The geochemistry and mineralogy of coal and coal-bearing strata from the Cannock Coalfield with special reference to chlorine
The project was conducted on four coals seams, the Shallow and Yard (Lower Coal Measures) from Lea Hall Colliery at Rugeley, and the Park and-Eight Feet seams from Littleton Colliery, near Cannock. Ultimate, proximate analyses and moisture contents showed them to be of high volatile bituminous 'B' coal-rank, and typical of high Cl coals. The Cl investigation showed a relationship with organic matter where ash is a dilutant, reaching c. 1% (by weight) in the coals, almost an order higher than in the associated mudrocks. It is related to the internal surface area and is thus highest in the vitrinite dominated 'bright rocks' and lowest in the 'dull coals'. Two types of Cl were identified, originating from saline ground waters, the former representing present ground water solutions trapped in the larger pores and readily water-soluble, and the latter held in organic combination within smaller or closed pores produced by Hercynian rank imposition. Varying levels of this Cl can be released by ion exchange with carbonates in leaching experiments. The mudrock and coal ash mineralogy was conducted on low temperature ashed material which suffered the side-effect of gypsum formation during oxidation. The mudrock mineralogy is dominated by detrital minerals, quartz and clays predominating. Diagenetic minerals rarely account for 7% of the normative minerals except in localised pyrite and siderite nodules., Climate played an important role in the detrital mineralogy, the lower seam floor measures being dominated by a tropically leached suite of kaolinite and quartz. Continental movement led to increased aridity characterised by illite and chlorite which dominate the higher seams. The intraseam dirt bands are composed of very fine clays, rapidly deposited over wide areas by minor base-level changes or river (ii) bank bursts. The roof measures show least evidence of leaching and are often highest in diagenetic siderite. Ba, Sr, Rb (illite) and Zr (zircon) are predominantly detrital trace elements whilst oxyhydroxide material was-the transporting media. for the diagenetically located elements, Ni, Pb (pyrite), Co and Mn (siderite). Cu is primarily associated with organic matter. The detrital coal mineralogy reflects the fine mudrock material and is usually highest associated with the dirt bands. Diagenetic minerals dominate the ash, reaching 95% (by weight), the major component being late diagenetic cleat. A paragenetic sequence of mineralisation follows a widespread trend from sulphides, silicates to carbonates reflecting changing ground water composition. The cleat fractures represent microjointing produced with stress release during uplift. Its frequence decreases with bed thickness, and the brittle nature of vitrinite causes it to have the earliest formed and most abundant cleat. The strength of multimaceral lithotypes such as durain is much greater and therefore fracture least, later änd with a greater dilation.