Oxalate Biominerals

Oxalate biominerals are widely distributed in nature. They are found in rocks, soil, and among a variety of living organisms, including plants and animals. Calcium oxalate is by far the most abundant biomineral of this type, and is especially common in plants and fungi. In mammals, including humans, calcium oxalate is formed as a pathological biomineral. Other oxalate biominerals have been characterized in recent years, usually during rock weathering processes by lichens. The structural and general physico-chemical properties of oxalic acid and metallic oxalates are briefly discussed. Chemical, morphological and structural characteristics of calcium oxalates in the plant kingdom are subsequently analyzed, including comments on their possible functions and on the biosynthesis of oxalic acid in plants. The presence of calcium oxalates in other forms of life, the main characteristics of pathological oxalates, the biochemical degradation of oxalate, and the impact of oxalate biominerals in the environment, are also reviewed. The most important characteristics of other oxalates (alkaline, alkaline-earth, ammonium, aluminum, lead, and transition metals) found in nature, are also presented and discussed.