An electron microscopic study of the formation of amorphous calcium phosphate and its transformation to crystalline apatite.

Abstract

Dried amorphous calcium phosphate (ACP) can exist in discoidal and spheroidal forms. The disk-shaped particles are most prominent in dried ACP specimens isolated immediately following precipitation. The spherical forms become dominant in dried specimens taken from older suspensions. The disk-like morphology is a result of sample drying. Spherules can also arise during the drying step but are present in the native suspension as well. Both the disks and spherules appear to have a common solution progenitor in the form of a highly hydrated ACP aggregate of unknown morphology. The formation of crystalline apatite in solution is intimately connected with ACP. The first crystals appear on the surface of the amorphous spherules and in contact with the amorphous disks. In the case of the spherules, the initial crystals increase in size and generate additional crystals by secondary nucleation until the spherules become enveloped by apatite. However, when conversion is completed, the apatite does not fill appreciably the space previously occupied by the dissolved amorphous spherules. These observations do not support the concept of anin situ solid state conversion in this case, but indicate a heterophase transition supported by a solution-mediated ion translocation process. The occurrence of crystals in the amorphous disks suggest that apatite can also form directly from the solution progenitor.