Chemistry:Whewellite

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Whewellite
Whewellite-md82a.jpg
A white Whewellite crystal from Schlema, Germany
General
CategoryOxalate minerals
Formula
(repeating unit)
CaC2O4·H2O
Strunz classification10.AB.45
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/c (no. 14)
Identification
ColorColorless, yellowish, brownish
Crystal habitEquant or stout prismatic crystals
Twinninge {101} twin plane
Cleavage{101} good, {010} imperfect, {001} indistinct, {110} indistinct
FractureConchoidal
TenacityBrittle
Mohs scale hardness2.5-3
|re|er}}Vitreous to pearly
DiaphaneityTransparent
Specific gravity2.23
Optical propertiesBiaxial (+), Colorless (transmitted light)
SolubilityInsoluble in water, soluble in acids
References[1][2][3][4]

Whewellite /ˈhjuːəlt/ is a mineral, hydrated calcium oxalate, formula Ca C2O4·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. However, it has also been found in hydrothermal deposits where a biological source appears improbable. For this reason, it may be classed as a true mineral.

Whewellite, or at least crystalline calcium oxalate, does also arise from biological sources. Small crystals or flakes of it are sometimes found on the surfaces of some cacti, and kidney stones frequently have the same composition.

Whewellite was named after William Whewell (1794–1866), an England polymath, naturalist and scientist, professor of moral philosophy at Cambridge and inventor of the system of crystallographic indexing.

Heat decomposition

Whewellite (calcium oxalate monohydrate) heat decomposition mass curve. The whewellite decomposes first to anhydrous calcium oxalate, then to calcium carbonate (losing carbon monoxide), and finally to calcium oxide (losing carbon dioxide).

Whewellite is used as a thermogravimetric analysis standard due to its well-known decomposition temperatures and products.

See also

References

Bibliography

  • Palache, P.; Berman H.; Frondel, C. (1960). "Dana's System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, Etc. (Seventh Edition)" John Wiley and Sons, Inc., New York, pp. 1099-1101.