Selenium Minerals: Structural and Chemical Diversity and Complexity

Chemical diversity of minerals containing selenium as an essential element has been analyzed in terms of the concept of mineral systems and the information-based structural and chemical complexity parameters. The study employs data for 123 Se mineral species approved by the International Mineralogical Association as of 25 May 2019. All known selenium minerals belong to seven mineral systems with the number of essential components ranging from one to seven. According to their chemical features, the minerals are subdivided into five groups: Native selenium, oxides, selenides, selenites, and selenates. Statistical analysis shows that there are strong and positive correlations between the chemical and structural complexities (measured as amounts of Shannon information per atom and per formula or unit cell) and the number of different chemical elements in a mineral. Analysis of relations between chemical and structural complexities provides strong evidence that there is an overall trend of increasing structural complexity with the increasing chemical complexity. The average structural complexity for Se minerals is equal to 2.4(1) bits per atom and 101(17) bits per unit cell. The chemical and structural complexities of O-free and O-bearing Se minerals are drastically different with the first group being simpler and the second group more complex. The O-free Se minerals (selenides and native Se) are primary minerals; their formation requires reducing conditions and is due to hydrothermal activity. The O-bearing Se minerals (oxides and oxysalts) form in near-surface environment, including oxidation zones of mineral deposits, evaporites and volcanic fumaroles. From the structural viewpoint, the five most complex Se minerals are marthozite, Cu(UO2)3(SeO3)2O2·8H2O (744.5 bits/cell); mandarinoite, Fe2(SeO3)3·6H2O (640.000 bits/cell); carlosruizite, K6Na4Na6Mg10(SeO4)12(IO3)12·12H2O (629.273 bits/cell); prewittite, KPb1.5ZnCu6O2(SeO3)2Cl10 (498.1 bits/cell); and nicksobolevite, Cu7(SeO3)2O2Cl6 (420.168 bits/cell). The mechanisms responsible for the high structural complexity of these minerals are high hydration states (marthozite and mandarinoite), high topological complexity (marthozite, mandarinoite, carlosruizite, nicksobolevite), high chemical complexity (prewittite and carlosruizite), and the presence of relatively large clusters of atoms (carlosruizite and nicksobolevite). In most cases, selenium itself does not play the crucial role in determining structural complexity (there are structural analogues or close species of marthozite, mandarinoite, and carlosruizite that do not contain Se), except for selenite chlorides, where stability of crystal structures is adjusted by the existence of attractive Se–Cl closed-shell interactions impossible for sulfates or phosphates. Most structurally complex Se minerals originate either from relatively low-temperature hydrothermal environments (as marthozite, mandarinoite, and carlosruizite) or from mild (500–700 °C) anhydrous gaseous environments of volcanic fumaroles (prewittite, nicksobolevite)

Raman spectroscopic characterization of the copper, cobalt, and nickel selenites: Synthetic analogs of chalcomenite, cobaltomenite, and ahlfeldite

<p>Raman spectroscopy has been used to study synthetic analogs of the minerals chalcomenite, cobaltomenite, and ahlfeldite occurring in nature. The results obtained are compared with the spectra of these minerals. In general, the majority of vibrational bands of synthetic species are in good agreement with natural chalcomenite, cobaltomenite, and ahlfeldite. The noticeable discrepancies are found for the bands assigned to the deformation mode of selenite groups. A better signal-to-noise ratio realized with synthetic species aids in comprehensive analysis of the spectra, especially in the region of water bands.</p

Mixed Uranyl Sulfate–Selenates: Evolution of Structural Topology and Complexity vs Chemical Composition

Phase formation in the aqueous system of uranyl nitrate, potassium hydroxide, and variable amounts of sulfuric and selenic acids has been investigated. Four different types of crystalline phases with variable S and Se contents were isolated and characterized using single-crystal X-ray diffraction (XRD) and IR spectroscopy. Topological analysis and information-based complexity calculations demonstrated the following: (a) the absence of a continuous solid solution in the system, (b) the absence of isotypic sulfate and selenate phases, and (c) the discovery of two layered topologies unprecedented among inorganic oxysalts

A Calorimetric and Thermodynamic Investigation of the Synthetic Analogue of Mandarinoite, Fe₂(SeO₃)₃·5H₂O

