Igor V. Pekov

In most cases, columbite-supergroup minerals are characterized by partial ordering of cations, which makes their identification based only on chemical composition and powder diffraction data difficult. Columbite-supergroup minerals with partially ordered cations were studied by means of electron microprobe analyses, powder and single-crystal X-ray diffraction, including crystal structure refinement in ixiolite-type and wolframine-type models. The samples originate from the Sakhanaiskiy granite massif, Eastern Siberia, Russia, (Sample 1) and from the Heftetjern pegmatite, Telemark, Norway (Sample 2). Their representative empirical formulae are (Fe²⁺_0.81Mn²⁺_0.53)_Σ1.34Fe³⁺_0.07(Ti_0.23Zr_0.11Sn_0.02)_Σ0.36(Nb_1.45Ta_0.26)_Σ1.71W_0.52O₈ (Sample 1) and (Mn²⁺_0.28Fe²⁺_0.25)_Σ0.53Sc_1.08(Sn_0.32Ti_0.04)_Σ0.36(Ta_1.41Nb_0.52)_Σ1.93W_0.10O₈ (Sample 2). Based on these data and the results of the crystal structure refinement, the studied samples can be considered as columbite-supergroup minerals in which cations are partially ordered in accordance with the wolframite mechanism. An approach is suggested according to which the degree of cation ordering in such columbite-supergroup minerals can be estimated based on the electron contents refined for different sites in a monoclinic model. According to this criterion, the degree of cation ordering of Samples 1 and 2 is 91% and 26%, respectively. Despite a significant degree of cation disordering, transition from the wolframite to ixiolite model results in a significant enhancement of the R-factor of the structure refinement (from 0.0365 to 0.0764 and from 0.0207 to 0.0610, respectively).

The new alluaudite-group mineral manganobadalovite (IMA 2020-035), ideally NaNaMn(MgFe³⁺)(AsO₄)₃, was found in the Arsenatnaya fumarole, the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption 1975–1976, Tolbachik volcano, Kamchatka peninsula, Far-Eastern Region, Russia. Manganobadalovite is a fumarolic mineral, and its aggregates are found overgrowing basalt scoria or exhalative hematite crystal crusts. Associated minerals are badalovite, hematite, cassiterite, sanidine, glauberite and metathénardite. Manganobadalovite occurs as prismatic to equant crystals up to 0.8 mm long typically combined in open-work clusters; it also forms grains that are irregular in shape and cavernous granular crusts up to 0.5 cm. The mineral is transparent, with vitreous luster, and its color varies from red to yellow. Manganobadalovite is brittle and has a noticeable cleavage in one direction and uneven fracture. The calculated density is 4.108 g cm⁻³. Manganobadalovite is optically biaxial (+), α = 1.790 (7), β = 1.800 (7), γ = 1.815 (8) and 2V_meas = 80 (5)°. Chemical composition (wt.%, electron-microprobe): Na₂O 8.75, K₂O 0.17, MgO 5.32, CaO 3.68, MnO 10.09, CuO 0.42, Al₂O₃ 0.18, Fe₂O₃ 13.90, V₂O₅ 0.42, As₂O₅ 56.75, total 99.68. The empirical formula calculated based on 12 O apfu is Na_1.69K_0.02Ca_0.39Mn_0.85Mg_0.79Cu_0.03Fe³⁺_1.04Al_0.02(As_2.96V_0.03)_∑2.99O₁₂. The crystal structure was solved using single-crystal XRD data, R = 2.30%. Manganobadalovite is monoclinic, C2/c, a = 12.1848(5), b = 12.8924(4), c = 6.6970(3) Å, β = 113.113(5)°, V = 967.60(7) ų and Z = 4. The strongest reflections of the powder XRD pattern are [d,Å(I)(hkl)]: 6.43(30)020, 3.589(32)(−131, 310), 3.215(38)(040, −112), 3.079(23)(221, 002), 2.941(32)(−312, −222, −331), 2.852(15)(041), 2.788(100)(330, 400, 240, 022), 2.649(22)(−402, 112), 2.626(25)(−132). Manganobadalovite is named as an analogue of badalovite NaNaMg(MgFe³⁺)(AsO₄)₃ with Mn²⁺ prevailing in the M(1) site.

