Precipitation of Amorphous Calcium Oxalate in Aqueous Solution

Inspired by the observation that crystalline calcium carbonate and calcium phosphate biominerals frequently form via amorphous precursors, a wide range of studies have been performed which demonstrate that many inorganic crystals can precipitate from solution via amorphous phases. This article considers the crystallization mechanism of calcium oxalate, which is a significant biomineral in many plants and the primary constituent of kidney stones in vertebrates, and shows that this can also precipitate via an amorphous precursor phase from aqueous solution. A range of approaches were employed to study calcium oxalate formation, including precipitation in bulk solution in the presence and absence of additives and in the spatially confined volumes offered by track etched membranes and a crossed cylinders apparatus. A freeze concentration method was also used to generate sufficient quantities of amorphous calcium oxalate (ACO) for analysis. The results show that amorphous calcium oxalate crystallizes rapidly in bulk solution but can be significantly stabilized through the concerted activity of additives and confinement. We also demonstrate that ACO has a composition of ≈CaC 2 O 4:H 2 O. These data suggest that calcium oxalate biominerals, in common with their carbonate and phosphate counterparts, may also precipitate via amorphous phases.