·2H2O) is a mineral form of calcium oxalate named for occurrences of millimeter-sized crystals found in bottom sediments of the Weddell Sea, off Antarctica. Occasionally, weddellite partially dehydrates to whewellite, forming excellent pseudomorphs of grainy whewellite after weddellite's short tetragonal dipyramids. It was first described in 1936 but only named in 1942.