Thermophysical and thermochemical calorimetric investigations were carried out on the synthetic analogue of mandarinoite. The low-temperature heat capacity of Fe 2 ( SeO 3 ) 3 &#183; 5 H 2 O ( cr ) was measured using adiabatic calorimetry between 5.3 and 324.8 K, and the third-law entropy was determined. Using these C p , m o ( T ) data, the third law entropy at T = 298.15 K, S m o , is calculated as 520.1 &#177; 1.1 J∙K&#8722;1∙mol&#8722;1. Smoothed C p , m o ( T ) values between T &#8594; 0 K and 320 K are presented, along with values for S m o and the functions [ H m o ( T ) &#8722; H m o ( 0 ) ] and [ &#934; m o ( T ) &#8722; &#934; m o ( 0 ) ] . The enthalpy of formation of Fe 2 ( SeO 3 ) 3 &#183; 5 H 2 O ( cr ) was determined by solution calorimetry with HF solution as the solvent, giving &#916; f H m o ( 298 &nbsp; K , &nbsp; Fe 2 ( SeO 3 ) 3 &#183; 5 H 2 O , &nbsp; cr ) = &#8722;3124.6 &#177; 5.3 kJ/mol. The standard Gibbs energy of formation for Fe 2 ( SeO 3 ) 3 &#183; 5 H 2 O ( cr ) at T = 298 K can be calculated on the basis on &#916; f H m o ( 298 &nbsp; K ) and &#916; f S m o ( 298 &nbsp; K ) : &#916; f G m o ( 298 &nbsp; K , &nbsp; Fe 2 ( SeO 3 ) 3 &#183; 5 H 2 O , &nbsp; cr ) = &#8722;2600.8 &#177; 5.4 kJ/mol. The value of &#916;fGm for Fe2(SeO3)3&#183;5H2O(cr) was used to calculate the Eh&#8315;pH diagram of the Fe&#8315;Se&#8315;H2O system. This diagram has been constructed for the average contents of these elements in acidic waters of the oxidation zones of sulfide deposits. The behaviors of selenium and iron in the surface environment have been quantitatively explained by variations of the redox potential and the acidity-basicity of the mineral-forming medium. These parameters precisely determine the migration ability of selenium compounds and its precipitation in the form of solid phases

A calorimetric and thermodynamic investigation of uranyl molybdate UO2MoO4

A thermophysical and thermochemical calorimetric investigation of UO2MoO4 was undertaken. The UO2MoO4 was synthesized by solid-state reaction of MoO3 and UO3 and characterized by X-ray powder diffraction and X-ray fluorescence methods. The temperature of fusion, Tfus = (1212 ± 2) K, and the enthalpy of fusion, ΔfusH° (1212 K, UO2MoO4, cr.) = (16 ± 1) kJ · mol−1, were determined using DTA methods. The low-temperature heat capacity, (Т), behaviour of UO2MoO4 was measured using adiabatic calorimetry from T = (6 to 301) K and the third-law entropy determined. S° = (192.8 ± 0.8) J · mol−1 · K−1 at T = 298.15 K is obtained. The enthalpy of formation of UO2MoO4 was determined using HF-solution calorimetry giving ΔfH°(298 K, UO2MoO4, cr.) = −(1985 ± 8) kJ · mol−1. From these data, the Gibbs energy of formation ΔfG°(298 K, UO2MoO4, cr.) = −(1836 ± 8) kJ · mol−1 is calculated. Smoothed (Т) values between T = (0 and 300) K are presented along with values for S° and the functions [H°(T) − H°(0)] and [G°(T) − H°(0)]

Preiswerkite: A First Occurrence in Marble Hosting Gem Spinel Deposits, Luc Yen, Vietnam

We report a new occurrence of preiswerkite, the rare sodium analog of eastonite, the trioctahedral mica, from marble-hosted noble spinel deposits of the Luc Yen district, northern Vietnam. It is found in marble for the first time. The preiswerkite is anhedral and associated with phlogopite, aspidolite, sadanagaite, pargasite, spinel, corundum, dolomite and calcite. The average compositions of preiswerkite is (Na0.88Ca0.08K0.01)&Sigma;0.97(Mg2.29Al0.72Fe0.04)&Sigma;3.05)[(Al1.95Si2.05)&Sigma;4.00O10](OH)2. The compositions of preiswerkite have a narrow range of Mg# values (0.96&ndash;0.99) and define a preiswerkite-aspidolite solid-solution series. Compared with other occurrences, the Luc Yen preiswerkite has a low iron content, which attains 0.09 atoms per formula unit (1.53 wt.% FeO). The formation of preiswerkite is favored by the proportion of Mg, Al and Si in the precursor rocks and the increased activity of sodium and H2O in the fluid phase