Ammonium-bearing mica and mica/smectite of several pottery stone and pyrophyllite deposits in Japan: their mineralogical properties and utilization

Many pottery stone and pyrophyllite deposits have been developed in southwestern Japan. These deposits are hydrothermal in origin and have produced dioctahedral mica (sericite) minerals usually used for porcelain, tiles, sanitary ware, etc. Sericite specimens from two pottery stone deposits (Izumiyama and Tobe) and three pyrophyllite deposits (Yagi, Mitsuishi and Horo mines) are mineralogically characterized in terms of utilization. These sericite clays are chemically characterized by extensive substitution of NH4 for K corresponding to the muscovite–tobelite series, resulting in the basal spacing being enlarged from 10.00 Å (muscovite) to 10.36 Å (tobelite), which is diagnostic of the interlayer composition. Infrared absorbance of the NH4 band (1430 cm−1) standardized to the OH band (3620 cm−1) is also useful for determination of the NH4 content. DTA of a NH4-dominant specimen from the Tobe deposit reveals the liberation of NH4 around 490°C and double steps dehydroxylation in the 600–800°C range, as well as crystallization of mullite and cristobalite at 1000°C. These thermal properties are considered to be effective in sintering and refractoriness of the sericite materials when fired. Most of the sericite specimens are interstratified mica/smectite with <20% smectite, which may be responsible for plasticity when treated in the wet refinement process. Of the sericite clays from the five deposits, the Tobe, Yagi and Mitsuishi specimens are characterized by a varied interlayer NH4/K ratio, interstratification and lower Fe and Ti content, which favor refractoriness, plasticity and whiteness, respectively: these are excellent qualities for their use as raw materials for porcelain.