Illite claystones (tonsteins) of Late permian coal-bearing formations are well developed and extensively distributed in southwestern China. Over the past decades it has been recognized that they are synsedimentary volcanic ash-falls in origin (altered tuff beds), based on the data derived from the comprehensive investigations on their stratigraphic and geographic distribution, petrographic types, chemical composition, accessory-mineral assemblage, as well as on their morphological characteristics. Recently, mineralogical and petrological identifications and X-ray diffraction analyses on these peculiar tonsteins further suggest that they can be classified mainly into three categories based on their clay-mineral constitution: kaolinitic, kaolinitic-illitic (transitional type), and illitic. As coal partings in most cases, these different types of tonsteins show a distinct zonal distribution on a regional scale. Their mineral constitution is profoundly modified by the combined effects of many factors during deuterogenic diagenesis, as indicated by the variation in volatile-component percentage (VM) of the adjacent coal seams (VM is calculated on a combustible-component basis). Those tonsteins intercalated in coal with a VM > 10% are dominated by kaolinite. But they contain increasing amount of illite at the expense of kaolinite when VM of coal decreases from 10% to 8%. When VM declines to less than 8% the clay minerals in these tonsteins are almost entirely illite with some sort of accompanying chloritization. It is clear that, from a regional point of view, the variation in clay-mineral constitution of these tonsteins is in response to the continual and progressive change in geological processes. In addition to the temperature and pressure conditions, the existence of a certain amount of alkali-metal ions and ferrous ions in solution is one of the controlling factors. © 1990.