The new mineral polyarsite, ideally Na₇CaMgCu₂(AsO₄)₄F₂Cl, was discovered in high-temperature incrustations of the active Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with aegirine, sanidine, ferrisanidine, hematite, halite, sylvite, cassiterite, evseevite, axelite, badalovite, johillerite, arsmirandite, aphthitalite, tridymite, potassic-magnesio-fluoro-arfvedsonite and litidionite. Polyarsite forms short-prismatic, equant or tabular crystals up to 0.15 mm across, their clusters up to 0.3 mm in size or crusts up to 0.5 mm across and up to 0.03 mm thick. Polyarsite is transparent, sky-blue to light blue, with vitreous lustre. It is brittle, no cleavage is observed and the fracture is uneven. D_calc. = 3.592 g cm⁻³. Polyarsite is optically biaxial (+), α = 1.624 (4), β = 1.645 (4), γ = 1.682 (4) (589 nm), 2V_meas. = 70 (10)°. The empirical chemical formula calculated based on 19 O+F+Cl apfu is Na_7.04Ca_1.00Mg_0.92Cu_2.06Fe³⁺_0.06(As_3.96S_0.05)_Σ4.01O_16.28F_1.66Cl_1.06. Polyarsite is monoclinic, space group I2/m, a = 8.4323(4), b = 10.0974(4), c = 10.7099(6) Å, β = 90.822(4)°, V = 911.79(8) ų and Z = 2. The crystal structure was determined based on SCXRD data, R = 0.0391. Polyarsite demonstrates a novel structure type. The structure is based on the (1 0 1) heteropolyhedral layers formed by Cu₂O₈Cl dimers built by CuO₄Cl tetragonal pyramids sharing common Cl vertex, AsO₄ tetrahedra and MgO₄F₂ octahedra. Adjacent layers are linked via CaO₈ cubes to form a pseudo-framework which hosts octahedrally coordinated Na cations. Polyarsite was named based on the Greek words πολύς, poly, “many” and due to belonging to arsenates: this arsenate contains many chemical components ordered between different positions in crystal structure.

Until 2021, experimental data on polysulfide groups in feldspathoids were limited to the properties of the S₃^•− chromophore center and the S₂^•− luminescence center in sodalite-group minerals. Interpretation of some spectroscopic data on other S-bearing groups in feldspathoids remained ambiguous because of significant differences between calculated data for isolated polysulfide species and experimental data for polysulfide groups in liquid sulfur, solutions, and matrix-isolated species published in different literature sources. For this reason, configurations of stable and metastable structures and parameters of the absorption spectra in the ultraviolet, visible, and near-infrared (UV-Vis-NIR) region and in the ESR and Raman spectra of various structure modifications of polysulfide S_n²⁻ anions, S_n^•− radical anions, and S_n neutral molecules (n = 2–6) as well as HS⁻ in the sodalite cage of sapozhnikovite have been calculated in frames of the density functional theory using the VASP and ORCA software packages. Taking into account the obtained results of theoretical calculations, spectroscopic properties of extra-framework polysulfide groups in natural tectosilicates belonging to the cancrinite and sodalite groups are discussed. The obtained results made it possible to confirm and partially clarify the interpretation of the experimental spectroscopic data for S-containing feldspathoids obtained by the authors of this work over the past five years.

