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Betzite, ideally Na6Ca2(Al6Si6O24)Cl4, a new cancrinite-group mineral, was discovered in a metasomatically altered (pyrometamorphosed) calcic xenolith, hosted by alkaline basalt at the Bellerberg paleovolcano in the Eastern Eifel region, Rhineland-Palatinate, Germany. The associated minerals are anorthite, phlogopite, diopside, grossular, fluorite, calcite, a tobermorite-like mineral, and vanadoallanite-(Ce). Betzite occurs as colorless hexagonal prismatic crystals up to 2 mm long and up to 0.5 mm thick. The new mineral is brittle, with a Mohs' hardness of 5½. Distinct cleavage on {100} and parting on {0001} are observed. The Dmeas = 2.38(2) g/cm3 and Dcalc = 2.363 g/cm3. Betzite is optically uniaxial (+) with ω = 1.528(2) and ε = 1.545(3). The IR spectrum is given. The chemical composition of betzite is (wt.%; electron microprobe, H2O determined by the modified Penfield method): Na2O 11.88, K2O 4.82, CaO 10.74, MgO 0.21, Al2O3 27.32, Fe2O3 0.68, SiO2 32.84, SO3 1.89, Cl 10.48, H2O 1.10, −O≡Cl −2.37, total 99.59. The empirical formula is Na4.22K1.13Ca2.11Mg0.06(Si6.01Al5.90Fe3+0.09O24)Cl3.25(SO4)0.26(H1.34O0.64). The crystal structure was determined using single-crystal X-ray diffraction data. It is hexagonal, space group P63, a = 12.8166(9) Å, c = 5.3562(3) Å, V = 761.95(12) Å3 (at a temperature of 100 K) and Z = 3. Betzite is a dimorph of quadridavyne, with a disordered distribution of extra-framework components occupying channels. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 11.14 (31) (100), 4.833 (93) (101), 3.715 (95) (300), 3.313 (100) (211), 2.787 (37) (400), 2.681 (56) (002, 131), 2.474 (35) (112, 401), 2.146 (24) (330). The mineral is named in honor of the German amateur mineralogist Volker Betz (b. 1947).

The new eudialyte-group mineral selsurtite, ideally (H3O)12Na3(Ca3Mn3)(Na2Fe)Zr3□Si[Si24O69(OH)3](OH)Cl⋅H2O, was discovered in metasomatic peralkaline rock from the Flora mountain, northern spur of the Selsurt mountain, Lovozero alkaline massif, Kola Peninsula, Russia. The associated minerals are aegirine, albite and orthoclase, as well as accessory lorenzenite, calciomurmanite, natrolite, lamprophyllite and sergevanite. Selsurtite occurs as brownish-red to reddish-orange, equant or flattened on (0001) crystals up to 2 mm across and elongate crystals up to 3 cm long. The main crystal forms are {0001}, {11$\bar{2}$0}, and {10$\bar{1}$1}. Selsurtite is brittle, with the Mohs’ hardness of 5. No cleavage is observed. Parting is distinct on (001). D(meas) = 2.73(2) and D(calc) = 2.722 g⋅cm–3. Selsurtite is optically uniaxial (–), with ω = 1.598(2) and ɛ = 1.595(2). The chemical composition is (wt.%, electron microprobe): Na2O 6.48, K2O 0.27, MgO 0.10, CaO 6.83, MnO 4.73, FeO 1.18, SrO 1.88, La2O3 0.57, Ce2O3 1.07, Pr2O3 0.20, Nd2O3 0.44, Al2O3 0.29, SiO2 50.81, ZrO2 13.50, HfO2 0.45, TiO2 0.61, Nb2О5 1.10, Cl 1.01, SO3 0.29, H2O 8.10, –O≡Cl –0.23, total 99.68. The empirical formula is H25.94Na6.03K0.16Mg0.07Ca3.51Sr0.52Ce0.19La0.10Nd0.08Pr0.03Mn1.91Fe0.47Ti0.22Zr3.16Hf0.06Nb0.24Si24.40Al0.16S0.10Cl0.82O79.13. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.0484. Selsurtite is trigonal, space group R3, with a = 14.1475(7) Å, c = 30.3609(12) Å, V = 5262.65(7) Å3 and Z = 3. Infrared and Raman spectra show that hydronium cations are involved in very strong hydrogen bonds and form Zundel- and Eigen-like complexes. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %)(hkl)] are: 11.38 (56)(101), 7.08 (59)(110), 5.69 (36)(202), 4.318 (72)(205), 3.793 (36)(303), 3.544 (72)(027, 220, 009), 2.970 (100)(315) and 2.844 (100)(404). The mineral is named after the discovery locality.

A new mineral paratobermorite with the ideal crystal-chemical formula Ca4(Al0.5Si0.5)2Si4O16 (OH)·2H2O·(Ca·3H2O) is a member of the tobermorite group within the tobermorite supergroup. It was found at the Bazhenovskoe chrysotile asbestos deposit, Central Urals, Russia, in cavities of grossular rodingite in association with prehnite, pectolite, thomsonite-Ca, and calcite. Paratobermorite occurs as well-shaped prismatic to acicular crystals up to 1 × 1.5 × 8 mm3 typically assembled in spray- or bush-like radial clusters or open-work aggregates up to 1.5 cm across, which form interrupted crusts up to 3 × 5 cm2. Paratobermorite is transparent, colorless, pale yellowish, pale beige, or pinkish, with a vitreous luster. The mineral is brittle, with the (001) perfect cleavage. The Mohs hardness is ca. 3½. Dmeas = 2.51 (2) and Dcalc = 2.533 g/cm3. Paratobermorite is optically biaxial (+), α = 1.565 (2), β = 1.566 (2), γ = 1.578 (2), 2 Vmeas = 25 (10)° and 2 Vcalc = 32° (589 nm). Optical orientation is: X = c, Y = b, Z = a. The chemical composition of paratobermorite (electron microprobe, H2O by selective sorption from gaseous products of heating) is Na2O 0.40, K2O 0.28, CaO 36.60, MnO 0.04, BaO 0.07, Al2O3 6.46, SiO2 42.32, H2O 14.10, total 100.27 wt%. The empirical formula calculated on the basis of 22 O atoms per formula unit and (O,OH)17·5H2O is Na0.09K0.04Ca4.72Al0.92Si5.09O15.69(OH)1.31·5H2O. Like other members of the tobermorite supergroup, paratobermorite displays OD character, with two MDO (maximum degree of order) structures: one (MDO1), with non-standard space group F2/d11 and the second (MDO2), just corresponding to the structure-type of the new mineral, with non-standard space group C1121/m; its unit-cell parameters obtained from single-crystal X-ray diffraction data are: a = 11.2220(4), b = 7.3777(2), c = 22.9425(8) Å, γ = 89.990(3)°, V = 1899.46(10) Å3, and Z = 4; polytype 2M. The structure of paratobermorite is solved on a single crystal, R = 8.36%. Like structures of other “tobermorites 11 Å,” it is based on the complex layer built of a sheet of sevenfold Ca-centered polyhedra with wollastonite-type chains of T tetrahedra attached to the Ca-sheet from both sides. The tetrahedral (T) sites T1 and T2 are fully occupied by Si, while alternating T3 and T4 sites are filled by Al and Si in the ratio 1:1. The chains of tetrahedra belonging to neighboring complex layers share common oxygen vertices of the bridging T3,4 tetrahedra to form xonotlite-type ribbons [Si6O17]∞. The heteropolyhedral Ca-T-O scaffolding appears as a microporous quasi-framework with wide channels, which contain additional Ca atoms and H2O molecules. The complex Ca-T-O layers in paratobermorite (so-called “complex modules of type A”) significantly differ in topology (mutual arrangement of T tetrahedra and Ca polyhedra) from the complex Ca-T-O layers in tobermorite (“complex modules of type B”). IR spectrum confirms the presence of nonequivalent H2O molecules and nonequivalent T-O-T angles involving T atoms of two neighboring wollastonite-type chains. Due to the original topological type of the structure and the presence of significant amount of Al, which substitutes Si, paratobermorite can be considered as a novel microporous material, a perspective cation-exchanger.

