In the urinary sediment, one can find two forms of calcium oxalate crystals. The most frequent form is the di-hydrated calcium oxalate. The mineralogical name of the calcium oxalate 2(H2O) is Weddellite. The second form is the mono-hydrated calcium oxalate whose mineralogical name is Whewellite. The two forms have different crystallographic characteristics. It seems that the calcium/magnesium ratio plays an important role in the formation of the calcium oxalate crystals. Crystals of calcium oxalate are found mainly in an acidic urine, but these can also be seen in slightly alkaline specimens.
Weddelites: Calcium oxalates 2(H2O)
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, an 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.
Weddelite crystals are usually of little clinical value. Many specimens develop weddelite crystals on standing.
Whewellites: calcium oxalates (H2O)
The whewellite crystal is a rare form of crystallization of calcium oxalate. In theory, the whewellite, or calcium oxalate mono-hydrate crystallizes in a monoclinic leave shape, but in the majority of cases, the former precipitates as an oval egg shape. The dumbbell structure is often erroneously associated to this form of oxalate. X-ray analysis have shown that the dumbbell structure can also represent weddelite crystals. Contrarily to the weddelite, the whewellite is found in situations of massive calcium oxalate precipitation. According to Berg, the abundance of oxalates formed of ovoid structures strongly agglutinated, twin structures, and microliths, is an indication of a pathological massive precipitation. Urines of patients with a calcium oxalate urolithiase have a tendency to have a sediment with some of the preceding characteristics.