Understanding the crystal structure of mixed-layer illite/smectite (I/S) is important to studies of petroleum migration1 and maturation 2, geopressure development3, sandstone cementation 4, and thermal histories of sedimentary5 and hydrothermally altered6 rocks. The crystallographic nature of I/S minerals is presently described by two competing theories, one based on Markov theory7 and the other based on transmission electron microscopy of thin crystallites, called 'fundamental particles'8. Here we present 29Si NMR data obtained using an inversion/recovery technique 9, which allows direct analysis of the Si/Al(4) ratios of individual illite and smectite components in I/S. The NMR results show that illite-rich I/S contains both high-charge and Al-rich (illite) and low-charge and Al-poor (smectite) structural environments. These results are best represented by the Markovian model of I/S. The NMR data are most consistent with the interpretation that the top and bottom tetrahedral sheets of both MacEwan crystallites and fundamental particles are smectitic. Fundamental particles are best interpreted as large crystallites of I/S that have been separated by osmotic swelling of Li+ and Na+ in smectite interlayers. Thermodynamic studies10 should consider illite-rich I/S as two phases, rather than as one phase, as suggested in previous investigations8,11-15.
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