Abstract The conversion of trioctahedral smectite to chlorite has been examined using drill core samples from the Hokuroku Kuroko mineralization area of Japan, where silicic pyroclastic rocks have undergone intensive hydrothermal and diagenetic alterations. The percentage of expandable layers in trioctahedral chlorite/smectite (C/S) decreases discontinuously with depth, with steps at 100–80%, 50–40%, and 15– 0%; thus the C/S exhibits a trimodal frequency in expandability throughout a given drill hole. These three types of C/S, having expandabilities of about 80%, 50% (corrensite), and 15%, coexist over a depth range of about 200 m. This discontinuous change in expandability contrasts with the continuous change from 100 to 0% expandability found for illite/smectite. As smectite converts to chlorite, C/S increases in tetrahedral Al, octahedral Fe, and exchangeable Mg and Fe and decreases in tetrahedral Si and exchangeable Na and K.