Physics
Scientific paper
Apr 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975gecoa..39..489b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 39, Issue 4, pp.489-494
Physics
48
Scientific paper
The seeded precipitation (crystal growth) of aragonite and calcite from sea water, magnesium-depleted sea water, and magnesium-free sea water has been studied by means of the steady-state disequilibrium initial rate method. Dissolved magnesium at sea water levels appears to have no effect on the rate of crystal growth of aragonite, but a strong retarding effect on that of calcite. By contrast, at levels less than about 5 per cent of the sea water level, Mg has little or no effect on calcite growth. Extended crystal growth on pure calcite seeds in sea water of normal Mg content resulted in the crystallization of magnesium calcite overgrowths, containing 7-10 mole % MgCO 3 in solid solution. This suggests that the rate inhibition by Mg is due to its incorporation within the calcite crystal structure during growth, which causes the resulting magnesian calcite to be considerably more soluble than pure calcite. The standard free energy of formation of 8.5 mole% Mg calcite calculated on this assumption is in good agreement with independent estimates of magnesian calcite stability. From the work of ( Geochim. Cosmochim. Acta 37 , 1563-1586, 1973), Plummer and ( Amer. J. Sci . 273 , 515-522, 1974), and the present paper, it can be predicted that the most stable calcite in Ca-Mg exchange equilibrium with sea water contains between 2 and 7 mole%MgCO 3 in solid solution. Likewise, calcites containing more than 8.5 mole% MgCO 3 are less stable, and those containing less than 8.5 mole% MgCO 3 are more stable than aragonite plus Ca and Mg in sea water.
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