Stabilities of synthetic magnesian calcites in aqueous solution: comparison with biogenic materials

Journal Article · Mon Jun 01 04:00:00 EDT 1987 · Geochim. Cosmochim. Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(87)90325-5· OSTI ID:6992631

Bischoff, W D; Mackenzie, F T; Bishop, F C

Free-drift dissolution data and inverse time plots were used to evaluate the stabilities of synthetic and biogenic magnesian calcites in aqueous solutions at 25/sup 0/C and 1 atm total pressure. Synthetic phases with MgCO/sub 3/ concentrations below 6 mole percent have stoichiometric ion activity products that are less than the value for calcite, whereas the values for phases with higher concentrations are greater than that of calcite. For synthetic phases, stability is a smooth function of composition, and all phases have values of ion activity products less than that for aragonite. Average sea water at 25/sup 0/ and 1 atm total pressure is supersaturated with respect to all synthetic phases in the compositional range studied. The difference in stability between biogenic materials and synthetic phases is due to greater variation in chemical and physical heterogeneities found for the biogenic samples. If it is assumed that the results of the dissolution experiments reflect only differences in Gibbs free energies of formation between synthetic phases and biogenic materials of similar Mg concentration, the biogenic materials are 200-850 j/mol less stable than the synthetic phases. Only the results of synthetic dissolution experiments should be used to model the thermodynamic behavior of the magnesian calcite solid solution. The results for the synthetic phases, however, may not be appropriate to use for interpreting diagenetic reaction pathways for magnesian calcites in modern sediments, except as a basis of comparison with the behavior of natural minerals.

• Research Organization: Wichita State Univ., KS (USA)
• OSTI ID: 6992631
• Journal Information: Geochim. Cosmochim. Acta; (United States), Journal Name: Geochim. Cosmochim. Acta; (United States) Vol. 51:6; ISSN GCACA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580400* -- Geochemistry-- (-1989)
ACTIVATION ENERGY
ALKALINE EARTH METAL COMPOUNDS
CALCITE
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CARBON COMPOUNDS
CARBONATE MINERALS
CARBONATES
CHEMISTRY
DISPERSIONS
DISSOLUTION
DOLOMITE
ENERGY
GEOCHEMISTRY
HYDROGEN COMPOUNDS
ION EXCHANGE
MAGNESIUM CARBONATES
MAGNESIUM COMPOUNDS
MINERALS
MIXTURES
OXYGEN COMPOUNDS
PHASE STUDIES
PRECIPITATION
SEAWATER
SEDIMENTS
SEPARATION PROCESSES
SOLID SOLUTIONS
SOLUTIONS
STABILITY
STOICHIOMETRY
WATER