Whewellite calcium oxalate

Figure 2.5. Scanning electron micrographs of (a) hyphae of Serpula himantioides mineralized with calcite (CaCOs) and whewellite (calcium oxalate monohydrate, CaC2O4-H2O), (b) calcite precipitated in proximity to and (c) on biomass of a Doratomyces sp. (From E. P. Burford and G.M. Gadd, unpublished Burford et al., 2006.)...

Calcium oxalate (whewellite, weddelite) Gallstones plant defenses... 

Both calcium oxalate monohydrate and dihydrate - whewellite and weddelite - contain eight-coordinated Ca2+ in distorted square-antiprismatic geometry, but there is one coordinated water in the former, two in the latter. Whewellite has a layer structure through the sharing of three edges of each coordination square-antiprism with three adjacent polyhedra weddelite has a chain structure (128). The trihydrate also contains eight-coordinated Ca2+ (129). 

Calcium oxalate (723) occurs as the monohydrate (whewellite, the thermodynamically stable form under ambient conditions (724)), the dihydrate (weddellite) in plant calcium stores and in sap, or the trihydrate (725). Calcium oxalate also plays a structural role in plants. Oxalate, for example from excessive amounts of rhubarb or spinach, inhibits absorption of Ca2+ from the GIT precipitation of calcium oxalate is the reason for the toxicity of oxalates. Calcium oxalate may also occur in man, where it can appear as minute star-shaped crystals in the urine. It is the main constituent of the majority of urinary calculi in man (726,727). The relationships between dietary calcium... 

In conclusion, any metal oxalate can be considered as a compound in a metastable equilibrium. The only sub-spontaneous diagenetic evolution of calcium oxalate is a possible transformation of weddellite into whewellite by dehydration (Frey-Wyssling, 1981 Verrecchia et al., 1993). Consequently, activation energy has to be provided for a complete oxidation of COM or COD, and life is the best and the most obvious candidate. Therefore, biogenic activity could be the key explanation for the absence of oxalate in paleosols as well as in the geological sedimentary record. 

Concentrations of LMW organic acids are high enough in some soils that salts of the acids are precipitated. Weddellite and whewellite (hydrated calcium oxalate) have been observed as coatings on fungal hyphae in forest soils (67). Oxalate, often one of the more abundant of the identifiable LMW organic acids, significantly enhances oxide dissolution in laboratory studies (22, 23). 

At the Toca do Serrote da Bastiana rock shelter in Brazil, a red, iron ochre pictograph of an anthropomorphic figure had become coated with a calcite accretion over time. Using X-ray diffraction and Fourier transform infrared spectroscopy, we determined that the accretion also contains whewellite, the monohydrate of calcium oxalate, in addition to calcium carbonate. An AMS radiocarbon date on the oxalate formed from contemporaneous carbon yielded a minimum date of... 

Iron(II) oxalate (or oxaUte), humboldtine (Fe[C204].2H20) is often found in association with the calcium oxalates, whewellite... 

Calcium Organo-calcium Calcium oxalate, whewellite type ... 

Carbonates Phosphates Silica Calcite Aragonite Vaterite Monohydrocalcite Amorphous Dahllite Francolite Amorphous calcium phosphate hydrogel Amorphous ferric phosphate hydrogel Opal Iron oxides Sulfates Halides Oxalates Magnetite Goethite Lepidocrocite Amorphous hydrates Celestite Barite Gypsum Fluorite Weddellite Whewellite... 

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