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High-Temperature Mineral Phases Generated in Natural Clinkers by Spontaneous Combustion of Coal

IUCA-Facultad de Ciencias, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
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Minerals 2019, 9(4), 213; https://doi.org/10.3390/min9040213
Received: 4 March 2019 / Revised: 26 March 2019 / Accepted: 29 March 2019 / Published: 1 April 2019
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Abstract

The aim of this study is to analyze natural clinkers (= calcined clays by coal combustion) from a lower Cretaceous coal outcrop in Ariño (Teruel, NE Spain) in order to describe mineral and textural transformations produced during the spontaneous combustion of coal. To achieve this aim, samples were analyzed using X-ray diffraction and optical and electron microscopy. Spontaneous combustion resulted in the melting of the surrounding clays, with the generation of an Al–Si-rich vitreous phase. Subsequently, high-temperature phases crystallized from this vitreous phase. These new minerals are interesting due to their similarity with those formed during ceramic processes, used in the manufacture of stoneware and ceramic tiles, as well as in refractory ceramics, and with natural events such as metamorphic and igneous processes. The studied natural clinkers are composed of vitreous phase mullite, hematite, hercynite, cristobalite, quartz, pyroxenes, cordierite, gypsum, pyrite, and calcium oxides. A trend from hematite to hercynite composition indicates compositional variations at sample scale, which evidence d-spacing differences in hercynite and may be related to the Al and Fe content in hercynite depending on its texture. The mullite shows higher Si/Al ratio (1.21) than the theoretical composition (0.35), indicating that this mullite is more Si-rich. Three pyroxene-type compositions (diopside-type, ferrosilite-type, and a Ca–Al-rich pyroxene) were found. Both the mullite and the pyroxenes are nonstoichiometric. View Full-Text
Keywords: natural clinkers; coal spontaneous combustion; electron microscopy; high-temperature mineral transformations natural clinkers; coal spontaneous combustion; electron microscopy; high-temperature mineral transformations
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Laita, E.; Bauluz, B.; Yuste, A. High-Temperature Mineral Phases Generated in Natural Clinkers by Spontaneous Combustion of Coal. Minerals 2019, 9, 213.

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