minerals Article A Novel Open-System Method for Synthesizing Muscovite from a Biotite-Rich Coal Tailing Hamid Khoshdast 1, *, Vahideh Shojaei 1 , Ahmad Hassanzadeh 2 , Tomasz Niedoba 3, * and Agnieszka Surowiak 3   Citation: Khoshdast, H.; Shojaei, V.; Hassanzadeh, A.; Niedoba, T.; Surowiak, A. A Novel Open-System Method for Synthesizing Muscovite from a Biotite-Rich Coal Tailing. Minerals 2021, 11, 269. https:// doi.org/10.3390/min11030269 Academic Editor: Stavros Kalaitzidis Received: 28 December 2020 Accepted: 2 March 2021 Published: 6 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Mining Engineering, Higher Education Complex of Zarand, 7761156391 Zarand, Iran; v.shojaei@uk.ac.ir 2 Independent Scholar, Am Apostelhof 7A, 50226 Frechen, Germany; a.hassanzadeh@gmx.de 3 Department of Environmental Engineering, Faculty of Mining and Geoengineering, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland; asur@agh.edu.pl * Correspondence: khoshdast@zarand.ac.ir (H.K.); tniedoba@agh.edu.pl (T.N.); Tel.: +48-126-172-056 (T.N.) Abstract: According to the wide application of muscovite in various industries, many studies have focused on its fabrication. However, the process of its synthesis faces long-standing challenges mainly related to the elevated temperature and pressure ambient, together with time and cost- consuming processes. This research work aims at synthesizing muscovite through a straightforward and direct wet thermal oxidation of an ash sample produced from biotite-rich coal tailings. For this purpose, the lab ash powder was mixed with 35% H 2 O 2 at the room temperature of 25 ◦ C while stirring at 480 rpm. Then, the temperature was gradually raised to 80 ◦ C, and the process ran for 180 min. The dried product and the raw lab ash were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) methods. The XRD results indicated that the biotite was efficiently converted to the muscovite as the number of relevant peaks was significantly increased in the synthesized product’s pattern. The SEM and FTIR results showed some structural changes, from pseudo-hexagonal in the starting material to amorphous pseudo-crystals in the synthetic product, as well as the growth of the product’s crystals. The crystallographic study and lattice parameter calculations revealed that the starting material and product peaks matched to International Center for Diffraction Data (ICDD reference patterns of 01-080-1110 and 01-082-2450 for the biotite and the muscovite, respectively. Moreover, the calculation of the mean crystallite size of the starting material and treated samples were obtained as 55 nm and 87 nm, respectively. Finally, according to the characterization properties of synthesized muscovite, the presented method was introduced as an effective technique. Therefore, we highly suggest it for further consideration and its development in future investigations. Keywords: synthetic muscovite; biotite; oxidation; coal tailing; crystal structure 1. Introduction Biotite and muscovite are two phyllosilicate minerals within the mica family that are commonly found in igneous and metamorphic rocks. Biotite has a small number of commercial uses, while muscovite has the greatest commercial value due to its special properties. Muscovite mineral is chemically inert, dielectric, elastic, flexible, hydrophilic, lightweight, reflective and refractive [1]. In addition, it is stable when exposed to electricity, light, moisture and extreme temperatures. Muscovite, either in the form of sheet or ground, is used in joint compound, paint, drilling mud, plastics, rubber, asphalt proofing and electronic devices [2]. The generalized chemical compositions for biotite and muscovite as a potassium- rich mica are K(Mg,Fe) 3 (AlSi 3 O 10 )(OH,F) 2 and KAl 2 (AlSi 3 O 10 )(OH) 2 , respectively [2]. Biotite is not very resistant to weathering and transforms into clay minerals. In contrast, muscovite is more resistant to weathering than biotite. In addition to weathering, the Minerals 2021, 11, 269. https://doi.org/10.3390/min11030269 https://www.mdpi.com/journal/minerals