Vol CLII, No 4 (2023)

The aim of this study is the synthesis of two iron(III) selenites: Fe₂(SeO₃)₃·5H₂O (an analogue of the mineral mandarinoite) and Fe₂(SeO₃)₃·3H₂O, as well as the study of their solubility. Identification of obtained samples was carried out using the X-ray powder diffractometry and the energy dispersive electron microprobe analysis. Solubility was determined by isothermal saturation at 25 °C and atmospheric pressure. To suppress the hydrolysis of Fe³⁺-ions and prevent the precipitation of iron hydroxide, the experiment was performed in solutions of nitric or sulfuric acid. Obtained compositions of saturated solutions were used to calculate the solubility product using the Geochemist’s Workbench software package (GMB 9.0, SpecE8 program). As a result of the calculation, lgKsp[Fe₂(SeO₃)₃·5H₂O] = –38.6 ± ± 0.5 and lgKsp[Fe₂(SeO₃)₃·3H₂O] = –39.0 ± 0.2.

Hematite from fumaroles of the Tolbachik volcano (Kamchatka, Russia) was studied in detail. There is characterized epitaxy of rutile-group minerals (cassiterite, rutile, and tripuhyite), oxide spinels (magnesioferrite, gahnite, and cuprospinel), and corundum on hematite. Fumarolic hematite contains significant admixtures of chalcophile elements: tin (up to 9.2 wt % SnO₂, the highest content known for natural hematite), copper (up to 4.7 wt % CuO), and antimony (up to 2.6 wt % Sb₂O₅) – which is a novel for this mineral. Other significant admixtures are represented by Ti, Mg, Mn, Al, and Cr. There were revealed previously unknown Sn-Cu- and Sn-Cu-Sb-bearing varieties of hematite and new for this mineral isomorphic schemes: Sn⁴⁺ + Cu²⁺ → 2Fe³⁺ and Sb⁵⁺ + 2(Cu,Fe)²⁺ → 3Fe³⁺. It is shown that in oxidizing-type fumaroles of Tolbachik hematite is the major concentrator of Sn and Sb. Hematite enriched in such admixtures was formed in the temperature range of 500–850 °C. Volcanic gas was the source of chalcophile elements (Sn, Sb, Cu, Zn), whereas a source of hardly volatile components (Mg, Al, Cr, Ti) was basalt surrounding fumarole chambers; a mixed source is suggested for Fe, Mn and V. The chalcophile specificity of admixed components is an indicative feature of fumarolic hematite. The finding of martite in Tolbachik fumaroles implies that the oxidizing potential increases in time during the fumarolic mineral formation.

Paralavas of combustion metamorphic complexes in Mongolia were formed in high-temperature conditions from carbonate-terrigenous rocks of sedimentary formations due to to multistage spontaneous underground coal fires. Melilite-nepheline paralavas contain phenocrysts of Fe-rich åkermanite-alumoåkermanite, clinopyroxene of the diopside–hedenbergite series containing up to 49 mol. % of kushiorite end-member, and basic plagioclase. Enstatite-ferrosilite is the rock-forming mineral in paralavas of the plagioclase-pyroxene ± ± indialite composition. Paralavas often contain xenoliths of thermally altered sedimentary rocks. Mineral associations in remnant xenoliths of marly limestone are composed of gehlenite, minerals of the monticellite–kirschsteinite series, perovskite, Al-rich clinopyroxene, spinel, and other minerals. They were formed both at the stage of the high-temperature metamorphism of sedimentary protolite, preceding the melting of carbonate-silicate rocks, and as a result of the reactionary interaction between xenoliths and pyrogenic silicate melts of different composition.

This paper presents the results of mineralogical and crystal chemical studies of leightonite K₂Ca₂Cu(SO₄)₄·2H₂O from the Kamchatka Peninsula (Russia), including an investigation of its thermal behavior in a wide temperature range (–180 ≤ T ≤ 325 °C). The following methods were used: energy dispersive X-ray spectral analysis, single-crystal and powder X-ray diffraction, and in situ powder X-ray diffraction (at room, negative and high temperatures). The temperature range of the existence of the mineral are established, the main values of the thermal expansion tensor are calculated, and a structural interpretation of thermal expansion is given. The mineral crystallizes in the monoclinic system, space group C2/c, a = = 11.734(2), b = 7.596(1), c = 10.176(1) Å, β = 124.85(1), V = 744.4(2) ų, Z = 2. The crystal structure was refined to R = 0.065. The average empirical formula is K_2.31Ca_1.99Cu_0.88Al_0.04S_3.97O₁₆·2H₂O. Leightonite is stable up to ~325 °C and above this temperature it decomposes onto high-temperature calciolangbeinite-C K₂Ca₂(SO₄)₃ (P2₁3) and chalcocyanite CuSO₄ phases. Thermal expansion coefficients are: α₁₁ = 8.3(9), α₂₂ = 3.6(4), α₃₃ = –4.7(5) × 10^–6 °C^–1 at –180 °C и α₁₁ = 41.6(5), α₂₂ =36.5(5), α₃₃ = –10.6(1) × 10^–6 °C^–1 at 325 °C).

Micro-sized crystals formed during the thermal oxidation of shungite have been studied using scanning electron microscopy and Raman spectroscopy. According to Raman spectroscopy, there were identified bultfonteinite, allanite, and cornubite, which were not detected before in study of the initial shungite samples. Conditions of their formation suggest that the mechanism of these crystals growth is the desublimation.