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Surface of a kidney stone.jpg
Scanning Electron Micrograph of the surface of a kidney stone showing crystals of weddellite emerging from the amorphous central part of the stone. Horizontal length of the picture represents 0.5 mm of the figured original.
Category Oxalate minerals
(repeating unit)
Strunz classification 10.AB.40
Crystal system Tetragonal
Crystal class Dipyramidal (4/m)
H-M symbol: (4/m)
Space group 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
Crystal habit Isolated crystals, may be corroded
Twinning Single or multiple
Cleavage Good on {010}
Fracture Conchoidal
Mohs scale hardness 4
Luster Vitreous
Streak White
Diaphaneity Transparent
Specific gravity 1.94
Optical properties Uniaxial (+)
Refractive index nω = 1.523 nε = 1.544
Birefringence δ = 0.021
Other characteristics Dehydrates on air exposure
References [1][2][3]

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 1942.[1]

Structural properties[edit]

Weddellite, or calcium oxalate dihydrate, crystallises in a tetragonal system: the classic crystal shape is the eight-face bipyramid. Using bright field microscopy, the weddellite crystals are recognised easily by their shape, reminiscent of a postal envelope. More complex shapes of weddellite are possible; the dumbbell shape is not rare and has no precise angles or sides. This form is, in reality, a microcrystalline agglomerate that takes the shape of a biconcave disc. Weddellite crystals are poorly birefringent and do not show any interference pattern under polarised light.

Biological role[edit]

Weddellite crystals are usually of little clinical value. Together, whewellite and weddellite are the most common renal calculi.


Weddellite occurs as authigenic crystals in sea floor mud. It also has been reported in peat bearing sediments and in calcite-bearing lacustrine sediments. It occurs with whewellite, urea, phosphammite and aphthitalite.[2]