Abstract: 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 A1 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.