The new mineral axelite, ideally Na14Cu7(AsO4)8F2Cl2, was found in 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 sylvite, halite, arsmirandite, bradaczekite, johillerite, tilasite, ericlaxmanite, lammerite, hematite, tenorite, cassiterite, pseudobrookite, aphthitalite-group sulfates, anhydrite, fluoborite, sanidine and fluorophlogopite. Axelite occurs as tabular, quadratic, rectangular or stronger distorted crystals up to 0.02 × 0.1 × 0.1 mm, sometimes combined in interrupted crusts up to 0.4 mm across overgrowing sylvite. It is transparent, sky-blue, with vitreous lustre. Cleavage was not observed. Dcalc is 3.662 g cm–3. Axelite is optically uniaxial (–), ɛ = 1.650(4) and ω = 1.678(4). Chemical composition (wt.%, electron microprobe data) is: Na2O 22.54, K2O 0.08, CaO 0.04, MgO 0.05, CuO 26.69, P2O5 1.75, V2O5 0.15, As2O5 44.14, SO3 0.04, F 1.57, Cl 3.60, –O=(F,Cl) –1.47, total 99.18. The empirical formula based on O+F+Cl=36 apfu is Na14.37K0.03Ca0.01Mg0.02Cu6.63P0.49V0.03As7.59S0.01O32.36F1.63Cl2.01. Axelite is tetragonal, P4bm, a = 14.5957(2), c = 8.34370(18) Å, V = 1777.51(6) Å3 and Z = 2. The strongest reflections of the powder X-ray diffraction (XRD) pattern [d,Å(I)(hkl)] are: 8.32(44)(001), 5.156(47)(220), 4.168(21)(002), 3.246(34)(222), 3.180(61)(331), 2.747(100)(402), 2.709(36)(511) and 2.580(29)(440). The crystal structure, solved from single-crystal XRD data (R = 4.50%), is unique. It is based on the heteropolyhedral chains built by clusters formed by CuO4Cl square pyramids connected with AsO4 tetrahedra. Adjacent chains are connected via common vertices of AsO4 tetrahedra with CuO4Cl pyramids to form a heteropolyhedral pseudo-framework. Axelite is remotely related, in both structural and chemical aspects, to lavendulan-like minerals and synthetic compounds. The mineral is named in honour of the outstanding Finnish–Russian crystallographer, mineralogist and material scientist Axel Gadolin (1828–1892).

A new mineral kalithallite, K3Tl3+Cl6⋅2H2O, was found in an active fumarole belonging to the Northern fumarole field at the First scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Kalithallite is a product of the relatively low-temperature (70–150°C) interactions involving high-temperature sublimate minerals, volcanic gas and atmospheric water vapour. The associated minerals are cryobostryxite, KZnCl3⋅2H2O, halite, sylvite, opal and gypsum. Kalithallite forms lamellar to tabular crystals up to 5 × 30 × 40 μm combined in open-work aggregates up to 1 mm across. It is transparent, colourless in individuals and white to pale cream coloured or pale beige in aggregates, with vitreous lustre. Dcalc = 3.01 g cm–3. Kalithallite is optically uniaxial (–), ɛ = 1.656(3) and ω = 1.662(3). The chemical composition (wt.%, electron-microprobe data, H2O calculated by stoichiometry) is: K 17.72, Zn 0.85, Tl 38.76, Cl 35.91, H2Ocalc 5.99, total 99.23. The empirical formula calculated on the basis of K+Zn+Tl+Cl = 10 apfu is K2.72Zn0.06Tl1.14Cl6.08⋅2H2O. Kalithallite is tetragonal, I4/mmm, a = 15.9333(5), c = 18.1088(7) Å, V = 4595.2(4) Å3 and Z = 14. The strongest reflections of the powder X-ray diffraction (XRD) pattern [d,Å(I)(hkl)] are: 5.98(100)(202); 5.64(36)(220); 3.984(20)(400); 3.528(30)(224); 3.315(22)(422); 2.890(15)(334); and 2.817(24)(206, 440). Kalithallite is isotypical to synthetic K3Tl3+Cl6⋅2H2O. The crystal structure was refined from the powder XRD data using the Rietveld method, RBragg = 0.55%, Rp = 0.56%, and Rwp = 0.75%. The structure contains Tl3+Cl6 octahedra and K-centred polyhedra of three types: KCl8, KCl8(H2O) and KCl7(H2O)2. The mineral is named as a kalium–thallium ordered compound.

This work is devoted to an investigation of elemental composition, crystal structure and thermal expansion of natural oxoborate azoproite from the Tazheran massif (Siberia, Russia) in the temperature range 5–1650 K. Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX). Its empirical formula based on five oxygen atoms is (Mg1.81Fe2+ 0.19)∑2.00(Fe3+ 0.36Ti0.26Mg0.26Al0.12)∑1.00O2(BO3). Local environment, oxidation states and ratio of Fe atoms are determined using Mössbauer spectroscopy and compared with EDX and single-crystal X-ray diffraction (SCXRD) data. A refinement of the crystal structure from SCXRD data collected at 293 K was provided for the first time. The structure could be described both in terms of cation- and anion-centered polyhedra. It is composed of vertex- and edge-sharing metal–oxygen [MO6] n − octahedra that form extended zigzag chains along the a axis building up a framework with the [BO3]3− triangles located in its distorted trigonal channels. From the other point of view, there are double chains consisting of oxocentred [OM 4] n + tetrahedra and [OM 5] n + tetragonal pyramids forming six-membered rings with the triangles in its cavities. Four non-equivalent Mn + sites are occupied by cations as follows: M(1) (2a) and M(2) (2d) – Mg, M(3) (4g) – Mg and Fe2+, M(4) (4h) – Fe3+, Ti4+, Mg and Al3+. According to differential scanning calorimetry, low- and high-temperature powder X-ray diffraction (LT- and HT-XRD) data, Mössbauer spectroscopy and magnetometry data (5 ≤ T ≤ 1650 K), there are no phase transitions obtained in the temperature range investigated. However, some anomalies in temperature dependencies of unit-cell parameters caused by a partial Fe2+ → Fe3+ oxidation are found in the range 873–1173 K. Azoproite melts at a temperature higher than 1600 K. Eigenvalues of the thermal expansion tensor are calculated for the oxoborate and thermal expansion is described in comparison with its crystal structure.

The new apatite-group mineral pliniusite, ideally Ca5(VO4)3F, was found in fumarole deposits at the Tolbachik volcano, Kamchatka, Russia, and in a pyrometamorphic rock of the Hatrurim Complex, Israel. Pliniusite, together with fluorapatite and svabite, forms a novel and almost continuous ternary solid-solution system characterized by wide variations of T5+ = P, As, and V. In paleo-fumarolic deposits at Mountain 1004 (Tolbachik), members of this system, including the holotype pliniusite, are associated with hematite, tenorite, diopside, andradite, kainotropite, baryte and supergene volborthite, brochantite, gypsum and opal. In sublimates of the active Arsenatnaya fumarole (Tolbachik), pliniusite–svabite–fluorapatite minerals coexist with anhydrite, diopside, hematite, berzeliite, schäferite, calciojohillerite, forsterite, enstatite, magnesioferrite, ludwigite, rhabdoborite-group fluoroborates, powellite, baryte, udinaite, arsenudinaite, paraberzeliite, and spinel. At Nahal Morag, Negev Desert, Israel, the pliniusite cotype and V-bearing fluorapatite occur in schorlomite-gehlenite paralava with rankinite, walstromite, zadovite-aradite series minerals, magnesioferrite, hematite, khesinite, barioferrite, perovskite, gurimite, baryte, tenorite, delafos-site, wollastonite, and cuspidine. Pliniusite forms hexagonal prismatic crystals up to 0.3 × 0.1 mm and open-work aggregates up to 2 mm across (Mountain 1004) or grains up to 0.02 mm (Nahal Morag and Arsenatnaya fumarole). Pliniusite is transparent to semitransparent, colorless or whitish, with a vitreous luster. The calculated density is 3.402 g/cm−3. Pliniusite is optically uniaxial (–), ω = 1.763(5), ε = 1.738(5). The empirical formulas of pliniusite type specimens calculated based on 13 anions (O+F+Cl) per formula unit are (Ca4.87Na0.06Sr0.03Fe0.02)Σ4.98(V1.69As0.66P0.45S0.12Si0.09)Σ3.01O11.97F1.03 (Mountain 1004) and (Ca4.81Sr0.12Ba0.08Na0.05)Σ5.06(V2.64P0.27S0.07Si0.03)Σ3.01O12.15F0.51Cl0.34 (Nahal Morag). Pliniusite has a hexagonal structure with space group P63/m, a = b = 9.5777(7), c = 6.9659(5) Å, V = 553.39(7) Å3, and Z = 2. The structure was solved using single-crystal (holotype) X-ray diffraction, R = 0.0254. The mineral was named in honor of the famous Roman naturalist Pliny the Elder, born Gaius Plinius Secundus (AD 23–79). It is suggested that the combination of high temperature, low pressure, and high oxygen fugacity favors the incorporation of V5+ into calcium apatite-type compounds, leading to the formation of fluorovanadates.

