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<sub>2</sub>O<sub>4</sub>:H<sub>2</sub>O. These data
suggest that calcium oxalate biominerals, in common with their carbonate
and phosphate counterparts, may also precipitate via amorphous phases