1. Whewellite (calcium oxalate monohydrate) crystals were found to induce epitaxially the heterogeneous nucleation of brushite (calcium monohydrogen phosphate dihydrate) from its metastable supersaturated solution in approximately one-quarter of the time required for spontaneous precipitation in the absence of added nucleating agents. Scanning electron-microscope observation of the crystalline phase showed brushite crystals originating from the whewellite seed crystals. 2. Crystal growth, upon nucleation, proceeded rapidly, and the metastable solutions quickly approached saturation. 3. Brushite crystals also induced the precipitation of calcium oxalate crystals in about one-quarter of the time required for spontaneous precipitation; however, the rate of crystal growth was considerably slower. In support of the chemical data, scanning electron micrographs showed few crystals of calcium oxalate nucleated on the surface of the brushite seed. 4. The results provide some insight into the cause of stones containing calcium oxalate or calcium phosphate (or both), which form in the normally acid environment of human urine.
Chemical kinetic data, complemented with scanning electron-microscope observations of the crystalline phase, show that seed crystals of hydroxyapatite have the ability to induce the growth of calcium oxalate monohydrate crystals epitaxially from a metastable supersaturated solution of calcium oxalate. The rate of growth of calcium oxalate crystals is dependent on the surface area of the seed material and follows a second-order rate law. It is suggested that there may be a causal relationship between the occurrence of apatite crystals in the urinary tract and the formation of both ‘pure’ and mixed urinary stones containing calcium oxalate. Under similar experimental conditions, however, seed crystals of calcium oxalate monohydrate appeared unable to induce epitaxially the growth of calcium phosphate crystals from a supersaturated calcium phosphate solution, indicating the absence of an epitaxial relationship between calcium oxalate monohydrate and the initially precipitating calcium phosphate phase(s).