Polytypes and expandability of sericite from the Itaya kaolin clay deposit, Northeast Japan.

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Polytypes and expandability of sericite from the Itaya kaolin clay deposit were examined by X-ray powder diffraction method, with the hope of relating these structural features to the hydrothermal alteration processes which took place in two major periods, Late Miocene and Quaternary. 1M polytype is found to be predominant throughout the Miocene alteration area, while 2M1 polytype with rare occurrence of 2M2 polytype occurs mainly in the central parts of the deposit, where the hydrothermal argillization was most intense. Amount of 2M1 polytype tends to decrease outward, being accompanied by the increase of 1M polytype. lMd structure is found most frequently in sheared zones with intense pyrite dissemination (“black fault”), which are thought to be the latest stage product of the Miocene alteration. Expandability of the sericite is presented by “expandability index”, which is defined in this paper for the apparent contraction value (A) of the basal 10A reflection upon treatment with ethylene glycol. The sericite in the “black fault” and that in fragments of Miocene altered tuffs taken into the Quaternary silicified breccia dikes show much higher expandability than those in major Miocene altered rocks. The per cent expandable layers of each polytype estimated from the expandability index increases in the order of 2M1 (1-3%, in average), 1M (2-5%), 2M2 (5-9%) and 1Md (7-10%). A considerable difference of the expandability between two variations of 2M polytype is observed. The results indicate that the polytype occurrence was basically ruled by the intensity of hydrothermal argillization and that the expandability was mainly controlled by secondary degradation processes and polytype species. The enrichment of expandable layers and lMd structure in the sericite in the “black fault” may be ascribed to the hydrothermal fluid of the latest stage of the Miocene alteration which caused the pyrite dissemination. The Quaternary silicification may have been responsible for the increase of amounts of expandable layers in the sericite captured by the silicified dikes. However, no other degrading effect by the Quaternary alteration has been observed.