|Oxalate(Oxalic acid) (Calcium oxalate dihydrate CaC2O4·2H2O )|
|Crystal habit||eight-face bi-pyramid (typical)|
|Mohs scale hardness||4|
|Refractive index||1.52-1.54 - DR +0.009 (B-G interval)|
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 weddellite 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 weddellite crystals are recognized easily by their shape that reminds a mail envelope. More complex shapes of weddellite 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. Weddellite crystals are poorly birefringent and do not show any interference pattern under polarized light.