Dissolution enthalpies of synthetic magnesian calcites: Comparison with biogenic phases

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:6000159

Bischoff, W D ^[1]; Wollast, R ^[2]

---

1. Wichita State Univ., KS (United States). Geology Dept.
2. Univ. Libre de Bruxelles (Belgium)

A series of synthetic magnesian calcites (0--15 mol% MgCO[sub 3]) were dissolved in weak acetic acid solutions to measure enthalpies of the reaction at 25 C. Heat released was 33.5 KJ/mol for calcite, decreasing to 33 KJ/mol for a Mg-Calcite of 2 mol% MgCO[sub 3], and increasing to 35 KJ/mol for a phase of 15 mol%. Excess enthalpies of formation Hf for these phases were calculated using calcite and magnesite as end-members. Values of Hf average about [minus]1 KJ/mol for the synthetic phases with a trend of increasingly negative values of Hf with Mg content. Excess entropies of formation Sf, corrected for excess entropies associated with ideal solid solutions, were calculated form available data on Gibbs free energies of formation and the values of Hf from this study. These values of Sf range from about [minus]2 J/mol-K for the phase with 2 mol% MgCO[sub 3] to [minus]5 J/mol-K at 15 mol%. The values of Sf suggest that some form of ordering (probably cation ordering) is obtained from these phases. In contrast, biogenic phases have positive values of Hf, increasing from about 1 KJ/mol for phases containing 5 mol% MgCO[sub 3] to 3 KJ/mol for those with 20 mol%. Values of Sf for the biogenic phases are positive, approximately 5 J/mol-K for a phase containing 15 mol% MgCO[sub 3]. For the biogenic phases, some form of disordering is suggested from the entropy calculations and may be the result of positional disordering is suggested from the entropy calculations and may be the result of positional disordering of the carbonate ion. The disorder in biogenic phases compared to the ordering in synthetic phases indicates significant structural differences that account for the higher solubilities of biogenic Mg-Calcites.

• OSTI ID: 6000159
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ACETIC ACID
ALKALINE EARTH METAL COMPOUNDS
CALCITE
CARBON COMPOUNDS
CARBONATE MINERALS
CARBONATES
CARBOXYLIC ACIDS
DISSOLUTION
ENERGY
ENTHALPY
ENTROPY
FORMATION FREE ENERGY
FORMATION HEAT
FREE ENERGY
MAGNESIUM CARBONATES
MAGNESIUM COMPOUNDS
MATERIALS
MICROSTRUCTURE
MINERALS
MONOCARBOXYLIC ACIDS
ORGANIC ACIDS
ORGANIC COMPOUNDS
OXYGEN COMPOUNDS
PHASE STUDIES
PHYSICAL PROPERTIES
REACTION HEAT
ROCKS
SOLUBILITY
SYNTHETIC MATERIALS
SYNTHETIC ROCKS
THERMODYNAMIC PROPERTIES