Kaznakhtite, ideally Ni6Co3+2(CO3)(OH)16⋅4H2O, is a new member of the hydrotalcite group within the hydrotalcite supergroup. The mineral was discovered at the Kaznakhtinskiy ultrabasic massif, Altai Republic, SW Siberia, Russia. It occurs as powdery aggregates forming flattened lenses up to 1.5 × 0.5 cm and veinlets up to 1 cm long and up to 1 mm thick in aggregates of chrysotile, lizardite, stichtite and dolomite. Other associated minerals include brucite, chromite, heazlewoodite, manganochromite, magnetite and magnesioferrite. Kaznakhtite aggregates are composed of tiny platy grains up to 0.01 mm across. Kaznakhtite is light green and translucent in aggregates. It has an earthy lustre and white streak. Cleavage is micaceous on {001}. Dcalc = 2.864 g cm–3. The mineral is optically uniaxial (–) with ɛ = 1.657(3) and ω = 1.676(3), and weakly pleochroic in greenish hues, ω > ɛ. Chemical composition (wt.%, electron microprobe, Co valence state determined by XANES spectroscopy, CO2 and H2O calculated by stoichiometry) is: MgO 2.15, NiO 47.40, ZnO 0.22, Al2O3 0.16, Cr2O3 0.98, Co2O3 17.42, Cl 0.63, CO2 5.05, H2O 24.60, –O=Cl –0.14, total 98.47. The empirical formula calculated based on the sum of all metal cations = 8 apfu is (Ni5.54Mg0.47Zn0.02)Σ6.03(Co3+1.83Cr0.11Al0.03)Σ1.97C1.00O2.99(OH)15.84Cl0.16⋅4H2O. Infrared spectroscopy confirmed the presence of CO32– anions, OH– groups and H2O molecules. The crystal structure was refined by the Rietveld method with RB = 0.19%. Kaznakhtite is trigonal, space group R͞3m, a = 3.0515 (3), c = 23.180 (3) Å, V = 186.93 (4) Å3 and Z = 3/8. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.72 (100) (003); 3.863 (24) (006); 2.630 (4) (101); 2.576 (10) (012); 2.294 (6) (015); 1.950 (4) (018); 1.526 (4) (110); and 1.497 (4) (113). Kaznakhtite is a Co3+ analogue of reevesite, Ni6Fe3+2(CO3)(OH)16⋅4H2O. The mineral is named after its type locality.

Reznitskyite, ideally CaMg(VO4)F, is a new mineral species of the tilasite group from the Arsenatnaya fumarole, Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It occurs as zones (usually up to 0.05 mm thick) in crystals of V- and P-containing tilasite or as homogeneous grains up to 0.1 mm across. It was found in polymineralic sublimate encrustations in association with minerals of the svabite–fluorapatite–pliniusite system and the schäferite–berzeliite, tilasite–isokite, wagnerite–arsenowagnerite and udinaite–arsenudinaite series. The mineral assemblage also includes calciojohillerite, diopside, forsterite, titanite, rhabdoborite-(V), rhabdoborite-(W), rhabdoborite-(Mo), ludwigite, powellite, scheelite, hematite, baryte and magnesioferrite. Reznitskyite is transparent or semi-transparent, colourless, with vitreous lustre. Dcalc. = 3.453 g cm–3. Under the microscope, in reflected light reznitskyite is grey, non-pleochroic, with very weak bireflectance (ΔR589 nm = 0.5%) and distinct anisotropy. Reznitskyite is the first vanadate with a titanite-type structure. It is monoclinic, space group C2/c, a = 6.6912(7), b = 8.9395(7), c = 7.0587(8) Å, β = 113.078(13)°, V = 388.43(8) Å3 and Z = 4. The strongest reflections of the powder X-ray diffraction pattern are [d in Å(I)(hkl)]: 3.082(100)(200), 3.250(66)($\bar{1}$12, 002), 2.631(44)(022), 2.854(34)($\bar{2}$02), 2.683(33)(130), 3.686(26)(021), 2.531(25)(220), 1.749(25)($\bar{3}$32, $\bar{2}$04) and 2.344(24)(131). Electron microprobe analysis gave (wt.%) MgO 20.44, CaO 26.83, P2O5 6.24, V2O5 21.09, As2O5 18.97, SO3 0.47, F 9.42, –O=F –3.97, with a total of 99.49. The empirical formula of reznitskyite, calculated on the basis of O+F = 5 atoms per formula unit, is: Ca0.97Mg1.03(V0.47As0.33P0.18S0.01)Σ0.99O3.99F1.01. Reznitskyite is a vanadate analogue of tilasite CaMg(AsO4)F and isokite CaMg(PO4)F. The mineral is named in honour of the outstanding Russian mineralogist Leonid Zinovievich Reznitsky (born 1938) who has made significant contribution to the mineralogy of vanadium.

Kainite, KMg(SO4)Cl⋅2.75H2O, is one of the most common hydrated sulfate minerals, and it plays an important role as a source of potassium. However, its properties and structure have, to date, been insufficiently studied. In our present work, kainite was investigated using multiple techniques, including single-crystal and powder X-ray diffraction, thermogravimetry, differential scanning calorimetry (DSC), and infrared spectroscopy (IR). The mineral is monoclinic, C2/m, a = 19.6742(2), b = 16.18240(10), c = 9.49140(10) Å, β = 94.8840(10)°, V = 3010.86(5) Å3 and Z = 16. The structure was refined to R1 = 0.0230 for 3080 unique observed reflections with |Fo| ≥ 4σF. The complex hydrogen bonding system for kainite is described for the first time. The structure of kainite contains seven symmetrically independent sites occupied by water molecules, six of which are strongly bonded to Mg2+ cations while the seventh resides in the framework cavities. The acceptors of the hydrogen bonds are either chloride anions, neighbouring water molecules or oxygens atoms of sulfate groups. A bifurcated hydrogen bond was described for one of the water molecules. Based on the analysis of the crystal structure, we have confirmed and propose the correct formula for kainite as KMg(SO4)Cl⋅2.75H2O. The thermal studies of kainite in the temperature range of –150°C to +600°C indicate its stability to 190°C. The decomposition products are K2Mg2(SO4)3, KCl and K2SO4. The thermal expansion was calculated, which for kainite has a character typical for monoclinic crystals and similar to the compressibility tensor described earlier.

Calciolangbeinite, ideally K2Ca2(SO4)3, exists in two modifications, cubic and, first described in the present paper, orthorhombic. They are topologically-similar polymorphs which can be designated as calciolangbeinite-C and calciolangbeinite-O. Calciolangbeinite-O is the first natural orthorhombic langbeinite-like sulfate. It clearly differs from calciolangbeinite-C in the powder X-ray diffraction pattern, optical data and Raman spectrum. Calciolangbeinite-O is found in sublimates of the active Arsenatnaya fumarole at the Tolbachik volcano, Kamchatka, Far Eastern Region, Russia and in pyrometamorphic rocks of the Hatrurim Complex at Jabel Harmun, Judean Desert, Palestinian Autonomy and Har Parsa, Negev Desert, both in Israel. Calciolangbeinite-C is known only in fumarole sublimates at Tolbachik. Calciolangbeinite forms a continuous solid-solution system with langbeinite K2Mg2(SO4)3. The majority of the system is represented by cubic phases, and only members with compositions K2(Ca2.0–1.9Mg0.0–0.1)(SO4)3 have orthorhombic symmetry under room-temperature conditions. The crystal structure of calciolangbeinite-O was studied on a single crystal, chemically very close to K2Ca2(SO4)3, from Tolbachik (R1 = 2.75%). The unit-cell parameters are: a = 10.3330(2), b = 10.5027(2), c = 10.1763(2) Å, V = 1104.37(4) Å3 and Z = 4; space group is P212121. Calciolangbeinite-O is a low-temperature modification of K2Ca2(SO4)3 belonging to the K2Cd2(SO4)3 structure type whereas calciolangbeinite-C (space group P213), a high-temperature modification, has the langbeinite-type structure. The significant Mg admixture in calciolangbeinite-C from Tolbachik probably stabilises its cubic structure at room temperature. In both high-temperature fumaroles and pyrometamorphic rocks calciolangbeinite crystallises in the cubic modification, and during cooling its chemical variety close to the end-member K2Ca2(SO4)3 undergoes phase transition to calciolangbeinite-O, whereas the Mg-enriched varieties of the mineral remain calciolangbeinite-C.

