minerals Article High Temperature Sulfate Minerals Forming on the Burning Coal Dumps from Upper Silesia, Poland Jan Parafiniuk * and Rafał Siuda Faculty of Geology, University of Warsaw, Zwirki˙ i Wigury 93, 02-089 Warszawa, Poland; [email protected] * Correspondence: j.parafi[email protected] Abstract: The subject of this work is the assemblage of anhydrous sulfate minerals formed on burning coal-heaps. Three burning heaps located in the Upper Silesian coal basin in Czerwionka-Leszczyny, Radlin and Rydułtowy near Rybnik were selected for the research. The occurrence of godovikovite, millosevichite, steklite and an unnamed MgSO4, sometimes accompanied by subordinate admixtures of mikasaite, sabieite, efremovite, langbeinite and aphthitalite has been recorded from these locations. Occasionally they form monomineral aggregates, but usually occur as mixtures practically impossible to separate. The minerals form microcrystalline masses with a characteristic vesicular structure resembling a solidified foam or pumice. The sulfates crystallize from hot fire gases, similar to high temperature volcanic exhalations. The gases transport volatile components from the center of the fire but their chemical compositions are not yet known. Their cooling in the near-surface part of the heap results in condensation from the vapors as viscous liquid mass, from which the investigated minerals then crystallize. Their crystallization temperatures can be estimated from direct measurements of the temperatures of sulfate accumulation in the burning dumps and studies of their thermal ◦ decomposition. Millosevichite and steklite crystallize in the temperature range of 510–650 C, MgSO4 Citation: Parafiniuk, J.; Siuda, R. forms at 510–600 ◦C and godovikovite in the slightly lower range of 280–450 (546) ◦C. These values High Temperature Sulfate Minerals are higher than those previously reported. Forming on the Burning Coal Dumps from Upper Silesia, Poland. Minerals Keywords: burning coal-dumps; sulfate minerals; exhalations; godovikovite; steklite; millosevichite 2021, 11, 228. https://doi.org/ 10.3390/min11020228 Academic Editors: Tomasz M. 1. Introduction Stawski and Alexander E.S. Van Driessche Sulfate minerals are commonly associated with a supergene environment as products of processes taking place on the Earth’s surface under low temperature conditions. The Received: 27 January 2021 vast majority of the more than 300 known sulfate minerals are formed in two environments: Accepted: 16 February 2021 as weathering products of oxidized sulfides and other ore minerals, and due to evaporation Published: 23 February 2021 of sea water or salt lakes. Also well-known are also such sulfates as barite, less often celestine and anhydrite or gypsum crystallizing from hydrothermal solutions of rather low Publisher’s Note: MDPI stays neutral to medium temperatures. Numerous sulfate minerals are also found in volcanic exhalation with regard to jurisdictional claims in products, where they crystallize from volcanic vapors over a wide temperature interval published maps and institutional affil- from several hundred to below 100 ◦C. These lower temperature minerals, such as members iations. of the alunite group, halotrichite, alunogen, etc., are also known from the weathering zone of ores. The fires of exposed coal seams or coal mines and burning coal-dumps located next to collieries in coal basins in many places around the world are interesting environments for Copyright: © 2021 by the authors. the formation of sulfate minerals [1]. Fires initiating spontaneously in the interior of heaps Licensee MDPI, Basel, Switzerland. generate streams of hot gases from which several dozen sulfate phases crystallize on or This article is an open access article near the dump surface. They are generally similar to those of volcanic exhalation products. distributed under the terms and Sulfate phases forming on burning coal dumps have been described from Russia, Czech conditions of the Creative Commons Republic, Germany, Poland, USA and other countries ([1] and literature therein) but the Attribution (CC BY) license (https:// conditions for their crystallization are still insufficiently understood. Mineralogical studies creativecommons.org/licenses/by/ were not favored by the fact that until recently they were treated as anthropogenic phases. 