KINETICS OF GROWTH AND DISSOLUTION OF FLUORAPATITE

Seeded growth and dissolution experiments with fluorapatite microcryst als in solutions with Ca:P:F = 10:6:2 and with constant values of pH i n the range 5.0 < pH < 6.5 are reported. The rates of both growth and dissolution appear to be controlled by a polynuclear surface mechanism in the supersaturation range studied: 2 < S < 7 for growth and 0.1 < S < 0.7 for dissolution. From this model the surface tension is found to be 120 mJ/m(2) from the growth results and 42 mJ/m(2) from the diss olution results. This difference can be explained by the imperfections of the surface being an advantage for the dissolution process. Using mean ion activities in the polynuclear expressions, the rate constants lead to values of the frequency for a building unit to enter a kink o f the order 10(7)-10(8) s(-1). These values are considered to be highe r than the values which can be expected for the molecular events invol ved. This leads to a revision of the model in the accompanying paper.