Physics
Scientific paper
May 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.1219v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 5, pp.1219-1234
Physics
7
Scientific paper
Seeded precipitation experiments were conducted in a pH/fluoride-stat system to study the crystal growth of fluorapatite ( FAP ) in carbonate-free NaCl-CaCl 2 -NaF-Na 2 HPO 4 solutions, at seawater calcium concentration, chlorinity, and pH. With increasing supersaturation, the dependence of the growth rate on the relative supersaturation changes from parabolic to exponential. This is interpreted as reflecting a transition in the crystal growth mechanism from growth at dislocation-induced surface steps to surface nucleation-controlled growth. The analysis of the kinetic data leads to a mineral-aqueous solution interfacial tension for FAP of 289 mJ / m 2 . The Arrhenius activation energy of the growth reaction in the temperature range 12 to 35°C is 47 kJ / mol . The inhibition of FAP growth by Mg 2+ ions was investigated over a range of total dissolved Mg of 0 to 60 mM . At dissolved magnesium concentrations typical of marine pore waters (40-60 mM ), the rate of FAP growth is 15 to 20 times slower than in the absence of Mg 2+ , for the same degree of supersaturation, at 25 °C and pH = 8. The inhibitory effect can be explained by the blocking of growth sites at the surface of FAP crystals by adsorbed Mg 2+ ions. A simple Langmuir adsorption model for the retardation effect of Mg 2+ is supported by the results. The effect of pH on FAP growth was tested for pH values from 7 to 8.5. In this range, growth of FAP is catalyzed by hydrogen ions. The apparent growth rate constant is proportional to ( a H +) m where m , the rate order with respect to H + , is a non-integral number which depends on pH. At identical degrees of supersaturation, the growth rate of FAP at pH = 7 is nearly twice that at pH = 8. When corrected for bottom water temperatures, pore water pH, and the retardation of Mg 2+ , the experimental growth rates predict that during burial in modern phosphatic sediments, apatite particles grow to sizes on the order of 0.1-10 rn. The relatively slow growth kinetics of FAP are consistent with the observed small particle sizes of marine sedimentary apatite.
Berner Robert A.
Van Cappellen Philippe
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