4.0/). Minerals 2021, 11, 228. https://doi.org/10.3390/min11020228 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 16 Minerals 2021, 11, 228 2 of 15 therein) but the conditions for their crystallization are still insufficiently understood. Mineralogical studies were not favored by the fact that until recently they were treated as anthropogenic phases. Currently, following the Commission on New Minerals, Nomen- Currently, following the Commission on New Minerals, Nomenclature and Classification clature and Classification of the International Mineralogical Association (CNMNC IMA) of the International Mineralogical Association (CNMNC IMA) decision, the naturally decision, the naturally forming phases crystallizing on the burning coal dumps have ob- forming phases crystallizing on the burning coal dumps have obtained the status of real tained the status of real minerals, which should favor an intensification of research [2]. minerals, which should favor an intensification of research [2]. The limitation, however, The limitation, however, is the diminishing availability of these environments due to the is the diminishing availability of these environments due to the liquidation of heaps and theliquidation closure ofof coal heaps mines and in the many closure countries. of coal Due mines to theirin many solubility countries. in water, Due theto their discussed solu- sulfatebility in minerals water, the are discussed available forsulfate study minerals only on are heaps available with activefor study fire centersonly on inside heaps them with andactive they fire quickly centers disappear inside them after and the they fire quic is extinguished.kly disappear after the fire is extinguished. TheThe focusfocus ofof thisthis workwork areare anhydrousanhydrous sulfatessulfates thatthat areare thethe firstfirst toto crystallizecrystallize fromfrom high-temperaturehigh-temperature firefire gasesgases onon heapsheaps ofof severalseveral collieriescollieries inin thethe UpperUpper SilesianSilesian coalcoal basinbasin nearnear Rybnik,Rybnik, Poland. Poland. The The main main aim aim is tois tryto totry determine to determine the conditions the conditions of their of formationtheir for- basedmation on based their on mode their ofmode occurrence, of occurrence, chemical chemical composition composition and thermal and thermal behavior. behavior. The resultsThe results of these of these analyses analyses were supplementedwere suppleme withnted field with observations field observations and gas and temperature gas tem- measurements.perature measurements. 2.2. Materials and Methods TheThe mineralmineral samplessamples forfor thisthis studystudy werewere collectedcollected onon selectedselected burningburning coal-dumpscoal-dumps inin thethe UpperUpper SilesianSilesian coalcoal basin,basin, SouthSouth Poland.Poland. Nowadays,Nowadays, mineral-formingmineral-forming processesprocesses cancan bebe mostlymostly observedobserved onon thethe dumpsdumps inin thethe westernwestern partpart ofof thethe coalcoal basinbasin inin thethe RybnikRybnik region,region, thereforetherefore ourour researchresearch hashas beenbeen limitedlimited toto thisthis area.area. ThreeThree burningburning heapsheaps locatedlocated inin Czerwionka-Leszczyny,Czerwionka-Leszczyny, RadlinRadlin andand RydułtowyRydułtowy werewere selectedselected forfor mineralogicalmineralogical studystudy ((FigureFigure1 1).) . ItIt isis worthworth emphasizingemphasizing thatthat nono industrialindustrial slagsslags oror municipalmunicipal waste were de- positedposited inin the the heaps heaps studied. studied. They They contain contain only only rock rock material material extracted extracted from undergroundfrom under- mineground workings. mine workings. Observations Observations of the formation of the formation of exhalation of mineralsexhalation in minerals these heaps in havethese beenheaps carried have been out forcarried over out decade for over and decade have covered and have the covered periods the of theperiods highest of the activity highest of firesactivity in their of fires interior. in their Currently, interior. the Currently, fires of these the heapsfires of are these in a decliningheaps are phase. in a declining Archival samples,phase. Archival kept in samples, collections kept for in over collections a dozen for years over or a dozen freshly years collected, or freshly were collected, used for thewere analysis. used for the analysis. FigureFigure 1.1. Location of the studied burning coal-dumpscoal-dumps ofof UpperUpper SilesiaSilesia inin thethe RybnikRybnik region.region. PowderPowder X-rayX-ray diffraction diffraction (PXRD) (PXRD) and and scanning scanning electron electron microscope microscope (SEM) with(SEM) energy with dispersiveenergy dispersive spectroscopy spectroscopy (EDS) attachment (EDS) attachme were usednt were to identifyused to minerals. identify minerals. The presence The ofpresence all minerals of all was minerals confirmed was confirmed by PXRD using by PXRD a powder using X-ray a powder diffractometer X-ray diffractometer X’Pert PRO MPDX`Pert (Almelo, PRO MPD The (Almelo, Netherlands). The Netherlands). The parameters Th ofe parameters the X-ray beam of the were