Mineralogical composition of the Ashibetsu coals in the Ishikari coalfield. 1. Silicate minerals.

Abstract

Silicate minerals in the seven Paleogene coal seams of the Ashibetsu colliery in the Ishikari coalfield, Hokkaido, were investigated by X-ray diffraction method for low temperature ash of coal, shale and tuff.
Quartz was detected from 86.5% of the coal samples with under 40% ash and from every shaly sample or tuff. Quartz is mostly detrital. However, the fact that it exists even in the extremely low ash coal like under 1% ash suggests that there may be vegetal quartz in origin.
Plagioclase is very abundant in Torakawa seam which has many intercalated tuffs. This mineral in coal is of tuff origin.
Kaolinite exists in 96% of the coal samples from extremely low ash to 40% ash coal, whereas clay minerals other than kaolinite are present in only 27% of the same samples. Although kaolinite is one of the major mineral components of shale and tuff, which have also the other major clay minerals, there are many coal samples which contain only kaolinite as clay mineral and ash content of most of them is under 10%. According to the result of measuring Hinckley's crystallinity index of kaolinite, kaolinites with high crystallinity are concentrated in these low ash coals and a few coal samples with over 10% ash also contain kaolinites with high crystallinity but there are no other clay minerals in them. Most kaolinites in the coals without the other clay minerals are thought to be authigenic.
Illite is present in the high ash coal and detrital from muddy sediments. The greater abundance of kaolinite and the less abundance of illite suggest the coal formations generally in the fresh water environment, though partly in the brackish water environment. Both the balance between the rate of accumulation of peat and the rate of subsidence of the coal basin, and the low flow rate of water must have also made illite less abundant within coals.
Illite/smectite mixed layer mineral is found in both tuff and shale. The mixed layer mineral found in the coals without illite in Torakawa seam is just like those of tuff. Another mixed layer is accompanied with illites in coal and shaly samples. The mixed layer mineral in tuffs, coals without illite and several shales was apparently identified by applying Watanabe's identification graph. The results show that their distribution on the graph forms a pattern of illitization by diagenesis and that the mixed layers of shales tend to take the area of higher illite ratio than those of tuffs and coals without illite. The latter fact seems to show that there appeared the variety of the effect on the diagenesis depending on the difference of the original sediment.
Both smectite and K-montmorillonite often occurred in the coals of Torakawa seam. As K-montmorillonite is also present in tuff, its source rock is simply thought to be tuff. However, smectite was not observed in tuff. Either Kmontmorillonite or illite/smectite mixed layer is thought to have been transformed into smectite in coal.
Trace amount of chlorite exists near the floors of two coal seams and in the two intercalated shale samples of one coal seam. Chlorite is unstable in the acidic environment like fresh water peat. Chlorire is rare in the Ashibetsu coal seams for that reason.