Precipitation of Amorphous Calcium Oxalate in Aqueous\nSolution

Abstract

Inspired by the observation that\ncrystalline calcium carbonate\nand calcium phosphate biominerals frequently form via amorphous precursors,\na wide range of studies have been performed which demonstrate that\nmany inorganic crystals can precipitate from solution via amorphous\nphases. This article considers the crystallization mechanism of calcium\noxalate, which is a significant biomineral in many plants and the\nprimary constituent of kidney stones in vertebrates, and shows that\nthis can also precipitate via an amorphous precursor phase from aqueous\nsolution. A range of approaches were employed to study calcium oxalate\nformation, including precipitation in bulk solution in the presence\nand absence of additives and in the spatially confined volumes offered\nby track etched membranes and a crossed cylinders apparatus. A freeze\nconcentration method was also used to generate sufficient quantities\nof amorphous calcium oxalate (ACO) for analysis. The results show\nthat amorphous calcium oxalate crystallizes rapidly in bulk solution\nbut can be significantly stabilized through the concerted activity\nof additives and confinement. We also demonstrate that ACO has a composition\nof ≈CaC₂O₄:H₂O. These data\nsuggest that calcium oxalate biominerals, in common with their carbonate\nand phosphate counterparts, may also precipitate via amorphous phases.