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.

[2] 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.

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.