COMPOSITIONAL VARIATIONS IN SMECTITES .1. ALTERATION OF INTERMEDIATE VOLCANIC-ROCKS - A CASE-STUDY FROM MILOS-ISLAND, GREECE

The chemistry of smectites from some bentonite deposits derived from intermediate rocks has been examined by electron microprobe methods. A large variation in chemical composition within very short distances, principally controlled by a well-defined negative relationship between Si and Al, and between Al(VI) and Fe3+ and Al(VI) and Mg has been observed. On the other hand, Mg does not vary systematically with either Si or Fe3+. In several bentonites beidellite coexists with montmorillonite and there is a compositional transition between the two smectite minerals, implying the existence of a possible solid-solution series. This transition occurs only when Cheto-type montmorillonites are present, being absent for Wyoming-type montmorillonites. No compositional transition between Wyoming- and Cheto-type montmorillonite was observed. It is believed that the compositional variations reflect initial chemical gradients originated during the devitrification of the volcanic glass, due to the migration of chemical components.