1991 年 41 巻 230 号 p. 351-366
Sericite is an important constituent of Roseki clay deposits, which are hydrothermally altered pyroclastics and lavas distributed regionally in the Chugoku province of western Japan. It is usually utilized as raw materials for refractories and ceramic wares as well as kaolin and pyrophyllite.
X-ray, chemical and thermal analyses have been made on sericite specimens from several Roseki clay deposits, which are classified into three types based on characteristic alteration minerals. Their X-ray properties differ considerably from deposit to deposit. Sericite in close association with andalusite and corundum in the so-called high temperature type deposit of the Uku mine (Yamaguchi Prefecture) is well crystallized, 2M 1 mica with a composition closer to muscovite. In typical Roseki deposits producing characteristic monomineralic sericite clay in the Yagi and Shinagawa-Mitsuishi mines (Mitsuishi area, Okayama Prefecture) and the Kayano deposit (Hiroshima Prefecture), however, it often occurs as interstratified mica/smectite with expandable layer less than 20%. Its polytype is determined to be predominantly 2M 2, though it is confirmed by two diagnostic reflections at 2.43Å and 2.09Å. The specimens in the Horo mine (Hiroshima Prefecture) are intermediate in crystallinity and polytype.
Substitution of NH4 for interlayer K in muscovite-tobelite series is very common and extensive in the specimens from the Yagi, Shinagawa-Mitsuishi and Horo mines, which can be correlated with enlargement of basal spacing from 10.0Å (muscovite) to 10.36Å (tobelite). Because of liberation of NH4 at 400°C, two steps of dehydroxylation in the range of 600-800°C and rapid transformation to mullite at 1000°C, these NH4-bearing sericites are similar to pyrophyllite rather than to muscovite (K-mica) in thermal behavior.
Sericite from typical Roseki clay deposits in the Mitsuishi area is used as excellent raw materials for ceramic wares: both adequate plasticity and high refractoriness are ascribed to interstratification with small amounts of smectite layer and interlayer NH4 composition, respectively.