Whewellite /ˈhjuːəlaɪt/ is a mineral, hydrated calcium oxalate 2O. Because of its organic content it is thought to have an indirect biological origin; this hypothesis is supported by its presence in coal. 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 frequently have the same composition.
Whewellite was named after William Whewell (1794–1866), an English and scientist , professor of moral philosophy at Cambridge and inventor of the system of crystallographic indexing.