The chemical compositions of chlorite-smectite mixed-layered minerals (C/S) from the Ohyu caldera (Inoue, 1985) are analyzed using M+-4Si-3R2+ diagrams. The assumed original saponite has the following composition: Si3.54Al0.46O10Al0.173Fe2+1.385Mg1.295Mn0.02(OH)2M+0.56. Random C/S minerals (100 to 80% expandable layers) are interpreted as an interstratification of the starting 2:1 smectite layer with a A1XR2+3−x interlayer. The 2:1 smectite layer charge remains constant but Ca, Na, K cations are replaced by a Al-R2+ complex ion. The brucitic layer (produced by the polymerization of the complex ions) and the 2:1 smectite layer form a 14 Å non-expandable phase having a composition different from a true chlorite.

The true chlorite layers first appear in the ordered (corrensite) phase composed of a high charge saponite: Si3.35A10.65O10R2+3(OH)2M+0.65 and an octahedral vacancy-free chlorite Si2.90A11.10O10Al1.10R2+4.90(OH)8. The recrystallization of the original trioctahedral smectite into a high-charge saponite decreases the b-dimension difference with the chlorite component.

From these data, it is suggested that the trioctahedral smectite-to-chlorite conversion is controlled by three reactions: 1. fixation and polymerization of Al-R2+ complex ions in the interlayer region of the original smectite producing a 14 Å non-expandable phase (the interlayering of this phase with the original smectite gives the randomly interstratified C/S mineral. 2. dissolution of these random mixed-layered minerals and precipitation of corrensite. 3. dissolution of corrensite and growth of Fe-rich chlorite.