Weddellite

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.

Weddellite
White weddellite crust from the Cerchiara mine, Borghetto di Vara, Liguria, Italy
General
CategoryOxalate minerals
Formula
(repeating unit)
CaC2O4·2H2O
IMA symbolWed
Strunz classification10.AB.40
Crystal systemTetragonal
Crystal classDipyramidal (4/m)
H-M symbol: (4/m)
Space groupI4/m
Unit cella = 12.371, c = 7.357 [Å]; Z = 8
Identification
ColorColorless to white, may be yellowish brown to brown from impurities
Crystal habitIsolated crystals, may be corroded
TwinningSingle or multiple
CleavageGood on {010}
FractureConchoidal
Mohs scale hardness4
LusterVitreous
StreakWhite
DiaphaneityTransparent
Specific gravity1.94
Optical propertiesUniaxial (+)
Refractive indexnω = 1.523 nε = 1.544
Birefringenceδ = 0.021
Other characteristicsDehydrates on air exposure
References
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