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Weddellite

Mineral form of calcium oxalate

Field	Value
name	Weddellite
category	Oxalate minerals
image	File:Weddellite-92799.jpg
caption	White weddellite crust from the Cerchiara mine, Borghetto di Vara, Liguria, Italy
formula	CaC2O4·2H2O
IMAsymbol	Wed
strunz	10.AB.40
system	Tetragonal
class	Dipyramidal (4/m)
H-M symbol: (4/m)
symmetry	I4/m
unit cell	a = 12.371, c = 7.357 [Å]; Z = 8
color	Colorless to white, may be yellowish brown to brown from impurities
habit	Isolated crystals, may be corroded
twinning	Single or multiple
cleavage	Good on {010}
fracture	Conchoidal
mohs	4
luster	Vitreous
refractive	nω = 1.523 nε = 1.544
opticalprop	Uniaxial (+)
birefringence	δ = 0.021
streak	White
gravity	1.94
diaphaneity	Transparent
other	Dehydrates on air exposure
references

H-M symbol: (4/m)

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. It was first described in 1936 but only named in 1942.

Structural properties

Weddellite, or calcium oxalate dihydrate, crystallises in a tetragonal system: the classic crystal shape is the eight-face bipyramid. Using bright field microscopy, 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.

Biological role

Weddellite crystals are usually of little clinical value. Together, whewellite and weddellite are the most common renal calculi.

Occurrence

Weddellite occurs as authigenic crystals in sea floor mud. It also has been reported in peat bearing sediments and in calcite-bearing lacustrine sediments. It occurs with whewellite, urea, phosphammite and aphthitalite.

If oxalic acid is used to clean any mineral sample that contains calcium, weddellite and whewellite may be produced on the sample.

References

Warr, L.N.. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine.
[http://www.mindat.org/min-4254.html Weddellite on Mindat.org]
[http://rruff.info/doclib/hom/weddellite.pdf Weddellite in the Handbook of Mineralogy]
[http://www.webmineral.com/data/Weddellite.shtml#.UsDa29LjW1U Weddellite data on Webmineral]

Info: Wikipedia Source

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calcium-minerals oxalate-minerals dihydrate-minerals tetragonal-minerals minerals-in-space-group-87 minerals-described-in-1942

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