Weddellite is a mineral form of calcium oxalate

Calcium oxalate

Calcium oxalate is a chemical compound that forms needle-shaped crystals, known in plants as raphides. A major constituent of human kidney stones, the chemical is also found in beerstone, a scale that forms on containers used in breweries...

 named for occurrences of millimeter-sized crystals found in bottom sediments of the Weddell Sea

Weddell Sea

The Weddell Sea is part of the Southern Ocean and contains the Weddell Gyre. Its land boundaries are defined by the bay formed from the coasts of Coats Land and the Antarctic Peninsula. The easternmost point is Cape Norvegia at Princess Martha Coast, Queen Maud Land. To the east of Cape Norvegia is...

, off Antarctica. Occasionally, weddellite partially dehydrates to whewellite

Whewellite

Whewellite is a mineral, hydrated calcium oxalate, formula CaC2O4·H2O. Because of its organic content it is thought to have an indirect biological origin and this is supported by it being found in coal and sedimentary nodules. However, it has also been found in hydrothermal deposits where a...

, forming excellent pseudomorph

Pseudomorph

In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form , resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another...

s of grainy whewellite after weddellite's short tetragonal dipyramids.

Structural properties

The weddellite or calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 oxalate

Oxalate

Oxalate , is the dianion with formula C2O42− also written 22−. Either name is often used for derivatives, such as disodium oxalate, 2C2O42−, or an ester of oxalic acid Oxalate (IUPAC: ethanedioate), is the dianion with formula C2O42− also written (COO)22−. Either...

 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.

Biological role

Weddellite crystals are usually of little clinical value. Many specimens develop weddellite crystals on standing. Together, whewellite

Whewellite

Whewellite is a mineral, hydrated calcium oxalate, formula CaC2O4·H2O. Because of its organic content it is thought to have an indirect biological origin and this is supported by it being found in coal and sedimentary nodules. However, it has also been found in hydrothermal deposits where a...

 and weddellite are the most common renal calculi.

Varieties and related minerals

Natroxalate, Whewellite

Whewellite

Whewellite is a mineral, hydrated calcium oxalate, formula CaC2O4·H2O. Because of its organic content it is thought to have an indirect biological origin and this is supported by it being found in coal and sedimentary nodules. However, it has also been found in hydrothermal deposits where a...

, Caoxite, Novgorodovaite, Oxammite, Glushinskite, Humboldtine, Minguzzite, Zhemchuzhnikovite

Zhemchuzhnikovite

Zhemchuzhnikovite is a mineral of organic origin; formula NaMgC2O4.8H2O. It forms smokey green crystals with a vitreous lustre and is found in Russian coal mines. It is named after Yury Zhemchuzhnikov , a Russian clay mineralogist.-References:**...

, Stepanovite, Moolooite, Wheatleyite, Coskrenite