Abstract
Coalification temperatures are often considered to be approximately 100–170 °C for bituminous coal and 170–275 °C for anthracite. However, our micropetrographic observations, solid state 27Al magic-angle spinning nuclear magnetic resonance measurements, interpretation of δ13C values for whewellite in pelosiderite concretions from Carboniferous sediments, and assessment of whewellite thermal stability show that coalification temperatures can be significantly lower. Also the temperatures of coal alteration may be substantially lower than is stated. Ordinarily, high-temperature alteration is reported, but microthermometric measurements of fluids temperatures and micropetrographic observations show that the coal alteration can take place at low temperatures. For this reason, coals from the Kladno–Rakovník Basin, part of Late Paleozoic continental basins of the Czech Republic, were analyzed. Regarding coalification, micropetrographic characterizations of unaltered coals, the presence of thermally unstable Al complexes in the coal organic mass documented using 27Al MAS NMR method, and proven occurrence of whewellite in pelosiderite concretions suggest a lower coalification temperature, max. ~ 70 °C. Regarding coal alteration, micropetrographic observations revealed (a) the weaker intensity of fluorescence of liptinite, (b) mylonitic structures and microbreccia with carbonate fluid penetration, and (c) high oxygen content in coals (37–38 wt.%). These phenomena are typical for thermal and oxidative alteration of coal. As the temperature of carbonate fluids inferred from fluid inclusion analysis was evaluated as ~ 100–113 °C, the temperature of coal alteration was suggested as ~ 113 °C; the alteration was caused by hot hydrothermal fluids.
Highlights
1.1 Temperatures of coalification and coal alterationElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.The temperatures of coalification are frequently reported as approximately 100–170 °C for bituminous coal (e.g. Taylor et al 1998) and 170–250 °C or even 200–275 °C for anthracite (Taylor et al 1998; Daniels et al 1994)
Special attention is paid to Al complexes with organic ligands, which are an integral part of organic matter and coal organic mass (Bouska, 1981; Bouska et al 2000; Straka and Klika 2006; Straka 2016)
The d13C values and thermal stability of whewellite from Carboniferous sediments of the Kladno–Rakovnık Basin were taken into account; due to limited thermal stability of whewellite, its proven existence implies that coalification could occur at significantly lower temperatures than expected
Summary
Special attention is paid to Al complexes with organic ligands, which are an integral part of organic matter and coal organic mass (Bouska, 1981; Bouska et al 2000; Straka and Klika 2006; Straka 2016) These complexes (see Supplementary material, point 1, Figs I and II) are thermally unstable and decompose at a temperature of ca 90–95 °C (Straka 2016), their positive identification in coal organic mass using an analysis of the 27Al MAS NMR spectra indicates that the mentioned temperature could not be attained during coalification. And oxidatively altered coals have much higher oxygen at the expense of carbon and hydrogen in comparison with unaltered ones, and, further, the thermal and oxidative altered coals have a little difference in the random vitrinite reflectance compared with that of unaltered coal
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