Terrestrial and space-grown HAP and OCP crystals: effect of growth conditions on perfection and morphology.

This paper reports comparative characterizations of calcium phosphate crystals grown on earth and in space. At the CaCl2 and KH2PO4 + K2HPO4 solution concentrations and the pH used, only hydroxyapatite (HAP) crystals grow under terrestrial condition while both HAP and octacalcium phosphate (OCP) crystals grew during the space experiment. The space-grown OCP crystals reach 3 mm in size, the space-grown HAP crystals reach sizes up to 100 times larger than the earth-grown crystallites. It was found also that the space-grown crystallites are more perfect than the terrestrial ones, being more stable under electron beam during HRTEM examination. Spherolites of hydroxyapatite consist of small and thin HAP crystals with different orientations. Space-grown OCP crystals containing almost pure OCP phase show strong striations along the c direction due to thickness variations. Terrestrial OCP crystals grown at lowest supersaturation on earth may be almost as large as the space-grown ones, possess a regular habit and are homogeneous in thickness. However, they always contain substantial regions of HAP structure. Also, in these crystals electron irradiation induces phase transformation from crystalline to amorphous (disordered) state during transmission electron microscopy observations. In the space-grown crystals, such transformation needs longer radiation time. We believe that the differences described above come from much lower supersaturation and different pH for crystals nucleating and growing in space compared to those formed on earth.

Calcium phosphate crystallization under terrestrial and microgravity conditions.

Calcium phosphate crystalline powders grown under terrestrial and space (EURECA 1992-1993 flight) conditions in the Solution Growth Facility are analyzed and compared by optical and electron microscopy (scanning and transmission), electron and X-ray microdiffraction and microanalyses. On earth, only small, micrometer size scale, spherolites of hydroxyapatite (HAP) grow. In space, the HAP spherolites reach hundreds of micrometer. Also, octacalcium phosphate (OCP) spherolites up to 3 mm have been obtained. Computer modelling of diffusion in a real chamber has been performed. It suggests high spatial supersaturation gradients at zero gravity which may provide much higher local supersaturations on earth, where convection takes place. The analyses suggest that the dramatic difference between the terrestrial and space samples should come from much lower supersaturation in space.

Crystallization of calcium phosphate in microgravity.

Dilute solutions of CaCl2 and KH2PO4 + K2HPO4 were diffusing from either side into a mixing chamber with KCl solution. The microgravity experiment yielded aggregates of large crystals of OCP (Ca8H2(PO4)3,5H2O) and spherolites of smaller, but still visible crystals of HAP (Ca5OH(PO4)3), the stable final phase. Ground-based experiments yielded submicroscopic HAP crystals. Results of calculations of diffusion and crystal growth on the basis of previous knowledge agree well with observations.