天草陶石の構成鉱物

抄録

The constituent minerals of the ores of the Amakusa pottery stone deposits have been examined by X-ray powder method, electron probe microanalysis, differential thermal analysis, polarizing microscopy, etc. The main constituent minerals of the pottery stones are quartz, sericites and kaolinite. Minor components of them are dickite, rectorite, tosudite, smectite, plagioclase, K-feldspar, calcite, siderite, pyrite, etc., among which the first three minerals occur only in the Sarayama vein. Feldspar, carbonate and pyrite are more abundant in the Kaigan and Murayama veins.
X-ray diffraction peaks of the quartz are broard. The cell dimensions are relatively large with α=4.9137-4.9154 Å and c=5.4047-5.4057 Å at 21°C. Most quartz samples show relatively high Al₂O₃ contents around 0.1wt% up to 1wt% in maximum, and are inhomogeneous in the Al₂O₃ content among grains. The high-low inversion peak in the DTA curve is broad and the peak temperature is lower than that of common quartz.
Sericites in the ores of the Sarayama vein comprises coarse-grained 2M₁ and 1M micas and finegrained irregularly interstratified 1M mica/smectite with small amounts of smectite layers. Sericites in the ores of the Kaigan and Murayama veins are mostly coarse-grained 2M₁ and 1M micas. These coarse-grained micas often form banded structure in which 2M₁ mica tends to grow inside. Tetrahedral and interlayer compositions of the 2M₁ mica are close to those of ideal muscovite although octahedral Fe and interlayer Na are appreciably contained. The 1M mica and fine-grained irregularly interstratified 1M mica/smectite have less tetrahedral Al and interlayer cations, and the contents of octahedral Fe and interlayer Na are very small. Such a difference in chemical composition between 2M₁ and 1M micas is most pronounced in those of the Sarayama ores. The observed variations of sericites in chemical composition, polytype and interstratification are discussed with their genesis.