Mineralogical and geochemical characteristics of pyrometamorphic rocks induced by coal fires in Junggar Basin, Xinjiang, China
Introduction
Coal fires are a global catastrophe, particularly in coal mining countries such as China, Russia, the United States, Indonesia, Australia, and South Africa (Song et al., 2015; van Dijk et al., 2011; Stracher, 2004, Stracher, 2007; Kuenzer et al., 2012). Large amounts of greenhouse gases (GHGs) and toxic gases (CO, Polycyclic aromatic hydrocarbons/PAHs, Hg, etc.) are emitted to the air (van Dijk et al., 2011; Kuenzer et al., 2012; Kuenzer and Stracher, 2012; Stracher and Taylor, 2004). In addition to environmental pollution, coal fires reshape geomorphology through small-scale surface fracturing (fissures, cracks, funnels, and vents), and large-scale surface subsidence (sinkholes, trenches, depressions, and slides) (Stracher and Taylor, 2004). Coal fires are triggered by natural phenomenon and by human activities (Stracher and Taylor, 2004; Song and Kuenzer, 2014; Zhang et al., 2015). With the increased scope of human activities and of the mining scale, the percentage of coal fires caused by the natural environment (lightning, forest fires, or strong solar heating) has decreased. Coal fire formation is an exothermic oxidation reaction process that occurs when coal is exposed to oxygen on the extraction face, coal storage piles, and coal waste piles and goafs (Kuenzer and Stracher, 2012); when temperatures reach 80–130 °C, the coal begins to burn (Song and Kuenzer, 2014).
Considerable amount of related literature has been published in many fields, including geological, geophysical, geochemical, environmental, numerical simulation, remote sensing, and fire-fighting (Song et al., 2015; van Dijk et al., 2011; Stracher and Taylor, 2004; Song and Kuenzer, 2014; Sun et al., 2012; Wang et al., 2003; Ribeiro et al., 2010), but only few papers about the changes of rock by coal fires are available. Combustion metamorphism is a common phenomenon in coal fires due to high temperatures (Bentor et al., 1981). Several authors (Ribeiro et al., 2010; Querol et al., 2008; Heffern and Coates, 2004) reported that coal burning temperatures reached up to 1300 °C. High temperatures induce changes on the mineral, textural, physical, and chemical characteristics of rocks in sediments with coal, gas, oil, or bitumen (Bentor et al., 1981; Ž'ček et al., 2005). New reactions (such as inversion of quartz to tridymite/cristobalite, and reaction rims with the non-crystalline matrix of silica grains) and rocks (clinker and paralava) are generated (Clark and Peacor, 1992; Kruszewski et al., 2014), which are an indicator of coal fires. Foit et al. (1987) reported the pyrometamorphic assemblages (sandstone, siltstone, and shale producing a multi-colored vesicular rock resembling slag) in near-surface combustion of the Healy coal seam near Buffalo, Wyoming. Querol et al., 1996, Querol et al., 1994 studied subbituminous coal, the inorganic matter, and its transformation by combustion experiments, and found some mineralogical and morphological characteristics of the atmospheric particulate matter may be used as tracers. Stracher (2005) investigated minerals (sulfates millosevichite, alunogen, coquimbite, voltaite, godovikovite, and an unidentified phase) and mineralization processes (condensation, hydrothermal alteration, crystallization from solution, fluctuating vent temperatures, boiling, and dehydration reaction) in coal-fire gas vents. Engle et al. (2012) detected the common minerals in soils and clastic sediments, including osumilite, cristobalite, hematite, quartz, calcite, and plagioclase. Gürdal et al. (2015) investigated the properties of coals contributing to spontaneous combustion and to the combustion by-products. They reported that the coal contained the pyrite, quartz, cristobalite, tridymite, kaolinite, and gypsum, but some geochemical properties of pyrometamorphic rocks were not addressed. Pone et al. (2007) focused on the coal-fire-gas minerals generated by the spontaneous combustion, noting that gas phases condensed to new minerals with decreasing temperature. The mineral paragenesis of the fired coal gangue (cristobalite, mullite, hematite, trydimite, cordierite) indicated a combustion temperature of 1200 °C (Ribeiro et al., 2010). Gatel et al. (2015) investigated the mineralogy and petrology of oil-shale slags in Lapanouse, France. They illustrated the mineralogical diversity in the slags, but the classification scheme is incomplete (e.g. without minerogenetic stages and complete combustion).
