佐賀県泉山陶石鉱床における熱水変質作用

石英とNH₄含有セリサイトの鉱物学的性質

抄録

Mineral parageneses and mineralogical properties of quartz and NH4-bearing sericite at the Izumiyama pottery stone deposit, Saga Prefecture, have been examined. The deposit was formed by hydrothermal alteration of a rhyolite intrusion. The deposit can be divided into four zones: 1) the sericite zone with sericite vein, 2) the sericite-kaolinite zone, 3) the weakly altered zone (I) and 4) the weakly altered zone (II), from the center to the outer of the alteration area. The typical Izumiyama ore of the sericite zone is composed mainly of quartz and sericite. The sericite-kaolinite zone consists of quartz, sericite and kaolinite. The weakly alterd zone (I) contains residual orthoclase. The weakly altered zone (II) contains residual sanidine and small amounts of mica/smectite with 20-50% smectite layers. Buddingtonite is also contained in some ores of the weakly altered zones.
X-ray diffraction peaks of the quartz are broad. The cell dimensions are considerably large with a=4.9147-4.9153Å and c=5.4054-5.4058Å at 18°C. The high-low inversion peak in the DTA curve is broad and the peak temperature is 10-14°C lower than that of standard quartz. The quartz sample in the center of the hydrothermal alteration has smaller cell dimensions and higher inversion temperature than the sample in the outer alteration zone.
Sericite in the inner two zones is mica with a little amount of smectite layer (0-5%). On the other hand, sericite in the weakly altered zone (I) is irregularly interstratified mica/smectite with 5-15% smectite layers, and the crystallite size is thinner than that in the inner zones. Basal spacing of sericite ranges from 10.063 to 10.174Å due to the extent of NH₄ incorporation in the interlayer site. Sericites having high content of interlayer NH₄ occur in the weakly altered zone (I), where buddingtonite is the main coexisting mineral. The distribution patterns of these NH₄-bearing minerals suggest that the enrichment of NH₄ in the Izumiyama pottery stone is a result of the interaction between the hydrothermal solution and sedimentary xenoliths in original rhyolite.