Chemical characteristics and origin of Ordovician K-bentonites along the Cincinnati Arch: a discussion

In a recent issue of this journal, Huff and Tiirkmenoglu (1981) concluded that montmorillonite layers alter to iUite layers in mixed-layer illite/smectite (I/S) having 18 to 27% smectite layers and having rectorite-type ordering. K-fixation was inferred to be controlled largely by substitution of octahedral Mg for Al, a conclusion of major significance because almost all similarly interlayered clay described in the literature (Weaver and Pollard, 1973, among many others) has mostly substitution of tetrahedral A1 for Si. Their calculation of structural formulae from which layer-charge distributions were inferred involves the assumptions that K is the only significant interlayer cation and that Na and Ca detected in chemical analyses of the clays can be ignored for two reasons: (1) the amounts are small and most likely do not represent structural site occupancy, and (2) Ca is a likely contaminant because the metabentonite beds occur in a massive carbonate section. They further justified these assumptions by the close agreement between calculated interlayer and total layer charges.