minerals Review The Thermodynamics of Selenium Minerals in Near-Surface Environments Vladimir G. Krivovichev 1,*, Marina V. Charykova 2 ID and Andrey V. Vishnevsky 2 1 Department of Mineralogy, Institute of Earth Sciences, St. Petersburg State University, 7/9 University Embankment, Saint Petersburg 199034, Russia 2 Department of Geochemistry, Institute of Earth Sciences, St. Petersburg State University, 7/9 University Embankment, Saint Petersburg 199034, Russia; [email protected] (M.V.C.); [email protected] (A.V.V.) * Correspondence: [email protected]; Tel.: +7-812-328-9481 Received: 18 August 2017; Accepted: 4 October 2017; Published: 6 October 2017 Abstract: Selenium compounds are relatively rare as minerals; there are presently only 118 known mineral species. This work is intended to codify and systematize the data of mineral systems and the thermodynamics of selenium minerals, which are unstable (selenides) or formed in near-surface environments (selenites), where the behavior of selenium is controlled by variations of the redox potential and the acidity of solutions at low temperatures and pressures. These parameters determine the migration of selenium and its precipitation as various solid phases. All selenium minerals are divided into four groups—native selenium, oxide, selenides, and oxysalts—anhydrous selenites (I) and hydrous selenites and selenates (II). Within each of the groups, minerals are codified according to the minimum number of independent elements necessary to define the composition of the mineral system. Eh–pH diagrams were calculated and plotted using the Geochemist’s Workbench (GMB 9.0) software package. The Eh–pH diagrams of the Me–Se–H2O systems (where Me = Co, Ni, Fe, Cu, Pb, Zn, Cd, Hg, Ag, Bi, As, Sb, Al and Ca) were plotted for the average contents of these elements in acidic waters in the oxidation zones of sulfide deposits. The possibility of the formation of Zn, Cd, Ag and Hg selenites under natural oxidation conditions in near surface environments is discussed. Keywords: selenium minerals; mineral systems; chemical weathering; oxidation zones; physicochemical modeling; Eh–pH diagrams 1. Introduction Selenium release and pollution is a worldwide phenomenon that results from a wide variety of anthropogenic activities, such as agriculture, mining, and other process industries [1]. Selenium is a potentially toxic element, and mining-related selenium release was a major concern during the last decade as high concentrations were reported at some mine sites [2,3]. Selenium contamination is vast, affecting both aquatic and terrestrial ecosystems, and has therefore attracted the attention of natural resource and water quality regulators around the world [4,5]. Due to the high mobility of selenium in oxidizing geochemical environments, the behavior of selenium is also important in safety analyses of radioactive waste repositories [6,7]. Khamkhash et al. [1] reported a brief introduction to selenium chemistry and toxicity, presented a detailed review of currently available techniques for removing selenium from industrial/mining wastewater, and discussed mining-related selenium contamination from Alaskan mines. Se mobility, bioavailability and toxicity are controlled by the various possible oxidation states (the most common being −2, 0, +4 and +6) that prevail under various conditions. These characteristics of selenium are affected by the redox conditions and the pH, which also plays a crucial role in selenium behavior in near-surface environments. Minerals 2017, 7, 188; doi:10.3390/min7100188 www.mdpi.com/journal/minerals Minerals 2017, 7, 188 2 of 18 The oxidation zone of selenide or selenium-bearing sulfide deposits is one of the prime contributors of selenium release into the environment. The mining of coal, precious metals (gold and silver), and metallic sulfides are key contributors of selenium from mining operations [2] exposing selenium-bearing compounds to air and water. This mobilizes selenium into aquatic systems where it bioaccumulates in the food chain, thereby escalating its hazardous effects [1]. Minerals 2017, 7, 188 2 of 18 Previously, we have characterized the selenium oxysalts (selenites and selenates) and evaluated the accuracy of the thermodynamic constants of the selenites [8]. The objective of this paper is to characterizeThe oxidation all selenium zone minerals of selenide as natural or selenium-bearing mineral systems sulfide and depositsto review isthe one thermodynamic of the prime contributorsconstants of the of selenium selenium release minerals, into which the environment. are unstable The or formed mining ofduring coal, the precious oxidation metals of (goldselenides and silver),and selenium and metallic-bearing sulfides sulfide areores, key and contributors to