The isomorphism of S-bearing feldspathoids belonging to the cancrinite, sodalite, tugtupite, vladimirivanovite, bystrite, marinellite and scapolite structure types has been investigated using a multimethodical approach based on infrared, Raman and electron spin resonance (ESR), as well as ultraviolet, visible and near infrared (UV–Vis–near IR) absorption spectroscopy methods and involving chemical and X-ray diffraction data. Sapozhnikovite Na₈(Al₆Si₆O₂₄)(HS)₂ and sulfite and thiosulfate analogues of cancrinite are synthesized hydrothermally and characterized by means of electron microprobe analyses, powder X-ray diffraction and Raman spectroscopy. The possibility of the incorporation of significant amounts of SO₄²⁻, S₄ and SO₃²⁻ in the crystal structures of cancrisilite, sulfhydrylbystrite and marinellite, respectively, has been established for the first time. Thermal conversions of S-bearing groups in the synthetic sulfite cancrinite and sapozhnikovite analogues as well as natural vladinirivanovite and S₄-bearing haüyne under oxidizing and reducing conditions have been studied using the multimethodical approach. The SO₄²⁻ and S²⁻ anions and the S₃^•– radical anion are the most stable S-bearing species under high-temperature conditions (in the range of 700–800 °C); their ratio in the heated samples is determined by the redox conditions and charge-balance requirement. The HS⁻ and S₅²⁻ anions are stable only under highly reducing conditions.

This second of two companion articles devoted to silicate mineralization in fumaroles of the Tolbachik volcano (Kamchatka, Russia) reports data on chemistry, crystal chemistry and occurrence of tectosilicates: sanidine, anorthoclase, ferrisanidine, albite, anorthite, barium feldspar, leucite, nepheline, kalsilite, sodalite and hauyne. Chemical and genetic features of fumarolic silicates are also summarized and discussed. These minerals are typically enriched with “ore” elements (As, Cu, Zn, Sn, Mo, W). Significant admixture of As⁵⁺ (up to 36 wt % As₂O₅ in sanidine) substituting Si is the most characteristic. Hauyne contains up to 4.2 wt % MoO₃ and up to 1.7 wt % WO₃. All studied silicates are hydrogen-free, including mica and amphiboles which are F-rich. Iron-bearing minerals contain only Fe³⁺ due to strongly oxidizing formation conditions. In Tolbachik fumaroles, silicates were formed in the temperature range 500–800 ^∘C as a result of direct deposition from the gas phase (as volcanic sublimates) or gas–rock interactions. The zonation in distribution of major silicate minerals observed in a vertical section of the Arsenatnaya fumarole, from deep (the hottest) to upper parts is diopside + forsterite + enstatite + andradite → diopside → fluorophlogopite + diopside → sanidine + fluorophlogopite → sanidine. This is in agreement with volatilities of major species-defining metals in volcanic gases. From a crystal-chemical viewpoint, this series corresponds to the following sequence of crystallization of minerals with temperature decrease: nesosilicates → inosilicates → phyllosilicates → tectosilicates.

This paper presents the nomenclature of the rhodonite group accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA). An overview of the previous studies of triclinic (space group P $\bar{1}$ ) pyroxenoids belonging to the rhodonite structure type, with a focus on their crystal chemistry, is given. These minerals have the general structural formula ^VIIM(5)^VIM(1)^VIM(2)^VIM(3)^VIM(4)[Si₅O₁₅]. The following dominant cations at the M sites are known at present: M(5) = Ca or Mn²⁺, M(1–3) = Mn²⁺; and M(4) = Mn²⁺ or Fe²⁺. In accordance with the nomenclature, the rhodonite group consists of three IMA-approved mineral species having the following the general chemical formulae: ^M(5)A^M(1–3)B₃^M(4)C[Si₅O₁₅], where A = Ca or Mn²⁺; B = Mn²⁺; and C = Mn²⁺ or Fe²⁺. The end-member formulae of approved rhodonite-group minerals are as follows: rhodonite CaMn₃Mn[Si₅O₁₅]; ferrorhodonite CaMn₃Fe[Si₅O₁₅]; and vittinkiite MnMn₃Mn[Si₅O₁₅].