The new alluaudite-group mineral paraberzeliite was found in the Arsenatnaya fumarole, Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. In the deepest zone of Arsenatnaya, paraberzeliite (holotype) is associated with anhydrite, diopside, hematite, svabite, berzeliite, schäferite, calciojohillerite, magnesioferrite, ludwigite, fluorapatite, powellite, baryte, and rhabdoborite-group and aphthitalite-group members. In the middle zone of the fumarole, paraberzeliite occurs with hematite, calciojohillerite, badalovite, johillerite, nickenichite, tilasite, svabite, fluorophlogopite, sanidine, cassiterite, anhydrite, metathénardite and belomarinaite. Paraberzeliite forms prismatic crystals up to 0.2 mm × 0.2 mm × 1 mm often occurring in open-work aggregates. It is transparent, brown (from light to dark brown, sometimes with purple or red hue) or green (from pale greenish to yellow–green). The mineral is brittle, cleavage was not observed. The Mohs hardness is 3½. Dcalc is 3.811 g cm–3. Paraberzeliite is optically biaxial (+), α = 1.718(4), β = 1.728(4), γ = 1.742(4) and 2Vmeas. = 85(5)°. The chemical composition (wt.%, electron-microprobe; holotype) is: Na2O 6.43, CaO 16.65, MgO 11.64, MnO 1.65, CuO 0.06, Fe2O3 2.45, V2O5 1.10, As2O5 59.46, total 99.44. The calculated empirical formula based on 12 O atoms per formula unit is (Na1.20Ca1.71Mg1.66Mn0.13Fe3+0.18)Σ4.88(As2.98V0.07)Σ3.05O12. Paraberzeliite is monoclinic, C2/c, a = 12.3143(7), b = 13.0679(5), c = 6.7717(4) Å, β = 113.657(7)°, V = 998.14(10) Å3 and Z = 4. The crystal structure was solved from single-crystal X-ray diffraction data, R = 0.0349. Paraberzeliite is isostructural with other alluaudite-group minerals. Its simplified crystal chemical formula is A(1)CaA(2)'NaM(1)CaM(2)Mg2(TAsO4)3. The idealised formula is NaCa2Mg2(AsO4)3, or, according to the actual nomenclature of alluaudite-group arsenates, NaCaCaMg2(AsO4)3. The name paraberzeliite reflects the dimorphism of this alluaudite-group mineral with the arsenate garnet berzeliite, ideally (Ca2Na)Mg2(AsO4)3.

This work is devoted to an investigation of the magnetic properties and thermal behaviour of the natural oxoborates vonsenite and hulsite in the temperature range 5–500 K. The local environment, the oxidation states of the Fe and Sn atoms, and the charge distribution were determined using Mössbauer spectroscopy and are in accordance with a refinement of the crystal structure of hulsite from single-crystal X-ray diffraction data (SCXRD) in anisotropic approximation for the first time. The magnetic properties were studied by vibrating sample magnetometry (VSM) (5 ≤ T ≤ 400 K) and are reported for the first time for iron-rich hulsite. Both oxoborates show a very complex magnetic behaviour. Cascades of magnetic transitions are revealed and the critical temperatures were determined. The sequences of magnetic transitions in both vonsenite and hulsite with increasing temperature were found to be as follows: magnetically ordered state → partial magnetic ordering → paramagnetic state. According to X-ray diffraction data (93 ≤ T ≤ 500 K), these processes are accompanied by anomalies in the unit-cell parameters and thermal expansion of the oxoborates at critical temperatures. A strong negative volume thermal expansion is observed for both oxoborates at temperatures below ∼120 K.

The mineralogy of carbonatites reflects both the diversity of the sources of their parent magmas and their unusual chemistry. Carbonatites contain diverse suites of both primary magmatic minerals and later hydrothermal products. We present a summary of the variety of minerals found in carbon-atites, and note the economic importance of some of them, particularly those that are major sources of “critical elements”, such as Nb and rare earth elements (REEs), which are essential for modern technological applications. Selected mineral groups are then discussed in detail: the REE carbonates, the alkali-rich ephemeral minerals that are rarely preserved but that may be important in the petrogenesis of carbonatites and their metasomatic haloes in adjacent rocks, and the Nb-rich oxides of the pyrochlore supergroup.

The new wermlandite-group mineral erssonite, ideally CaMg7Fe3+2(OH)18(SO4)2⋅12H2O (or [Mg7Fe3+2(OH)18][Ca(SO4)2]⋅12H2O), was discovered in a late-stage, low-temperature assemblage in cavities of a magnetite-bearing dolomitic rock from the Långban deposit, Värmland county, Bergslagen ore province, Sweden. The associated minerals are dolomite, calcite, members of the magnetite–magnesioferrite solid-solution series, phlogopite, chrysotile, pyroaurite and norbergite. Erssonite has a vitreous lustre and forms colourless, platy hexagonal crystals flattened on [0001], up to 0.5 mm across and up to 10 μm thick, occurring mainly as aggregates in cavities of dolomitic rock. Erssonite is malleable; separate crystals are flexible and non-elastic, with a perfect, mica-like cleavage on {0001}. The calculated density is equal to 2.02 g⋅cm–3. Raman spectroscopy shows the presence of typical bands for S–O bonds attributed to intercalated SO42– anions and structural OH– anions together with the absence of C–O bonds, attributed to carbonate anions. The chemical composition is (wt.%, electron microprobe, H2O content is calculated from structure data): MgO 28.67, CaO 2.76, Al2O3 0.23, Cr2O3 0.23, Fe2O3 16.00, SiO2 0.48, SO3 14.80, H2O 35.58, total 98.75. The empirical formula based on 38 O atoms is H41.48Ca0.52Mg7.47Fe3+2.11Al0.05Cr0.03S1.94Si0.08O38. The ideal formula is CaMg7Fe3+2(OH)18(SO4)2⋅12H2O or {Mg7Fe3+2(OH)18}{[Ca(H2O)6](SO4)2(H2O)6}. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.093. Erssonite is trigonal, P$\bar{3}$c1, with a = 9.3550(6), c = 22.5462(14) Å, V = 1708.8(2) Å3 and Z = 2. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %)(hkl)] are: 11.22 (90)(002), 5.63 (64)(004), 4.670 (100)(110, 104, 014), 2.626 (64)(032, 302), 2.435 (66)(034, 304) and 1.951 (45)(038, 308). The mineral is named in honour of the Swedish amateur mineralogist Dr. Anders Ersson (b. 1971).

A new mineral nishanbaevite, ideally KAl2O(AsO4)(SO4), was found in 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 euchlorine, alumoklyuchevskite, langbeinite, urusovite, lammerite, lammerite-β, ericlaxmanite, kozyrevskite, and hematite. Nishanbaevite occurs as long-prismatic or lamellar crystals up to 0.03 mm typically combined in brush-like aggregates and crusts up to 1.5 mm across. It is transparent, colourless, with vitreous lustre. Dcalc = 3.011 g cm− 3. Nishanbaevite is optically biaxial (–), α = 1.552, β ≈ γ = 1.567. The chemical composition (average of seven analyses) is: Na2O 3.79, K2O 8.01, CaO 0.10, CuO 0.21, Al2O3 30.08, Fe2O3 0.50, SiO2 1.62, P2O5 0.66, As2O5 32.23, SO3 22.59, total 99.79 wt.%. The empirical formula calculated based on 9 O apfu is: (K0.57Na0.41Ca0.01)Σ0.99(Al1.99Fe3+0.02Cu0.01)Σ2.02(As0.95S0.95Si0.09P0.03)Σ2.02O9. Nishanbaevite is orthorhombic, Pbcm, a = 15.487(3), b = 7.2582(16), c = 6.6014(17) Å, V = 742.1(3) Å3 and Z = 4. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 15.49(100)(100), 6.56(30)(110), 4.653(29)(111), 3.881(54)(400), 3.298(52)(002), 3.113(29)(121), and 3.038(51)(202, 411). The crystal structure, solved from single-crystal XRD data (R = 7.58%), is unique. It is based on the complex heteropolyhedral sheets formed by zig-zag chains of Al-centred polyhedra (alternating trigonal bipyramids AlO5 and octahedra AlO6 sharing edges) and isolated tetrahedra AsO4 and SO4. Adjacent chains of Al polyhedra are connected via AsO4 tetrahedra to form a heteropolyhedral double-layer. Its topological peculiarity is considered and compared with those in structurally related compounds. The (K,Na) site is located in the interlayer space between SO4 tetrahedra. The position of nishanbaevite among the arsenate-sulfates and their specific structural features are discussed. The mineral is named in honour of the Russian mineralogist Tursun Prnazorovich Nishanbaev (1955–2017).

The crystal structure of Mg-rich pyroxmangite from sanidinite of the Laach Lake (Laacher See) Volcano, Eifel paleovolcanic region, Rhineland-Palatinate, Germany has been solved based on single-crystal X-ray diffraction data, the final R indices for all data are: R1 = 0.0302, wR2 = 0.0617. Parameters of the triclinic unit cell are: a = 9.6410(4), b = 10.4328(6), c = 17.3419(9) A, α = 112.256(5), β = 102.806(4), γ = 82.935(4)°, V = 1572.72(15) A3. Space group is C-1. The empirical formula of the studied sample is Mn3.48Mg1.78Fe1.61Ca0.13(Si7.00O21), and the refined crystal-chemical formula is M1[(Mn,Fe)0.90Mg0.10] M2[(Mn,Fe)0.93Mg0.07] M3[(Mn,Fe)0.84Mg0.16] M4[(Mn,Fe)0.63Mg0.37] M5[(Mn,Fe)0.74Mg0.26] M1[(Mn,Fe)0.69Mg0.31] M1[(Mn,Fe)0.54Mg0.46] [Si7O21]. This sample is characterized by a stronger disordering of Mg among the M sites as compared to Mg-rich members of the pyroxmangite-pyroxferroite solid-solution series from xenoliths hosted by basalt of the Bellerberg volcano, Eifel. The genetic factors which could influence to the cation ordering in pyroxmangite-type minerals are discussed.

