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1 Texaco Upstream Technology, 3901 Briarpark, Houston, Texas 77042, USA
2 Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA
E-mail of corresponding author: mccardk1{at}texaco.com
The three-dimensional crystal structures of illite-smectite (I-S) in K-bentonite samples from the Appalachian Basin are characterized by rotational disorder in the stacking sequence of 2:1 illite layers, different proportions of n60° rotations (as opposed to n120°) in the rotated layers, and layers with centrosymmetric trans-vacant (tv) octahedral sites that are randomly interstratified with noncentric cis-vacant (cv) layers. The proportion of cv interstratification in the I-S increases with tetrahedral Al and decreases with octahedral Mg and Fe content. The I-S minerals in the northern Appalachian basin K-bentonites are characterized by high (79% average) proportions of cv (Pcv) layers. In contrast, I-S from equivalent K-bentonites from the southern Appalachian basin has low Pcv values (38% average). These values do not correlate with expandability or rotational disorder. The geographic distribution of these I-S structural parameters may have resulted from possibly short-term, hot, and advective fluid migrations that differed in Mg concentrations and/or other physical and chemical parameters.
Key Words: Bentonite • Cis-Vacant Octahedra • Crystal Structure • Illite-Smectite • Polytype • Rotational Disorder • X-ray Diffraction
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