Computer Science
Scientific paper
May 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50..733w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 5, pp.733-739
Computer Science
4
Scientific paper
Experimentally reversed portlandite and brucite solubilities were determined between 300° and 600°C and 1 to 3 kbar. In the portlandite runs the molality of Ca decreases with increasing pressure at constant temperature. For instance, at 2 kbar log molalities at 300°, 400°, 500° and 600°C give values of -2.34, -2.71, -3.18 and -4.18, respectively. At 500°C, pressures of 1, 2 and 3 kbar yield values of -4.40, -3.18 and -2.65. Distribution of species in solution can be calculated with the aid of data from Helgeson and co-workers assuming Ca ++ is the dominant Ca species. These calculated Ca concentrations are within ± 0.2 log units of experimental values in most cases. The solubility reaction is, in all likelihood: 2 H + + Ca ( OH ) 2 a 3 Ca ++ + 2 H 2 O . Although the computed pH's are close to 2 units greater than neutral, the solutions apparently contained no significant Ca(OH) + or Ca(OH) 2 sq . Concentrations of Mg in the brucite runs show a sigmoidal behavior at 2 kbar as a function of temperature with log molalities of Mg of -4.00, -4.28, -4.14 and -4.60 at 350°, 450°, 550° and 600°C, respectively. Values at 1 kbar are lower and decrease monotonically from 350° to 550°C. Based on available thermodynamic data for Mg ++ it appears that Mg(OH) + is the dominant Mg species in solution. The solubility reaction is proposed to be: H + + Mg ( OH ) 2 a 3 Mg ( OH ) + + H 2 O . With the aid of data of Helgeson and co-workers values of the equilibrium constant for H 2 O + Mg ++ a 3 Mg ( OH ) + + H + necessary to account for the measured solution compositions can be calculated. These calculations indicate Mg(OH) + becomes dominant at temperatures above 450°C at 2 kbar and above 360°C at 1 kbar at neutral pH.
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