Whewellite is a mineral, hydrated calcium oxalate, formula CaC₂O₄·H₂O. Because of its organic content it is thought to have an indirect biological origin and this is supported by it being found in coal and sedimentary nodules. However, it has also been found in hydrothermal deposits where a biological source appears improbable. For this reason it can be classed as true mineral.

Whewellite, or at least crystalline calcium oxalate, does also arise from natural sources. Small crystals or flakes 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 mineralogist.

Whewellite heat decomposition

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

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