Vivid blue inclusions in white whewellite or weddellite, occurring within the medulla of four lichen species growing on copper-bearing rocks, have been identified as the hydrated copper oxalate, moolooite, CuC2O4. nH2O (n ∼ 0.4–0.7), by infrared spectroscopy and X-ray powder diffraction. The mineral is believed to have been formed by reaction between oxalic acid secreted by the lichen and ground or surface water containing copper.

“Envelope crystals” collected during The Great Barrier Reef Expedition in May 1929 have been studied using low-temperature synchrotron single-crystal X-ray diffraction. The crystals are shown to be weddellite, with the largest content of zeolitic water reported to date. A new H2O site has been located within the crystal structure. Study of the crystals show that the end-member formula for weddellite should be reported as CaC2O4·(2.5 – x)H2O, where 0≤x≤ 0.25, instead of CaC2O4·(2H2O or CaC2O4·(2 + x )H2O. This is also the first report of weddellite occurring in a coral reef.

The factors influencing the predominance of one of the two mineral forms of calcium oxalate (CO), the monohydrated whewellite (COM) and the di-hydrated weddellite (COD), forming the pruina of the upper cortex of lichens, have been investigated through a simple, sensitive histochemical assay: toluidine blue O (TBO), a metachromatic staining test. The differential reactivity of 43 thalli of 17 pruinose foliose species, supplemented by X-ray diffraction analysis and observations with polarizing and scanning electron microscopy, suggests that the histochemical reactivity of hyphal walls and cementing substances of the upper cortex are related to the density of anionic charges. These factors are probably due to the occurrence of polyuronic acid substances that strongly affects the mineralization of CO. Di-hydrated wedellite is always associated with TBO metachromatic reactivity, and COM with orthochromatic reactivity. When the material has an ambiguous ortho/metachromatic reactivity, COD and COM may occur together. This study presents the first experimental evidence that in lichens CO biomineralization is at least partially biologically controlled.