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.

Structural properties
The weddelite or calcium oxalate di-hydrate crystallizes in the tetragonal system. The classic crystal shape is the eight-face bi-pyramid. In bright field microscopy, the weddelite crystals are recognized easily by their shape that reminds a mail envelope. More complex shapes of weddelite are possible. The dumbbell shape is not rare. The former has no precise angles or sides. This form is, in reality, a microcrystalline agglomerate that takes the shape of a biconcave disc. Weddelite crystals are poorly birefringent and do not show any interference pattern under polarized light.

Biological role
Weddelite crystals are usually of little clinical value. Many specimens develop weddelite crystals on standing. Together, apatite, whewellite, and weddellite are probably the most common urinary stones.

Varieties and related minerals
Natroxalate, Whewellite, Caoxite, Novgorodovaite, Oxammite, Glushinskite, Humboldtine, Minguzzite, Zhemchuzhnikovite, Stepanovite, Moolooite, Wheatleyite, Coskrenite