Weddellite (
Ca C2 O4·2
H2O) is a mineral form of
calcium oxalate 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 ...
, 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, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
. Occasionally, weddellite partially dehydrates to
whewellite
Whewellite is a mineral, hydrated calcium oxalate, formula Ca C2 O4· 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 sedimentary nodules. Howev ...
, 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 with the 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 t ...
oxalate
Oxalate (IUPAC: ethanedioate) is an anion with the formula C2O42−. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl ...
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 becomes a rectangular prism with a squar ...
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 att ...
, 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
Polarization ( also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of t ...
.
Biological role
Weddellite crystals are usually of little clinical value. Together, whewellite
Whewellite is a mineral, hydrated calcium oxalate, formula Ca C2 O4· 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 sedimentary nodules. Howev ...
and weddellite are the most common renal calculi
Kidney stone disease, also known as nephrolithiasis or urolithiasis, is a crystallopathy where a solid piece of material (kidney stone) develops in the urinary tract. Kidney stones typically form in the kidney and leave the body in the urine s ...
.
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 minerals form during sedimentation by precipitation or recrys ...
crystals in sea floor mud. It also has been reported in peat
Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficient ...
bearing sediments and in calcite-bearing lacustrine sediments. It occurs with whewellite, urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important ...
, 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", fo ...
.[
If oxalic acid is used to clean any mineral sample that contains ]calcium
Calcium is a chemical element with the 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 t ...
, weddellite and whewellite
Whewellite is a mineral, hydrated calcium oxalate, formula Ca C2 O4· 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 sedimentary nodules. Howev ...
may be occurrenced on the sample. [
]
References
{{Reflist
Calcium minerals
Oxalate minerals
Tetragonal minerals
Minerals in space group 87
Minerals described in 1942