Igneous Intrusives into Coal-bearing Formations and Thermal Metamorphism of Coal in the Chikuho Coal Field

Part 3-Thermal Metamorphism and Metamorphic Temperature of the Coal

Abstract

The thermal metamorphism of coal seams by the intrusion of sills or dykes of "don" produced metamorphosed coals of several types such as cinder coal, anthracitic coal ("muen"), and semi-anthracitic coal ("hakuen").This thermal metamorphism did not always occur in the same manner, but instead is characterised by two types of metamorphism based on differences in the nature of the coal.Among these differences the coking power of the coal had the greatest influence on the mode of metamorphism.For this reason the writer has divided the thermal metamorphism of the coal into two types, one of which had stronger coking power and the other with weaker or no coking power.In both types, we can recognize several zones of thermal metamorphism;the mineralogical, petrological, and chemical properties of all are described in detail.
The degree of thermal metamorphism of coal may be best indicated by the values and variations of fuel ratios.It is surprizing, however, that the cinder coal at the contact of coal with the"don"has a fairly low fuel ratio, compared to that of the cinder coals of the outer zones of the contact. This apparent lower fuel ratio may be ascribed to the relatively abundant content of carbonates in the contact cinder coals.So the writer corrected the fuel ratios by recalculating the volatile components, that is by subtracting carbon dioxide in carbonates from the volatile components analysed.The amount of the carbonates have been obtained from the contents of MgO, CaO, and FeO of the analysed ashes.
The temperatures of the thermal metamorphism were estimated by comparison of the fuel ratios in natural coals with those of artificial cokes prepared at definite temperatures.The maximum temperature at the contact was determined to lie between 670 and 700°C.From the maximum temperature, the writer estimated the temperature of the final solidification of magma of the"don" using the equationss and tables of Jaeger; the result shows that it may lie between 700 and 800°C.