Three new isostructural minerals, alexkuznetsovite-(La), ideally La2Mn(CO3)(Si2O7), alexkuznetsovite-(Ce) Ce2Mn(CO3)(Si2O7) and biraite-(La) La2Fe2+(CO3)(Si2O7), were discovered in polymineralic nodules from the Mochalin Log REE deposit, South Urals, Russia. The new minerals form anhedral grains up to 0.3 mm × 0.4 mm [alexkuznetsovite-(La)], 0.5 mm × 0.9 mm [alexkuznetsovite-(Ce)] and 0.2 mm × 1.2 mm [biraite-(La)] embedded in granular aggregates consisting of different REE minerals [allanite-(Ce)/-(La), bastnäsite-(Ce)/-(La), fluorbritholite-(Ce), perbøeite-(Ce)/-(La), percleveite-(Ce)/-(La) and törnebohmite-(Ce)/-(La)]. All three new species are brown to dark brown, translucent in thin fragments, with white streak, vitreous lustre and Mohs’ hardness of ~5. Dcalc = 4.713 [alexkuznetsovite-(La)], 4.687 [alexkuznetsovite-(Ce)] and 4.682 [biraite-La)] g⋅cm–3. Their empirical formulae, calculated on the basis of 2 Si and 10 O apfu, are: alexkuznetsovite-(La): (La0.98Ce0.89Nd0.10Pr0.05)Σ2.02Mn2+0.50Fe2+0.29Ca0.12Mg0.03(CO3)0.94(HCO3)0.06 (Si2O7); alexkuznetsovite-(Ce): (Ce0.96La0.78Nd0.16Pr0.07)Σ1.97Th0.01Mn2+0.50Fe2+0.33Ca0.14Mg0.02(CO3)0.93(HCO3)0.07(Si2O7); and biraite-(La): (La0.95Ce0.87Nd0.08Pr0.04)Σ1.94Th0.01Ca0.12Fe2+0.44Mn2+0.38Mg0.07(CO3)0.88(HCO3)0.12(Si2O7). The new minerals are monoclinic, P21/c and Z = 4. The unit-cell parameters of alexkuznetsovite-(La)/alexkuznetsovite-(Ce)/biraite-(La) are: a = 6.5642(3)/6.5764(4)/6.5660(10), b = 6.7689(3)/6.7685(4)/6.7666(11), c = 18.7213(10)/18.7493(15)/18.698(3) Å, β = 108.684(6)/108.672(8)/108.952(16)° and V = 788.00(7)/790.66(10)/785.7(2) Å3. The crystal structures are solved from single-crystal X-ray diffraction data; R = 0.0628 [alexkuznetsovite-(La)], 0.0589 [alexkuznetsovite-(Ce)] and 0.1193 [biraite-(La)]. A new biraite group is defined; it includes isostructural biraite-(Ce) and the three new minerals described herein. The rootname alexkuznetsovite is given in honour of the Russian mineral collector Alexey M. Kuznetsov (born 1962) who provided samples in which all three new minerals were found. The Levinson's suffix-modifier -(La) or -(Ce) indicates the predominance of La or Ce among REE in the mineral. Biraite-(La) is named as an analogue of biraite-(Ce) with La prevailing among REE.

Six different exsolution types are found in crystals of aphthitalite-group alkali sulfates from exhalations of the active Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. The coexisting minerals in these exsolutions are metathénardite, ideally Na2SO4 (P63/mmc), and vanthoffite, Na6Mg(SO4)4 (P21/c) (Type I); metathénardite and belomarinaite, KNaSO4 (P3m1) (Type II); thénardite, Na2SO4 (Fddd), and aphthitalite, K3Na(SO4)2 (Pm1) (Type III); aphthitalite and arcanite, K2SO4 (Pnma) (Type IV); metathénardite and natroaphthitalite, KNa3(SO4)2 (Pm1) (Type V); and two chemical varieties of metathénardite (Type VI). The exsolution processes occur in crystals belonging to the high-temperature, hexagonal Na2SO4(I) (= metathénardite, P63/mmc) structure type with different K:Na ratios formed at temperatures higher than 500 °C. The similarity and hexagonal close-packed nature of the crystal structures of the coexisting phases, all representatives of aphthitalite-like structure types, cause the coherent conjugation of domains during diffusion and cation ordering in the parent phase. The breakdown of solid solution can be facilitated by the mosaic character of crystals of a parent phase (incoherent grain boundaries) and the presence of coherent twin boundaries. The heating of samples with exsolution Types II and V up to 700 °C over 24 h shows that diffusion of K and Na through the domain borders does not result in the complete disorder of these cations and the extinction of domains with different crystal structures.

The solid-solution system between natural arsenates, vanadate, and phosphate of the tilasite group are characterized. The material originates from sublimates of the active Asenatnaya fumarole, Tolbachik volcano, Russia and paleofumaroles of Thomas Range, Utah, USA. The tilasite-group minerals were formed under the oxidizing conditions at temperatures > 300°C. The minerals belong to the titanite structure type and have the general formula AM(TO4)X in which A are Ca and Na in sevenfold coordination, M are octahedrally coordinated Mg, Al, Fe3+, and Ti, T are tetrahedrally coordinated pentavalent As, P, and V, and X are O or F. The studied minerals show wide-range isomorphous substitutions at all these sites except of O atoms coordinating T. Maxwellite NaFe3+(AsO4)F and durangite NaAl(AsO4)F form a solid-solution series with only homovalent substitution Fe3+ ↔ Al. Two systems with heterovalent substitutions are observed: (1) with the major substitution Mg ↔ Fe3+ between tilasite CaMg(AsO4)F, maxwellite and a hypothetic end-member CaFe3+(AsO4)O and (2) with the major substitution M3+ ↔ Ti4+ between arsenatrotitanite NaTi(AsO4)O, durangite, maxwellite and CaFe3+(AsO4)O. In both cases the charge balance is achieved by coupled substitutions ANa+ ↔ ACa2+ and XF− ↔ XO2−. Two novel mineral varieties belong to these solid solutions, namely a MFe3+-dominant variety of tilasite with the formula close to (Ca2/3Na1/3)(Fe3 + 2/3Mg1/3)(AsO4)(F2/3O1/3) and an XO-dominant variety of maxwellite with the formula (Na2/3Ca1/3)(Fe3 + 2/3Ti1/3)(AsO4)(O2/3F1/3). Another solid solution forms between reznitskyite CaMg(VO4)F, tilasite and isokite CaMg(PO4)F with the major substitutions TAs5+ ↔ TV5+ ↔ TP5+. All these solid solutions are novel and highlight the specific crystal chemistry of fumarolic oxysalts with the titanite-type structure.

New data on the isomorphism of extra-framework components (including chromophores) in two- and multilayer minerals belonging to the cancrinite and sodalite groups, are obtained using chemical and single-crystal X-ray diffraction data as well as infrared, Raman, ESR, UV–Vis–near IR absorption and photoluminescence spectroscopy methods. It is shown that the blue color of these minerals may be due to CO3•− or S3•− radical anions, whereas yellow and pink shadings are typically due to the presence of S2•− radical anions and S4•− or S4, respectively. Two kyanoxalite varieties are distinguished: (1) with predominantly acid oxalate groups and (2) predominantly neutral oxalate groups. Zundel cation H5O2+ and CO2 molecules are shown to be typical impurities in nosean. The Zundel cation is also detected in kyanoxalite and in the 12-layer, cancrinite-related mineral marinellite. Wide isomorphic series involving substitutions of SO4− for SO32− and CO32−, as well as OH− for H2O and F−, are common for eight-layer, cancrinite-group minerals with an afghanite-type framework.

