Formation and stability of endmember illite: I. Solution equilibration experiments at 100-250°C and P v,soln

Details Formation and stability of endmember illite: I. Solution equilibration experiments at 100-250°C and P v,soln Formation and stability of endmember illite: I. Solution equilibration experiments at 100-250°C and P v,soln

Jun 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.1873y&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 60, Issue 11, pp.1873-1883

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Scientific paper

A hydrothermal study was undertaken to determine the stability of endmember illite with respect to muscovite at 100-250°C. Experiments were conducted using Brazilian (BR) muscovite or San Juan (RM30a, SG4a) illite, kaolinite, or microcline and quartz or amorphous silica with an equal weight of 2 M KCl/HCl solution from 100-250°C and P v,soln to determine solution compositions in equilibrium with solids. Approach to equilibrium was verified by starting with low -low a H 4 SiO 4 , high -low a H 4 SiO 4 , and high -high a H 4 SiO 4 solutions. Post-experiment solutions were separated from solids and analyzed for pH, m K + , and m H 4 SiO 4 . Solids were analyzed by XRD in this study; TEM investigations of the solid products of complementary experiments will be reported in a subsequent paper. Experimental data were used to construct isothermal, isobaric log vs. log a H 4 SiO 4 diagrams at 100, 150, 200, and 250°C. These data define univariant lines which represent equilibrium between mica-like solubility-controlling phases and kaolinite, microcline, or diaspore. Slopes of univariant lines were used to infer the compositions of solubility-controlling phases: 0.28 ± 0.04, 0.51 ± 0.04, and 0.88 ± 0.04K/O 10 (OH) 2 . A 0.69K/O 10 (OH) 2 solubility-controlling phase may also exist but a solubility-controlling phase with a composition close to that of endmember muscovite [1.0K/O 10 (OH) 2 ] (Aja et al., 1991; Aja, 1991) was not observed. Solubility-controlling phases with compositions of 0.28, 0.51, and 0.69K/O 10 (OH) 2 appear to represent discrete, metastable steps encountered in the formation of the stable phase, endmember illite (0.88K/O 10 (OH) 2 ). Mechanisms proposed for the conversion of smectite to illite based on natural occurrences fail to account for discrete, illitic solubility-controlling phases. The discrepancy between natural and experimentally-based mechanisms for the conversion of smectite to illite may, in part, reflect the different methods used to characterize I/S in these studies. Bulk methods (e.g., XRD) provide a macroscopic view of the phases present in natural samples, whereas solubility experiments may be used to infer the presence of minute quantities of solubility-controlling phases that exist within bulk samples.

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