Weddellite ( Ca C₂ O₄·2 H₂O) is a mineral form of

calcium oxalate Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula or . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydr ...

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

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 C ...

, off

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

. Occasionally, weddellite partially dehydrates to

whewellite Whewellite is a mineral, hydrated calcium oxalate, formula calcium, Ca carbon, C2oxygen, O4·water, H2O. Because of its organic content it is thought to have an indirect biological origin; this hypothesis is supported by its presence in coal and ...

, forming excellent

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

s of grainy whewellite after weddellite's short tetragonal dipyramids. It was first described in 1936 but only named in 1942.

Structural properties

Weddellite, or

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

oxalate Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as ...

dihydrate, crystallises in a

tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the Cube (geometry), cube becomes a rectangular Pri ...

system: the classic crystal shape is the eight-face bipyramid. Using

bright field microscopy Bright-field microscopy (BF) is the simplest of all the optical microscopy illumination techniques. Sample illumination is transmitted (i.e., illuminated from below and observed from above) white light, and contrast in the sample is caused by a ...

, the weddellite crystals are recognised easily by their shape, reminiscent of a postal envelope. More complex shapes of weddellite are possible; the dumbbell shape is not rare and 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

polarised light , or , is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. One example of a polarize ...

Biological role

Weddellite crystals are usually of little clinical value. Together,

and weddellite are the most common renal calculi.

Occurrence

Weddellite occurs as

authigenic Authigenesis is the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described as authigenic. Authigenic sedimentary Mineral, minerals form during or after sedimentation by Precipita ...

crystals in sea floor mud. It also has been reported in

peat Peat is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. ''Sphagnum'' moss, also called peat moss, is one of the most ...

bearing sediments and in calcite-bearing lacustrine sediments. It occurs with whewellite,

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

, phosphammite and

aphthitalite Aphthitalite is a potassium sulfate mineral with the chemical formula: (K,Na)3Na(SO4)2. It was first described in 1835 for an occurrence on Mount Vesuvius, Italy. The name is from the Greek άφθητος, "unalterable", and άλας, "salt", for ...

. If

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and chemical formula , also written as or or . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name i ...

is used to clean any mineral sample that contains

, weddellite and

may be produced on the sample.

References

{{Reflist Calcium minerals Oxalate minerals Dihydrate minerals Tetragonal minerals Minerals in space group 87 Minerals described in 1942