Geology topics
Publications and source records attributed to George A. Parks.
XAFS and TEM analyses show that the magnetite surface passivates at neutral pH by conversion to maghemite, limiting its potential use for aqueous Cr(VI) reduction.
Considerable disparity exists in the published thermodynamic data for selected species in the Cd(II)-CO 2 -H 2 O system near 25°C and 1 atm pressure. Evaluation of published experimental and estimated data for aqueous cadmium-carbonate species suggests an association constant, pK , of −3.0 ± 0.4 for CdCO 3 0 , about −1.5 for CdHCO 3 + , and −6.4 ± 0.1 for Cd(CO 3 ) 2 2− ( T = 298.15 K ; P = 1 atm; I = 0). Examination of all available data for cadmium-hydrolysis species and β - Cd ( OH ) 2( s ) ) confirms that the consistent set of constants presented by Baes and Mesmer ( Hydrolysis of Cations , 1976) is the best available. The solubility of synthetic otavite, CdCO 3(s) , has been measured in KClO 4 solutions where I ≤ 0.1 M . We calculated pK sp = 12.1 ± 0.1 ( T = 25.0° C ; P = 1 atm; I = 0) from measured concentrations of Cd 2+ , measured P C 02 and pH, our selected set of equilibrium constants, and activity corrections estimated using the Davies equation. Values at 5 and 50°C were 12.4 ± 0.1 and 12.2 ± 0.1, respectively. Based on the new solubility data and the CODATA key values for Cd 2+ and CO 3 2− , a new set of thermodynamic properties is recommended for otavite: ΔG f 0 = −674.7 ± 0.6 kJ / mol ; ΔH f 0 = −751.9 ± 10 kJ / mol ; S 0 = 106 ± 30 J/mol K; and ΔG r 0 for the reaction Cd 2+ + CO 3 2− ⇌ CdCO 3 ( s ) is −69.08 ± 0.57 kJ / mol.
Reported solubility data for a synthetic scorodite and the techniques used to prepare the synthetic phase are questioned. (A.M. 70-838, 72-852)-J.A.Z.
The point of zero charge of forsterite shifts from an estimated 8.9 for the unaltered mineral to 8.4 and 8.0, respectively, after being weathered artificially for 1 h and 4 h. These results corroborate the presence of magnesium-deficient surfaces on the weathered minerals, also indicated by chemical analyses of the solutions. H + and OH − are shown to be potential-determining ions in this system