The new mineral yurgensonite, ideally K2SnTiO2(AsO4)2, the first natural arsenate with species-defining tin, and the continuous isomorphous series between yurgensonite and katiarsite KTiO(AsO4) are described from sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Yurgensonite and a Sn-bearing variety of katiarsite are associated closely with one another and with badalovite, pansnerite, yurmarinite, achyrophanite, arsenatrotitanite, hatertite, khrenovite, svabite, sanidine, hematite, cassiterite, rutile and aphthitalite-group sulfates. Yurgensonite occurs as sword-shaped crystals up to 0.01 mm × 0.05 mm × 1 mm or acicular to hair-like individuals up to 1 mm long, typically forming radial aggregates up to 2 mm across. It is transparent, colourless, white or pale beige, with vitreous lustre. The mineral is brittle, cleavage was not observed. Dcalc is 3.877 g cm-3. Yurgensonite is optically biaxial (–), α = 1.764(6), β = 1.780(6), γ = 1.792(6) and 2Vmeas. is large. Chemical composition (wt.%, electron-microprobe; holotype) is: Na2O 0.51, K2O 16.27, Rb2O 0.12, Al2O3 0.26, Fe2O3 4.33, SiO2 0.29, TiO2 10.17, SnO2 22.01, P2O5 0.14, V2O5 0.19, As2O5 40.20, Sb2O5 4.88, SO3 0.28, total 99.65. The empirical formula based on 10 O apfu is (K1.92Na0.09Rb0.01)Σ2.02(Sn0.81Ti0.71Fe3+0.30Sb5+0.17Al0.03)Σ2.02(As1.945Si0.03S0.02P0.01V0.01)Σ2.015O10. Yurgensonite is orthorhombic, Pna21, a = 13.2681(6), b = 6.6209(3), c = 10.8113(5) Å, V = 949.74(7) Å3 and Z = 4. The crystal structure was solved from single-crystal X-ray diffraction data, R = 5.02%. Yurgensonite belongs to the KTP-structure type. It is a Ti,Sn-ordered analogue of katiarsite. The structure contains chains of corner-linked alternating crystallographically non-equivalent octahedra M(1) and M(2). In yurgensonite, Sn4+ prevails in the M(2)O6 octahedron whereas the M(1) site is Ti4+-dominant. The new mineral is named in honour of the Russian mineralogist, geochemist and specialist in studies of ore deposits Professor Georgiy Aleksandrovich Yurgenson (born 1935).

This article provides data on cristobalite and tridymite from the Arsenatnaya active fumarole of the Tolbachik volcano, Kamchatka, Russia. The minerals occur in associations with fumarolic sylvite, sanidine, cassiterite, hematite, pseudobrookite, johillerite, tilasite, and badalovite. Fumarolic cristobalite is tetragonal (α-modification); the unit-cell parameters for one sample are: а = 4.975 (7) A, с = 6.944 (13) A, and V = 171.89 A3. There are two types of tridymite, that is, monoclinic (MC) and orthoorthorhombic (PO-10), in the Arsenatnaya fumarole. The unit-cell parameters of these tridymite modifications are: a = 18.553 (5) A, b = 5.006 (1) A, с = 25.952 (10) A, β = 117.68 (2)°, V = 2134.3 (11) A3 (MC); a = 9.941 (2) A, b = 17.165 (4) A, с = 82.362 (18) A, and V = 14053.4 (29) A3 (PO-10). Mineral assemblages of cristobalite and tridymite indicate high-temperature formation conditions of these minerals not lower 450–500°С with considerable participation of HCl and HF in process of basalt alteration by fumarolic gas. The surrounding basalt was a source of silicon. This element was probably transported in the form of SiX4, where X = F, Cl.

The crystal structure of a potentially new mineral, the Fe2+-dominant analogue of armalcolite with the idealized formula Fe2+Ti2O5 has been solved. The sample studied originates from a pneumatolytic association related to the lamproite complex of SE Spain. The mineral is orthorhombic, space group Cmcm, the unit-cell parameters are a = 3.7325(1) A, b = 9.7649(4) A, c = 9.9902(3) A, V = 364.12(2) A3. The crystal-chemical formula $${{^{M}}^{1}}{{({\text{M}}{{{\text{g}}}_{{0.19}}}{\text{Fe}}_{{0.25}}^{{2 + }}{\text{Fe}}_{{0.26}}^{{3 + }}{\text{T}}{{{\text{i}}}_{{0.30}}})}^{{2.86 + }}}$$ $${{{{[}^{M}}^{2}{{({\text{T}}{{{\text{i}}}_{{0.65}}}{\text{Fe}}_{{0.27}}^{{3 + }}{\text{Fe}}_{{0.08}}^{{2 + }})}^{{3.57 + }}}]}_{2}}{{{\text{O}}}_{5}}$$ (Z = 4) is in good agreement with the chemical composition of the mineral.

The new mineral vasilseverginite, ideally Cu9O4(AsO4)2(SO4)2, was found in 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 tenorite, lammerite, stranskiite, lammerite-β, langbeinite, dolerophanite, sanidine, hematite, and gahnite. Vasilseverginite occurs as prismatic crystals up to 0.02 × 0.02 × 0.06 mm3 combined in groups or interrupted crusts up to 1 × 2 cm2 in area and up to 0.1 mm thick. It is transparent, bright green, with vitreous luster. Dcalc is 4.41 g·cm−3. Vasilseverginite is optically biaxial (–), α 1.816(5), β 1.870(5), γ 1.897(5), estimated 2V is 30(15)°. Chemical composition (wt%, electron-microprobe) is: CuO 64.03, ZnO 0.79, Fe2O3 0.25, P2O5 0.05, As2O5 20.83, SO3 14.92, total 100.87. The empirical formula calculated on O = 20 apfu is (Cu8.78Zn0.11Fe0.033+)Σ8.92As1.98P0.01S2.03O20. Vasilseverginite is monoclinic, P21/n, a = 8.1131(4), b = 9.9182(4), c = 11.0225(5) Å, β = 110.855(2)°, V = 828.84(6) Å3, and Z = 2. The strongest reflections in the powder XRD pattern [d,Å(I)(hkl)] are: 7.13(41)(101), 5.99(70)(110, 111), 5.260(100)(101), 4.642(46)(111), 3.140(31)(031), 2.821(35)(023), 2.784(38)(132, 032), 2.597(35)(204), and 2.556(50) (231, 212). The crystal structure, solved using single-crystal X-ray diffraction data, R1 = 0.025, is based upon complex [O4Cu9]10+ layers parallel to (101) that are composed of edge- and corner-sharing (OCu4) tetrahedra. The topology is unprecedented in inorganic structural chemistry. The crystal structure can be considered a hybrid of the structures of popovite Cu5O2(AsO4)2 and dolerophanite Cu2O(SO4) according to the scheme Cu9O4(AsO4)2(SO4)2 = Cu5O2(AsO4)2 + 2Cu2O(SO4). The chemical hybridization does not result in a significant increase in chemical complexity of vasilseverginite compared to the sum of those of popovite and dolerophanite, whereas the structural hybridization leads to the doubling of structural information per unit cell. The mineral is named in memory of the outstanding Russian mineralogist, geologist, and chemist Vasiliy Mikhailovich Severgin (1765–1826).

An inclusion of graphite in zircon from the diamond- and tourmaline-bearing rocks of the Kokchetav massif (Northern Kazakhstan) has been studied. The inclusion, associated with diamond crystals, was identified within the marginal part of the zircon grain. The Raman spectrum of the graphite inclusion is characterized by the presence of an intense band at 1350 cm–1, which is indicative of a high degree of disorder in its structure. An investigation of this inclusion by means of transmission electron microscopy (TEM) allowed us to identify nine randomly oriented high-ordered submicron grains of graphite without relics of diamond. Thus, the presence of the intense band at 1350 cm–1 in the Raman spectra of graphite is not a necessary evidence of its low degree of crystallinity and particularly its formation as a result of diamond graphitization. The formation of the polycrystalline aggregates, consisting of submicron grains of well-ordered graphite, might have been related to rapid crystallization from a high-density C–O–H fluid.

Microporous silicate tiettaite from the Khibiny alkaline massif (Kola Peninsula, Russia) has been studied using X-ray diffraction analysis, electron probe microanalysis, and Raman spectroscopy of single crystals and powders. Its chemical formula is refined to be K4Na12Fe3+Si16O41(OH)4 ⋅ 2H2O. The Raman spectrum of tiettaite contains lines at 3650, 3450, 1097, 1066, 980, 920, 681, 635, 569, 499, 458, 404, 272, 144 cm–1. The orthorhombic unit-cell parameters are a = 28.866(7) A, b = 10.707(3) A, c = 17.425(7) A, V = 5385(3) A3, Z = 4, sp. gr. Cmcm. The tiettaite crystal structure has been solved for the first time from single crystal and refined by the Rietveld method to the final Rwp = 2.47% and RBragg = 1.38%. The structure based on a new-type microporous heteropolyhedral framework formed by one-dimensional modules [Si28Na16Fe4K4O110]76– elongated along the c axis. Each module consists of complex ribbons with compositions of [Si14O39]22– and [Fe2K2Na6O32]50– compose a framework via shared vertices of half-occupied silicon–oxygen tetrahedra Si5 and Si6. Tiettaite structure contains a three-dimensional system of crossed channels with effective channel widths of 2.61 × 7.50, 2.46 × 2.92, and 3.60 × 3.90 A2, running along the [100], [010], and [001] directions, respectively. The presence of a system of broad channels allows one to consider tiettaite as promising ion exchanger. Regarding the information complexity parameters, tiettaite can be assigned to minerals with very complex structure.

