Citation: Sachkov, V.I.; Nefedov, R.A.; Medvedev, R.O.; Amelichkin, I.V.; Sachkova, A.S.; Shcherbakov, P.S.; Solovyev, V.S.; Leonov, D.I.; Biryukov, D.A. Content and Forms of Radioactive Elements in Orthite (Allanite). Minerals 2023, 13, 366. https://doi.org/10.3390/ min13030366 Received: 23 November 2022 Revised: 16 February 2023 Accepted: 22 February 2023 Published: 6 March 2023 Copyright: © 2023 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/). minerals Article Content and Forms of Radioactive Elements in Orthite (Allanite) Victor Ivanovich Sachkov 1 , Roman Andreevich Nefedov 1 , Rodion Olegovych Medvedev 1 , Ivan Vyacheslavovich Amelichkin 1 , Anna Sergeevna Sachkova 2 , Pavel Sergeevich Shcherbakov 1, *, Vladislav Sergeevich Solovyev 1 , Daniil Igorevich Leonov 1 and Danil Aleksandrovich Biryukov 2 1 Laboratoryof Chemical Technology, Faculty of Chemistry, National Research Tomsk State University, 634050 Tomsk, Russia 2 School of Nuclear Science & Engineering, National Research Tomsk Polytechnic University, 634050 Tomsk, Russia * Correspondence: xcrbgc@gmail.com Abstract: The present work analyzes the specimens of orthite (allanite) mineral ores obtained from the Vernadsky mine (Slyudyanka River, Irkutsk region) and Yuzhno-Bogatyrskoye occurrence (Kuznetsk Alatau). The elemental chemical composition of the ore was determined by electronic microscopy of the specimen’s surface. The study establishes that uranium and thorium in orthite ores of the Yuzhno-Bogatyrskoye occurrence are mainly represented by their separate minerals, while the content of the orthite phase in the form of their isomorphic inclusion is small. Keywords: orthite; mineral; radioactive elements; uranium; thorium 1. Introduction Orthite (allanite) is a mineral of the epidote group or complex silicates of calcium, aluminum and iron. The common formula is A 2 M 2 Si 3 O 12 [OH], where A is a cation of Ca 2+ or Sr 2+ and rare-earth elements (REEs); M is Al 3+ , Fe 3+ , Mn 3+ , Fe 2+ , Mg 2+ or other [1]. Another formula of orthite or hydrosilicate of REEs can be (La, Ca) 2 (Al, Fe) 3 Si 3 O 12 (O, OH) [2]. Orthite in ores can also represent both disseminated crystals and veins of various widths, ranging from several millimeters to some meters. An orthite grain is black, having a submetallic luster, uneven surface and average roundness [3]. The maximum content of rare-earth elements (REEs) in allanite, as a rule, is 5% of rare-earth oxides; however, for some sources, it can vary from 3 to 51%. The content of thorium is also different: ranging from traces to up to 3% [4]. Uranium often is not detected as an inclusion. The reason for the radioactivity of REE minerals, in particular allanite, is the similarity of Th, U and REE behavior in endogenous processes [5]. The similar structure of the outer electron shells of U, Th, and REE is responsible for the common patterns of their behavior in natural processes and their lithophilicity. The proximity of the ionic radius of these elements causes their isomorphism in minerals. Thorium is closest to cerium, samarium, neodymium and gadolinium, with which it is associated in high-temperature deposits. Joint occurrence of thorium, uranium and rare-earth elements in endogenous processes is also predetermined by the fact that they have a common analogue among petrogenic elements, such as calcium, whereas calcium minerals are the main carriers of these elements. The incorporation of uranium and thorium in calcium minerals depends largely on the presence of rare-earth elements in them. As is known, all high-temperature rare-earth accessory minerals of pegmatites (monazite, orthite, titanium-tantal-niobates) are constantly enriched in thorium and uranium. As the temperature decreases in the postmagmatic stage, thorium, uranium and rare-earth elements are separated due to changes in the acid-alkali characteristic of the process. Minerals 2023, 13, 366. https://doi.org/10.3390/min13030366 https://www.mdpi.com/journal/minerals