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Title: A CHEMICAL AND THERMODYNAMIC MODEL OF DIOCTAHEDRAL 2:1 LAYER CLAY MINERALS IN DIAGENETIC PROCESSES: DEHYDRATION OF DIOCTAHEDRAL ALUMINOUS SMECTITE AS A FUNCTION OF TEMPERATURE AND DEPTH IN SEDIMENTARY BASINS
Author(s): Barbara Ransom, Harold C. Helgeson
Annotation: The standard molal thermodynamic properties of dehydration of dioctahedral aluminous smectites have been computed for temperatures to 300°C and pressures to 5 kb. By combining values of the standard molal Gibbs free energies of the dehydration reaction at 25°C and 1 bar (Ransom and Helgeson, 1994a) with those of the standard molal entropy, heat capacity, and volume of dehydration generated from the thermodynamic properties of interlayer and bulk H2O (Ransom and Helgeson, 1994b; Johnson and Norton, 1991), the equilibrium hydration states of dioctahedral aluminous smectites can be predicted as a function of temperature and pressure in geologic systems. Calculations of this kind indicate that smectite gradually dehydrates with increasing temperature and burial depth. Combining the results of such calculations with equations that take into account the fact that smectites in nature contain more than one type of interlayer cation make it possible to estimate the amount of interlayer H2O released to pore spaces as smectites are buried along crustal geotherms. Calculations indicate that at 100°C and 3.5 km along US Gulf Coast geotherms, smectites contain ~ 15 to 65 percent less interlayer H2O than when they were deposited, depending on the interlayer cations present. Calculations also indicate that with increasing burial monova-lent cations are increasingly preferred over divalent cations in smectite interlayers, a trend verified by transmission electron microscope studies (Ahn and Peacor, 1986). Calculation of reaction properties indicate that both the volume and enthalpy of smectite dehydration are positive along crustal geotherms. For geologic systems for which the volume change is suppressed, a sudden pressure release such as failure along a fault could trigger rapid smectite dehydration resulting in an episode of rapid fluid generation accompanied by a short term depression of the local geothermal gradient.
Bibliographical description: A CHEMICAL AND THERMODYNAMIC MODEL OF DIOCTAHEDRAL 2:1 LAYER CLAY MINERALS IN DIAGENETIC PROCESSES: DEHYDRATION OF DIOCTAHEDRAL ALUMINOUS SMECTITE AS A FUNCTION OF TEMPERATURE AND DEPTH IN SEDIMENTARY BASINS / Barbara Ransom, Harold C. Helgeson - AMERICAN JOURNAL OF SCIENCE, VOL. 295, MARCH, 1995, P. 245-281
Publication's type: статья
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