The crystal structure of a new mineral philoxenite (K,Na,Pb)4(Na,Ca)2(Mg,Cu)3(Fe $$_{{0.5}}^{{3 + }}$$ Al0.5)(SO4)8 from fumarole exhalations of the Tolbachik Volcano (Kamchatka, Russia) has been determined by single-crystal X-ray diffraction analysis (R1 = 5.67%). Philoxenite is triclinic (sp. gr. P $$\bar {1}$$ ) with the following parameters: a = 8.8410(3) A, b = 8.9971(3) A, c = 16.1861(5) A, α = 91.927(3)°, β = 94.516(3)°, γ = 90.118(3)°, V = 1282.77(7) A3, and Z = 2. The mineral has a unique structure based on a new-type microporous heteropolyhedral framework formed by SO4 tetrahedra and M(1–4)O6 octahedra of four types. It is pierced with a three-dimensional network of channels with large cations occupying seven different sites А(1–7). The philoxenite structural formula (Z = 1) is A(2,6)(K0.90Na0.07Pb0.03)4 A(5)(K0.69Na0.28Pb0.03)2 A(7)(K0.85Pb0.15)2 A(1)(Na0.61Ca0.39)2- A(3)(Na0.72Ca0.28)A(4)(Na0.81Ca0.19)M(2)(Mg0.60Cu $$_{{0.40}}^{{2 + }}$$ )2M(1)(Mg0.56Cu $$_{{0.44}}^{{2 + }}$$ )2M(4)(Mg0.43Al0.35Zn0.22)2- M(3)(Fe $$_{{0.42}}^{{3 + }}$$ Al0.40Mg0.18)2(SO4)16.

Single crystals of “malakhovite” (an anthropogenic silico-oxide from burnt dumps of the Chelyabinsk coal basin; sp. gr. P $$\bar {1}$$ , a = 10.555(4) A, b = 10.928(3) A, c = 9.059(3) A, α = 106.338(12)°, β = 95.87(7)°, γ = 124.40(2)°, V = 781.0(5) A3, Z = 1, R1 = 0.0378 have been investigated by electron-probe microanalysis, Raman spectroscopy, and X-ray diffraction analysis. The idealized formula of the mineral is Ca4(Fe $$_{{8.56}}^{{3 + }}$$ Mg1.72Ca0.82Fe $$_{{0.55}}^{{2 + }}$$ Mn0.22Ti0.13)Σ12.00(Fe $$_{{6.48}}^{{3 + }}$$ Al2.82Si2.71)Σ12.01O40. It is shown that malakhovite is not only chemical but also crystal chemical anthropogenic analogue of khesinite (Ca4Mg2Fe $$_{{10}}^{{3 + }}$$ O4Fe $$_{{10}}^{{3 + }}$$ Si2O36). The relationships between the malakhovite and the minerals from the dorrite–khesinite series of the rhonite group in the sapphirine supergroup are discussed.

—In the $${\text{Cu}}_{3}^{{2 + }}$$ (T5+O4)2 (T = As, V or P) natural system arsenates are trimorphous (lammerite, lammerite-β, and unnamed triclinic Cu3[(As,V)O4]2), as well as vanadates (mcbirneyite, pseudolyonsite, and borisenkoite), while phosphates are represented by a poorly studied lammerite-like Cu3[(P,As)O4]2 phase. In active oxidizing-type fumaroles related to the Tolbachik volcano in Kamchatka, Russia, all these minerals crystallized at temperature no lower than 250°C. The crystal structure of natural triclinic Cu3[(As,V)O4]2 was first studied on a single crystal from the Yadovitaya fumarole at Tolbachik; R = 4.89%. It is isostructural to stranskiite and mcbirneyite and crystallizes in the P-1 space group; the unit-cell parameters are: a = 5.0701(8), b = 5.4047(8), c = 6.3910(8) A, α = 70.091(13), β = 86.828(12), γ = 68.399(15)°, V = 152.61(4) A3, and Z = 1. This paper also contains new data on mcbirneyite (its novel As-enriched variety is characterized), lammerite, and lammerite-β. Original and previously published materials on natural members of the Cu3(T5+O4)2 system are summarized and isomorphism and other crystal chemical features of minerals belonging to this system are discussed in comparison with the data on related synthetic compounds.

— In situ high-temperature powder X-ray diffraction (HTPXRD), differential scanning calorimetry (DSC), and thermogravimetry (TG) of three natural borosilicates have been performed in the temperature range of 25–1200°С. Axinite-(Mn) melts incongruently at 900°С with the formation of anorthite and bustamite. Leucosphenite decomposes at 850°С to form fresnoite and cristobalite. According to DSC data, kornerupine decomposes at 1177°С to form sapphirine, indialite, and spinel as the products of heating kornerupine. The values of the thermal expansion tensor and its orientation were determined using the HTPXRD data. The study showed that the three borosilicates expand weakly and almost isotropically. The average bulk thermal expansion coefficients are 21.3, 22.7, and 32.9 × 10–6°С–1 for axinite-(Mn), kornerupine, and leucosphenite, respectively. Leucosphenite has the maximum volumetric expansion, most likely due to the pronounced layered character of its crystal structure. The least symmetric structure of axinite-(Mn) has the maximum anisotropy of thermal expansion in the temperature range of 600–900°С.

A new mineral, deltalumite, which is an analogue of the spinel-type synthetic δ-Al2O3, the second natural modification of alumina after corundum, α-Al2O3, has been found in products of two eruptions of the Ploskiy Tolbachik Volcano, Kamchatka, Russia. It occurs in the pores of basalt lava and basalt scoria altered by fumarolic gas. The mineral forms roundish segregations up to 0.2 mm across, which consist of blocky coarse prismatic individuals of up to 0.03 mm. Deltalumite is pale yellowish, pale beige, or white, translucent, with a vitreous luster. The mineral is brittle. Dcalc = 3.663 g/cm3. Deltalumite is optically uniaxial, negative, ω = 1.654(2), e = 1.653(2) (λ = 589 nm). The chemical composition (electron microprobe data) is, wt %: 99.74 Al2O3 and 0.04 SiO2; the total is 99.78. The strongest reflections of the powder X-ray diffraction pattern [d, A(I)(hkl)] are: 2.728(61)(202), 2.424(51)(212), 2.408(49)(213), 2.281(42)(206), 1.993(81)(1.0.11, 220, 221), 1.954(48)(0.0.12), and 1.396(100)(327, 400, 2.1.14). The mineral is tetragonal, with the P-4m2 space group (similar to synthetic δ-Al2O3). The unit-cell dimensions are a = 5.608(1), c = 23.513(7) A, V = 739.4(4) A3, and Z = 16. Deltalumite belongs to the spinel subgroup within the oxy-spinel group; its structural formula can be written as (Al0.67◻0.33)Al2O4, where ◻ is a vacancy. The new mineral can be clearly distinguished from other modifications of alumina using its powder X-ray diffraction pattern or IR spectrum.

An unusual highly hydrated and Na-depleted variety of elpidite was identified in a hydrothermally altered peralkaline pegmatite at Mt. Yukspor in the Khibiny alkaline complex, Kola Peninsula, Russia. It differs from ‘ordinary’ elpidite, ideally Na2ZrSi6O15⋅3H2O, in its crystal chemical features, infrared spectrum and optical characteristics. The chemical composition (wt.%, electron microprobe, H2O by TGA) is: Na2O 5.45, K2O 0.67, CaO 0.05, SiO2 60.32, TiO2 1.34, ZrO2 18.43, Nb2O5 0.65, H2O 12.80, total 99.71. The empirical formula calculated on the basis of 6 Si and 15 O atoms is [(Na1.05K0.08Ca0.01)Σ1.14(H3O)0.74]Σ1.88(Zr0.89Ti0.10Nb0.03)Σ1.02Si6O15⋅3.47H2O; the H2O:H3O ratio was calculated from the charge balance requirement, taking into account the results of crystal structure refinement. The highly hydrated variety of elpidite is orthorhombic, Pma2, a = 14.5916(6), b = 7.3294(3), c = 7.1387(2) Å, V = 763.47(5) Å3 and Z = 2. The crystal structure was solved from single-crystal X-ray diffraction data, R1 = 3.43%. The structure is based upon an elpidite-type heteropolyhedral Zr–Si–O framework with Na+ and H3O+ cations and H2O molecules in the zeolitic channels. Hydronium cations substitute for water molecules in one of the extra-framework sites. This variety of elpidite could be considered as an intermediate product of natural ion-exchange reaction between ‘ordinary’ elpidite and a low-temperature hydrothermal fluid.

