Whewellite

Whewellite
A white Whewellite crystal from Schlema, Germany
General
Category	Oxalate minerals
Formula (repeating unit)	CaC2O4·H2O
Strunz classification	10.AB.45
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Identification
Color	Colorless, yellowish, brownish
Crystal habit	Equant or stout prismatic crystals
Twinning	e {101} twin plane
Cleavage	{101} good, {010} imperfect, {001} indistinct, {110} indistinct
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	2.5-3
Luster	Vitreous to pearly
Diaphaneity	Transparent
Specific gravity	2.23
Optical properties	Biaxial (+), Colorless (transmitted light)
Solubility	Insoluble in water, soluble in acids
References

Whewellite 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 English 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

• List of minerals approved by IMA
• List of minerals named after people

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.