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>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
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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
The standard molal thermodynamic properties of dehydration of dioctahedral aluminous smectites have been computed for temperatures to 300℃ and pressures to 5 kb. By combining values of the standard molal Gibbs free energies of the dehydration reaction at 25℃ 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 H_2O (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 H_2O released to pore spaces as smectites are buried along crustal geotherms. Calculations indicate that at 100℃ and 3.5 km along US Gulf Coast geotherms, smectites contain ～15 to 65 percent less interlayer H_2O than when they were deposited, depending on the interlayer cations present. Calculations also indicate mat with increasing burial monova- of rapid fluid generation accompanied by a short term depression of the local geothermal gradient.
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机译：计算了温度为300℃，压力为5 kb时，八面体铝蒙脱石脱水的标准摩尔热力学性质。通过将在25℃和1 bar下脱水反应的标准摩尔Gibbs自由能的值与标准摩尔熵，热容量和由中间层和热力学性质产生的脱水量的值相结合。大量的H_2O（Ransom and Helgeson，1994b; Johnson and Norton，1991），可以预测地质环境中二八面体铝蒙皂石的平衡水化状态。这种计算表明，蒙脱石随着温度和埋藏深度的增加而逐渐脱水。将这样的计算结果与考虑了自然蒙脱石包含一种以上类型的层间阳离子的事实的方程相结合，使得可以估计出当蒙脱石沿地壳地热埋藏时释放到孔隙中的层间H_2O的量。计算表明，在沿美国墨西哥湾沿岸地热的100℃和3.5 km处，蒙脱石的层间H_2O比沉积时少约15％至65％，这取决于存在的层间阳离子。计算还表明，随着流体的快速生成，埋藏物的单价增加，伴随着局部地热梯度的短期下降。
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