propose ofdata selenium at the standard from mining state operations(25 °C, 1 bar [2 pressure).] exposing selenium-bearing compounds to air and water. This mobilizes selenium into aquatic systems where it bioaccumulates2. Materials and in Methods the food chain, thereby escalating its hazardous effects [1]. Previously, we have characterized the selenium oxysalts (selenites and selenates) and evaluated 2.1. Mineral Systems of Selenium Minerals the accuracy of the thermodynamic constants of the selenites [8]. The objective of this paper is to characterizeThe selenium all selenium minerals minerals are relatively as natural rare mineral in nature; systems only and118 tomi reviewneral species the thermodynamic are presently constantsknown (Supplementary of the selenium Table minerals, S1). As which shown are [9,10] unstable, any ormineral formed can during be assigned the oxidation to a specific of selenides system, andeach selenium-bearingcomponent of which sulfide is a species ores, and-defining to propose chemical data atelement the standard (cf. [11– state13]) (25determined◦C, 1 bar by pressure). the rules of the new mineral species definition [14–16]. For mineral coding, we used the sequence of species- 2.defining Materials chemical and Methods element symbols according to the so-called “thermochemical” sequence of 2.1.chemical Mineral elements Systems (Figure of Selenium 1) [9,10] Minerals. For example, giraudite, Cu6Cu4Zn2(AsSe3)4S, responds to the system SSeAsZnCu, while prewittite, K2Pb3Zn2Cu12(SeO3)4O4Cl20, responds to the system OClSePbZnCuK.The selenium minerals are relatively rare in nature; only 118 mineral species are presently knownAs (Supplementaryin all contemporary Table mineral S1). Asclassifications, shown [9,10 selenium], any mineral minerals can is beclearly assigned divided to ainto specific four system,groups— eachnative component selenium, of oxide, which selenides, is a species-defining and oxysalts chemical—anhydrous element selenites (cf. [11 –(I)13 ])and determined hydrous byselenites the rules and selenates of the new (II). mineral Within each species of these definition groups, [14 minerals–16]. For can mineral be classified coding, according we used to the sequenceminimum of number species-defining of componen chemicalts required element for symbolstheir formation according [9,10] to. the The so-called proposed “thermochemical” classification of sequenceselenium mineral of chemical systems elements is given (Figure in Table1)[ 9 ,1.10 ].An Forimportant example, advantage giraudite, of Cuthis6Cu classification4Zn2(AsSe3) 4ofS, respondsselenium mineral to the system systems SSeAsZnCu, is the ability while to use prewittite, computer Ktechnology2Pb3Zn2Cu to12 (SeOorganize,3)4O4 storeCl20, and responds retrieve to thethermodynamic system OClSePbZnCuK. data. Figure 1. 1.ThermochemicalThermochemical sequence sequenceof chemical ofelements chemical and corresponding elements and single corresponding-component single-componentsystems. systems. As in all contemporary mineral classifications, selenium minerals is clearly divided into four groups—native selenium, oxide, selenides, and oxysalts—anhydrous selenites (I) and hydrous selenites and selenates (II). Within each of these groups, minerals can be classified according to the minimum number of components required for their formation [9,10]. The proposed classification of selenium mineral systems is given in Table1. An important advantage of this classification of selenium mineral systems is the ability to use computer technology to organize, store and retrieve thermodynamic data. Minerals 2017, 7, 188 3 of 18 0 Table 1. Mineral systems and DfG 298 of selenium minerals. 0 n System Mineral Chemical Formula DfG 298, kJ/mol Native Elements 1 Se Selenium Se 0 Oxides 2 OSe Downeyite SeO2 −171.80 ± 0.62 Selenides SeAs Laphamite As2Se3 −83.9 ± 4.2 SeSb Antimonselite Sb2Se3 −127.4 ± 3.7 Nevskite BiSe −46.8 ± 5.7 Guanajuatite Bi Se −146.6 ± 10.1 SeBi 2 3 Paraguanajuatite Bi2Se3 – Laitakarite Bi4Se3 – SePb Clausthalite PbSe −97.9 ± 7.7 SeMo Drysdallite MoSe2 – Achávalite FeSe −70.1 ± 4.0 SeFe Dzharkenite (cub) FeSe2 – Ferroselite (orth) FeSe2 −101.3 ± 15.0 Bornhardtite CoCo2Se4 – SeCo Freboldite CoSe −56.3 ± 6.5 Trogtalite CoSe2 −100.4 ± 15.0 Sederholmite NiSe −69.8 ± 1.6 Mäkinenite NiSe – Penroseite NiSe −112.4 ± 7.0 2 SeNi 2 Kullerudite NiSe2 – Trüstedtite NiNi2Se4 – Wilkmanite Ni3Se4 – Verbeekite PdSe2 – SePd Oosterboschite Pd7Se5 – Palladseite Pd17Se15 – Sudovikovite PtSe – SePt 2 Luberoite Pt5Se4 – Klockmannite CuSe −36.8 ± 0.6 Krut’aite CuSe2 – Petˇríˇcekite CuSe2 – Bambollaite CuSe2 – SeCu Bellidoite Cu2Se – Berzelianite Cu2Se – Umangite Cu3Se2 – Athabascaite Cu5Se4 Geffroyite Cu9Se8 – SeAg Naumannite Ag2Se −46.9 ± 1.3 SeZn Stilleite ZnSe −172.5 ±