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Dedolomitization in Na-Ca-Cl brines from 100/sup 0/ to 200/sup 0/C at 300 bars

Journal Article · · Geochim. Cosmochim. Acta; (United States)
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Dedolomitization experiments were run at 300 bars for 7 to 8 weeks at 100/sup 0/, 150/sup 0/, and 200/sup 0/C in chloride brines containing 2 m (molal) Na, 0.5 m Ca, and 0.02 m Sr. The ordered dolomite dissolved and low-Mg calcite back precipitated as a replacement and/or rim cement of equant spar calcite crystals 10 to 50 ..mu..m in diameter. Between 100/sup 0/ and 200/sup 0/C the mode of calcite formation changed from direct precipitation into void space produced by dissolving dolomite to pseudomorphic replacement of dolomite. The dedolomitization textures formed in these experiments are similar to those found in the rock record. Solution compositions were monitored as a function of experimental time and imply that calcite-dolomite equilibrium was approached at 100/sup 0/C and reached at 150/sup 0/ and 200/sup 0/C. The final aqueous molality ratios of Ca:Mg were 14.2, 23.5, and 35.4, respectively, at 100/sup 0/, 150/sup 0/, and 200/sup 0/C. The respective activity ratios, computed using Pitzer's model, were 14.3, 23.6, and 37.0, representing estimated constants for the dissolution of dolomite. The estimated standard state enthalpy of reaction is 3.6 kcal between 150/sup 0/ and 200/sup 0/C. The empirical Sr distribution coefficient for the precipitated calcite was determined from micro-probe analyses of the calcite and solution data. The average coefficient values increased with increasing temperature, being 0.027, 0.048, and 0.063, respectively, at 100/sup 0/, 150/sup 0/, and 200/sup 0/C.
Research Organization:
Univ. of New Orleans, LA (USA)
OSTI ID:
7188784
Journal Information:
Geochim. Cosmochim. Acta; (United States), Journal Name: Geochim. Cosmochim. Acta; (United States) Vol. 51:4; ISSN GCACA
Country of Publication:
United States
Language:
English

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Related Subjects

58 GEOSCIENCES
580400* -- Geochemistry-- (-1989)
ACTIVATION ENERGY
ALKALI METAL COMPOUNDS
ALKALINE EARTH METAL COMPOUNDS
BRINES
CALCITE
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CARBON COMPOUNDS
CARBONATE MINERALS
CARBONATES
CHEMICAL COMPOSITION
CHEMISTRY
CHLORIDES
CHLORINE COMPOUNDS
CRYSTALLIZATION
DISSOLUTION
DOLOMITE
ENERGY
ENTHALPY
EQUILIBRIUM
GEOCHEMISTRY
HALIDES
HALOGEN COMPOUNDS
MAGNESIUM CARBONATES
MAGNESIUM COMPOUNDS
MINERALIZATION
MINERALS
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PHYSICAL PROPERTIES
PRECIPITATION
SEPARATION PROCESSES
SODIUM COMPOUNDS
STRONTIUM COMPOUNDS
TEMPERATURE EFFECTS
THERMODYNAMIC PROPERTIES
THERMODYNAMICS