Abstract
Six specimens of sericite minerals collected from wall-rock alteration zones of the Kuroko deposits of the Hanaoka, Kosaka and Shakanai mines, Akita Prefecture, have been examined by x-ray, chemical, infrared absorption and differential thermal analyses. The results reveal that one specimen is sericite, and the other five are interstratified minerals of sericite and montmorillonite with 5 to 40 percent montmorillonite layers. Polytypes identified in the specimens are 2M1, 1M and 1Md.
Chemically the specimens are characterized by high aluminium and low magnesium and iron contents, and have a dioctahedral structure. The slight differences in octahedral composition due to substitution of aluminium by magnesium and iron can be correlated with their polytypes: the 2M1 type has a composition close to that of muscovite, but the 1M type is phengitic. The d(060) spacings and the vibration frequencies of the IR bands at about 530 cm-1 also indicate these chemical variations. The differences in appearance of the endothermic peaks between 500° and 700°C of the DTA curves seem to be related to the octahedral composition.