Various names have been applied to the heated rocks. Cosca et al. (1989) called unmelted rocks “burnt rocks” or “clinker”, in which rock color changes from its initial color to orange or red, and the solidified melt resembles igneous rocks called “paralava”. Liu (1959) claimed that protolith rocks should be considered in naming combustion metamorphic rocks, such as burnt-mudstone. Huang and Liu (2014) noted that pyrometamorphic rocks should be named as paralavas and clinkers. Zhang et al. (2016) classified them into baked rocks, baked–melted rocks, and melted rocks. Grapes et al., 2009, Grapes et al., 2010 and Grapes, 2010, Grapes, 2006 redefined the burnt, unmelted, reddish rocks as clinker, and the melted glassy rocks as paralava. In terms of detailed research of pyrometamorphic rocks, Clark and Peacor (1992) described the changes of pyrometamorphism and partial melting of shales during combustion metamorphism in terms of mineralogy and texture. Pyrometamorphic rocks were discussed terms of petrography and mineralogy with respected to the oxidation of a pyritic lignite seam in the Erin Formation, SW Trinidad (Baboolal et al., 2018). Ciesielczuk et al. (2015) conducted three types of heating experiments to reveal crystallization processes of the particular pyrometamorphic minerals for shales and carbonate rocks. Changes in mineralogy, textures, and glass composition were discussed in paralava and clinker in Shanxi Province, China, and lithologies and geochemistry also were used to characterize the paralava and clinker (Grapes et al., 2009). Surface thermal anomalies, pyrometamorphic rocks, and barren patches of land or soil with locally different emissivities are common in coal fire areas (Kuenzer and Stracher, 2012; Stracher, 2005). New techniques are being used to capture structural features such as rock and coal (Takagi et al., 2004; Okolo et al., 2015; Roberts et al., 2015; Everson et al., 2008; Song et al., 2017; Yu et al., 2018). Sokol et al., 1998, Sokol et al., 2007 studied the mineralogy of annealed and fused waste rocks via optical microscopy, X-ray diffraction (XRD), X-ray Fluorescence (XRF) and microprobe analyses, and some rare minerals were found (tridymite, crystobalite, mullite, K-bearing cordierite, K-Mg-osumilite, and Fe3+- and Al-rich Caclinopyroxene, as well as hexagonal and orthorhombic analogues of anorthite).
Major elements and rare earth elements (REEs) also are used to evaluate clinkers and paralavas by quantitative chemical analyses of the synthesized mixtures. Ciesielczuk et al. (2015) proved that mineralogies are similar to the rocks found in the burning coal-mine dumps. Concentrations of major, trace, and rare elements in the coal gangue and in the different lithologies were used to illustrate environmental characteristics of coal gangue dumps (Querol et al., 2008). The distribution and concentration of REEs are significant in geochemistry, which provide information about material sources, diagenetic environment, sedimentary environment, origin, and evolution (Zhu et al., 2014; Chen et al., 2003; Yang et al., 2017). Chondrites are considered the original chemical mixture from which the Earth was formed (Lunine, 2013). Taking such a mixture as a standard for construction can clearly reflect the sample change from the original composition of the earth (Taylor and McLennan, 1985). The smoothness of a normalized distribution pattern (to Upper Continental Crust, UCC; and North American Shale Composite, NASC) (Taylor and McLennan, 1985) provides a simple, but reliable, basis for testing the quality of REE chemical analyses of sedimentary rocks (Dai et al., 2016).
In this study, samples from eight clinkers and paralavas produced by coal combustion in Xinjiang, China, were selected for X-ray diffraction (XRD), optical microscopy, and X-ray Fluorescence (XRF). The study provides detailed information about pyrometamorphic rocks caused by coal fires, and contributes to the knowledge of coal fire by determining location and burning intensity.
Section snippets
Geological setting
Xinjiang is the largest provincial autonomous region in north-west China with an area of 1.66 million km2, accounting for one sixth in land area of China, located at 34°25′–48°10′ north and 73°40′–96°18′ east (Fig. 1). There are many coal fire areas in China, especially in northern China, extending ~5000 km in an E-W direction and ~750 km in an N-S direction (Grapes et al., 2009). The coal accumulation process in Xinjiang occurred from the Paleozoic Carboniferous to the Mesozoic Jurassic, among
Macroscopic characteristics of combustion metamorphic rocks
Combustion metamorphic rocks are evidently different from regional and magmatic thermal metamorphic rocks. Types of mineralization were divided into high-temperature (>800 °C), mid-temperature (~500–800 °C), and low-temperature (<500 °C) (Kruszewski et al., 2018). For the high-temperature stage, the new rocks (clinkers, buchites, parabasalts, and slags) are generated; for mid-temperature stage, gas condensation and gas-waste interaction are generated; and for the low-temperature, supergene
SiO2 polymorphs
Tridymite, cristobalite, and quartz are SiO2 polymorphs. SiO2 polymorphs have a low content in clinkers shown in Fig. 7a), however, high-SiO2 polymorphs are shown in paralava Fig. 7b). Higher contents of quartz and lower amounts of tridymite are present in clinkers than paralava. The quartz transforms to the cristobalite and tridymite in high-temperature processes (Matjie et al., 2012). Cosca et al. (1989) indicated that even minor (0.1–0.3 wt%) additional components (Al, Fe, and Ti) in the SiO2
Conclusions
In this paper, the products of coal fires (pyrometamorphic rocks) are discussed and classified, the main conclusions are as follows:
- (1)
Tridymite, cristobalite, and quartz (SiO2 polymorphs) were also determined in clinker and paralava, indicating that clinker formed in the temperature ranging from 767 to 1125 °C. Sekaninaite and cordierite are rich in paralava, which revealed that the pyrometamorphic rocks experienced a high temperature (>1000 °C). At temperatures >300 °C, pyrite can transform into
Declaration of competing interest
The authors declared that they have no conflicts of interest to this work.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Nos. 41672153 and 41430640) and Strategic Priority Research Program-Climate Change: Carbon Budget and Related Issues of the Chinese Academy of Sciences, Grant No. XDA05030201. Thanks are also due to candidate Dr. Xiaoyun Yan (China University of Mining & Technology, Beijing) and Congjun Huang (Chengdu University of Technology) their technical assistance.
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