Natroaphthitalite K(Na,K)₂Na(SO₄)₂, a new aphthitalite-group mineral, was found in exhalations of the active Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka Peninsula, Russia. The associated minerals are anhydrite, langbeinite, hematite, tenorite, sanidine, johillerite, tilasite, and belomarinaite. Natroaphthitalite occurs as translucent yellow to colorless roundish single-crystal individuals up to 1 cm in diameter, sometimes combined in crusts up to 2 cm across, and as aqua-transparent colorless tabular to lamellar hexagonal crystals up to 2 mm across, sometimes skeletal, typically combined in parallel intergrowths or brush-like aggregates. D_calc is 2.664 g/cm³. Natroaphthitalite is optically uniaxial (+), ω = 1.488(2), ε = 1.490(2). The chemical composition (wt.%, electron microprobe) is: Na₂O 22.54, K₂O 26.39, SO₃ 51.78, total 100.71. The empirical formula calculated based on 8 O apfu is K_1.735Na_2.252S_2.002O₈ (average of 15 spot analyses). The crystal structures of natroaphthitalite and the associated aphthitalite-group mineral belomarinaite, ideally KNaSO₄, from the Arsenatnaya fumarole were solved using single-crystal X-ray diffraction and refined to R₁ = 0.0283 and 0.0261, respectively. The unit-cell parameters, Z, and space groups are a = 5.6014(3), c = 7.1507(5) Å, V = 194.30(2) ų, Z = 1, and for natroaphthitalite and a = 5.6088(3), c = 7.1858(4) Å, V = 195.77(1) ų, Z = 2, and P3m1 for belomarinaite. The crystal chemical formulae of natroaphthitalite and belomarinaite (Z = 1) are ^XK_1.00^Y(Na_1.28K_0.72)_Σ2.00^MNa_1.00(^TSO₄)₂ and ^XK_1.00^Y1(Na_0.96K_0.04)_Σ1.00^Y2(K_0.94Na_0.06)_Σ1.00^MNa_1.00(^TSO₄)₂, respectively. Natroaphthitalite is isostructural with aphthitalite K₃Na(SO₄)₂ = ^XK^YK₂^MNa(^TSO₄)₂ (both have centrosymmetric structures) with prevailing Na over K at the Y site of the crystal structure. Belomarinaite ^XK^Y2K^Y1Na^MNa(^TSO₄)₂ is the fully cation-ordered species, with acentric structure, in the series of natural aphthitalite-like K-Na sulfates.

Ferrisanidine, K[Fe³⁺Si₃O₈], the first natural feldspar with species-defining iron, is an analogue of sanidine bearing Fe³⁺ instead of Al. It was found in exhalations of the active Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Fissure Tolbachik Eruption, Tolbachik volcano, Kamchatka Peninsula, Russia. The associated minerals are aegirine, cassiterite, hematite, sylvite, halite, johillerite, arsmirandite, axelite, aphthitalite. Ferrisanidine forms porous crusts composed by cavernous short prismatic crystals or irregular grains up to 10 μm × 20 μm. Ferrisanidine is transparent, colorless to white, the lustre is vitreous. D_calc is 2.722 g·cm⁻³. The chemical composition of ferrisanidine (wt. %, electron microprobe) is: Na₂O 0.25, K₂O 15.15, Al₂O₃ 0.27, Fe₂O₃ 24.92, SiO₂ 60.50, in total 101.09. The empirical formula calculated based on 8 O apfu is (K_0.97Na_0.03)_Ʃ1.00(Si_3.03Fe³⁺_0.94Al_0.02)_Ʃ3.99O₈. The crystal structure of ferrisanidine was studied using the Rietveld method, the final R indices are: R_p = 0.0053, R_wp = 0.0075, R₁ = 0.0536. Parameters of the monoclinic unit cell are: a = 8.678(4), b = 13.144(8), c = 7.337(5) Å, β = 116.39(8)°, V = 749.6(9) ų. Space group is C2/m. The crystal structure of ferrisanidine is based on the sanidine-type “ferrisilicate” framework formed by disordered [SiO₄] and [Fe³⁺O₄] tetrahedra.