A multimethodic approach based on infrared, Raman, electron spin resonance and photoluminescence spectroscopy, absorption spectroscopy in near infrared, visible and ultraviolet regions, single-crystal X-ray diffraction as well as electron microprobe analyses was applied to the characterization of a new commensurately modulated cubic haüyne analogue with the modulation parameter of 0.2 and unit-cell parameter of 45.3629(3) Å (designated as haüyne-45Å) from the Malobystrinskoe lazurite deposit, in the Baikal Lake area, Siberia, Russia, as well as associated SO₃²⁻-bearing afghanite. Haüyne-45Å is the second member, after vladimirivanovite, of the sodalite group with a commensurately modulated structure. The average structure is based on the tetrahedral aluminosilicate sodalite-type framework with sodalite cages of different sizes. The simplified formula of haüyne-45Å is Na₆Ca_2−x(Si₆Al₆O₂₄)(SO₄²⁻,HS⁻,S₂^●−,S₄,S₃^●−,S₅²⁻)_2−y. The structural modulations of the haüyne-45Å framework are presumably related to the regular alternation of SO₄²⁻ anions with polysulfide S₂^●−, S₃^●−, S₄, and S₅²⁻ groups detected by the spectroscopic methods. Mechanisms of thermal conversions of S-bearing groups in haüyne-45Å under oxidizing and reducing conditions at temperatures up to 800 °C are studied, and their geochemical importance is discussed.

The new mineral achyrophanite (K,Na)₃(Fe³⁺,Ti,Al,Mg)₅O₂(AsO₄)₅ was found in high-temperature sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with aphthitalite-group sulfates, hematite, alluaudite-group arsenates (badalovite, calciojohillerite, johillerite, nickenichite, hatertite, and khrenovite), ozerovaite, pansnerite, arsenatrotitanite, yurmarinite, svabite, tilasite, katiarsite, yurgensonite, As-bearing sanidine, anhydrite, rutile, cassiterite, and pseudobrookite. Achyrophanite occurs as long-prismatic to acicular or, rarer, tabular crystals up to 0.02 × 0.2 × 1.5 mm, which form parallel, radiating, bush-like, or chaotic aggregates up to 3 mm across. It is transparent, straw-yellow to golden yellow, with strong vitreous luster. The mineral is brittle, with (001) perfect cleavage. D_calc is 3.814 g cm^–3. Achyrophanite is optically biaxial (+), α = 1.823(7), β = 1.840(7), γ = 1.895(7) (589 nm), 2V (meas.) = 60(10)°. Chemical composition (wt.%, electron microprobe) is: Na₂O 3.68, K₂O 9.32, CaO 0.38, MgO 1.37, MnO 0.08, CuO 0.82, ZnO 0.48, Al₂O₃ 2.09, Fe₂O₃ 20.42, SiO₂ 0.12, TiO₂ 7.35, P₂O₅ 0.14, V₂O₅ 0.33, As₂O₅ 51.88, SO₃ 1.04, and total 99.40. The empirical formula calculated based on 22 O apfu is Na_1.29K_2.15Ca_0.07Mg_0.34Mn_0.01Cu_0.11Zn_0.06Al_0.44Fe³⁺_2.77Ti_1.00Si_0.02P_0.02S_0.14V_0.04As_4.90O₂₂. Achyrophanite is orthorhombic, space group P222₁, a = 6.5824(2), b = 13.2488(4), c = 10.7613(3) Å, V = 938.48(5) ų and Z = 2. The strongest reflections of the PXRD pattern [d,Å(I)(hkl)] are 5.615(59)(101), 4.174(42)(022), 3.669(31)(130), 3.148(33)(103), 2.852(43)(141), 2.814(100)(042, 202), 2.689(29)(004), and 2.237(28)(152). The crystal structure of achyrophanite (solved from single-crystal XRD data, R = 4.47%) is unique. It is based on the octahedral-tetrahedral M-T-O pseudo-framework (M = Fe³⁺ with admixed Ti, Al, Mg, Na; T = As⁵⁺). Large-cation A sites (A = K, Na) are located in the channels of the pseudo-framework. The achyrophanite structure can be described as stuffed, with the defect heteropolyhedral pseudo-framework derivative of the orthorhombic Fe³⁺AsO₄ archetype. The mineral is named from the Greek άχυρον, straw, and φαίνομαι, to appear, in allusion to its typical straw-yellow color and long prismatic habit of crystals.