In vitro formation of dicalcium phosphate dihydrate, CaHPO4 . 2H2O (DCPD).

Dicalcium phosphate dihydrate, CaHPO4 . 2H2O (DCPD) crystals were grown in solutions and in different types of gels (silica, agar, collagen) at different pH's, in the presence of other ions, and on different Ca-bearing substrates (enamel, dentine, bone, calcite). Hydrolysis of DCPD to other calcium phosphates and calcium oxalates were made in solutions containing different ions at 37 degrees C, 24h. Results showed that the type of media (solution or gel), type of gel, pH, presence of other ions affect the morphology (size/shape) and growth of DCPD. In systems of changing pH, the more basic calcium phosphate (e.g., apatite, OCP) formed in the first zone, DCPD in the last zone, regardless of the initial Ca/P of the system. Growth of DCPD was suppressed in the presence of F-(which favored growth of apatite), of Cd2+, Pb2+ (which favored OCP). Hydrolysis of DCPD to apatite was promoted in solutions containing Ca2+, F-, CO3=, and inhibited by Mg2+, P2O74-. Hydrolysis of DCPD to calcium oxalate occurs in presence of C2O4=. Results indicate that the variation in morphology of DCPD occurring in various human tissue calcinosis and the co-existence of DCPD with other calcium phosphates and oxalates in urinary and dental calculi are due to variations in pH, and presence of different ions in the media.

Apatite crystallites: effects of carbonate on morphology.

Carbonate is a substituent in the apatite structure; when present, it limits the size of the growing apatite crystals and so influences their shape that they grow more equiaxed than needle-like. The tendency for carbonate apatites to be equiaxed is related to the nature of the chemical bonds formed in the crystal. The interference of carbonate with the good crystallization of apatite, and its weakening effect on the bonds in the structure, increase the dissolution rate and the solubility, thereby presumably contributing to the susceptibility to caries of dental apatites containing carbonate.