The new triazolate mineral bojarite (IMA2020-037), Cu3(N3C2H2)3(OH)Cl2⋅6H2O, is found in a guano deposit located at the Pabellón de Pica Mountain, Iquique Province, Tarapacá Region, Chile. Associated minerals are salammoniac, halite, nitratine and belloite. Bojarite occurs as blue fine-grained porous aggregates up to 1 mm × 3 mm × 5 mm combined typically in interrupted earthy crusts. The mineral is brittle. The Mohs hardness is 2.Dcalc= 2.057 g cm–3. The IR and Raman spectra show the presence of the 1,2,4-triazolate anion and H2O molecules. Bojarite is optically isotropic andn= 1.635(2) (λ = 589 nm). The chemical composition (electron-microprobe data for Na, Mg, Fe, Cu and Cl; H, C and N contents measured by gas chromatography on products of ignition at 1200°C; wt.%) is: Na 0.22, Mg 0.74, Fe 0.99, Cu 29.73, Cl 13.62, N 20.4, C 11.6, H 3.3, O (calculated by stoichiometry) 19.93, total 100.53.The empirical formula is (Cu2.68Mg0.17Fe0.10Na0.05)Σ3(N3C2H2)2.755[(OH)][Cl2.19(H2O)3.77(OH)0.04]Σ6⋅2.3H2O. The idealised formula is Cu3(N3C2H2)3(OH)Cl2⋅6H2O. The crystal structure of bojarite was refined based on powder X-ray diffraction data, using the Rietveld method. The final agreement factors are:Rp= 0.0225,Rwp= 0.0310 andRobs= 0.0417. The new mineral is cubic, space groupFd$\bar{3}$c;a= 24.8047(5) Å,V= 15,261.6(5) Å3andZ= 32. The strongest reflections of the powder X-ray diffraction pattern [d, Å (I,%)(hkl)] are: 8.83 (31)(220), 7.19 (100)(222), 6.23 (35)(400), 5.077 (28)(422), 4.194 (28)(531), 3.584 (23)(444), 2.865 (28)(660, 751) and 2.723 (22)(753, 842).

The new mineral percleveite-(La) (IMA2019–037), ideally La2Si2O7, was found in polymineralic nodules of the Mochalin Log REE deposit, Chelyabinsk Oblast, South Urals, Russia. It is associated with allanite-(Ce), allanite-(La), bastnäsite-(Ce), bastnäsite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperbøeite-(Ce), ferriperbøeite-(La), fluorbritholite-(Ce), hydroxylbastnäsite-(Ce), perbøeite-(Ce), perbøeite-(La), törnebohmite-(Ce) and törnebohmite-(La). Percleveite-(La) occurs as isolated anhedral grains commonly up to 0.2 mm × 0.4 mm and very rarely up to 1 mm × 1 mm. The new mineral is transparent with greasy lustre. The mineral is very pale yellow to colourless in thin fragments to light yellow in aggregates. It is brittle, with imperfect {001} cleavage and an uneven fracture. Mohs’ hardness is ca. 6. Dcalc = 5.094 g cm–3. Under the microscope, percleveite-(La) is non-pleochroic, optically uniaxial (+), ω = 1.825(10) and ɛ = 1.835(10). The Raman spectrum is given. Chemical data (wt.%, electron-microprobe) are: La2O3 36.80, Ce2O3 31.22, Pr2O3 1.57, Nd2O3 2.96, SiO2 26.73, total 99.28. The empirical formula based on 7 O apfu is (La1.02Ce0.86Nd0.08Pr0.04)Σ2.00Si2.00O7. Percleveite-(La) is tetragonal, P41; the unit-cell parameters are: a = 6.8482(3), c = 24.8550(13) Å, V = 1165.64(11) Å3 and Z = 8. The strongest reflections in the powder X-ray diffraction pattern [d, Å(I)(hkl)] are: 4.194(18)(113), 3.564(16)(106), 3.349(16)(201,202), 3.157(100)(203,116,008), 3.043(22)(211), 2.934(39)(122), 2.893(29)(213) and 2.864(21)(117). The crystal structure of percleveite-(La) is solved from the single-crystal X-ray diffraction data [R = 0.0617 for 2831 unique reflections with I > 2σ(I)]. The new mineral is named as an analogue of percleveite-(Ce) with La predominance over the rare-earth elements.

The rhodonite-group mineral with the idealised, end-member formula MnMn4[Si5O15] and the crystal chemical formulaVIIM(5)MnVIM(1–3)Mn3VIIM(4)Mn[Si5O15] (Roman numerals indicate coordination numbers) is defined as a valid mineral species named vittinkiite after the type locality Vittinki (Vittinge) mines, Isokyrö, Western and Inner Finland Region, Finland. Vittinkiite is an isostructural analogue of rhodonite, ideally CaMn4[Si5O15], with Mn2+> Ca at theM(5) site. Besides Vittinki, vitiinkiite was found in more than a dozen rhodonite deposits worldwide, however, it is significantly less common in comparison with rhodonite. The mineral typically forms pink to light pink massive, granular aggregates and is associated with quartz, rhodonite, tephroite, pyroxmangite and Mn oxides. Vittinkiite is optically biaxial (+), with α = 1.725(4), β = 1.733(4), γ = 1.745(5) and 2Vmeas= 75(10)° (589 nm). The chemical composition of the holotype (wt.%, electron microprobe) is: MgO 0.52, CaO, 0.93, MnO 51.82, FeO 1.26, ZnO 0.11, SiO246.48, total 101.12. The empirical formula calculated based on 15 O apfu is Mn4.71Ca0.11Fe0.11Mg0.08Zn0.01Si4.99O15. Vittinkiite is triclinic, space groupP$\bar{1}$, witha= 6.6980(3),b= 7.6203(3),c= 11.8473(5) Å, α = 105.663(3), β = 92.400(3), γ = 94.309(3)°,V= 579.38(7) Å3andZ= 2. The crystal structure is solved on a single crystal toR1= 3.85%. Polymorphism of MnSiO3(rhodonite-, pyroxmangite-, garnet- and clinopyroxene-type manganese metasilicates) is discussed, as well as the relationship between vittinkiite and pyroxmangite, ideally Mn7[Si7O21], and the application of infrared spectroscopy for the identification of manganese pyroxenoids.

The new ettringite-group mineral chiyokoite, ideally Ca3Si(CO3)[B(OH)4]O(OH)5·12H2O, was found in a hydrothermally altered calc-silicate skarn at the Fuka mine, Okayama Prefecture, Japan. Associated minerals are calcite, henmilite, and tacharanite. Chiyokoite occurs as hexagonal prismatic crystals up to 30 μm long and up to 20 μm thick. The major forms are the hexagonal prism {100} and monohedra {0001} and {000}. The crystals are combined in clusters which form friable nests up to 1 cm across. The mineral is pink to colorless with white streak and vitreous luster. The cleavage is parallel to {100} and {0001}, good. The fracture is stepped. Dmeas is 1.85(1) g/cm3, Dcalc is 1.85 g/cm3. Chiyokoite is optically uniaxial (–), ω = 1.523(2) and ε = 1.492(3) (589 nm). The infrared spectrum is reported. The chemical composition (wt.%) is CaO 27.56, B2O3 3.47, Al2O3 3.05, Fe2O3 0.12, As2O3 4.77, MnO2 0.32, SiO2 6.55, SO3 0.76, H2O 46.3, CO2 7.30, total 100.2. The empirical formula calculated on the basis of 3 Ca apfu is H31.37Ca3(Si0.67Al0.37Mn4+0.02Fe3+0.01)Σ1.07(C1.01B0.61As3+0.29S0.06)Σ1.97O24.19. The simplified general formula is Ca3(Si,Al)(CO3,AsO3)[B(OH)4,AsO3](OH)6·12H2O. Chiyokoite is hexagonal, P63, a = 11.0119(5), c = 10.5252(6) Å, and V = 1105.3(1) Å3. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 9.53(100)(100), 5.50(24)(110), 4.618(11)(102), 3.812(23)(112), 3.412(15)(211), 2.726(14)(302), 2.521(19)(123), and 2.172(13)(320,402,223). The crystal structure, refined from single-crystal X-ray diffraction data [R1(F) = 0.042], is based on [Ca3(Si,Al)(OH)6(H2O)12] columns parallel to the c axis with B(OH)4– and CO32– and admixed AsO33– anionic groups in channels between the columns. The mineral is named in honor of Professor Chiyoko Henmi (1949–2018).

The new sulfosalt chukotkaite, ideally AgPb7Sb5S15, was discovered in the valley of the Levyi Vulvyveem river, Amguema river basin, Iultin District, Eastern Chukotka, Chukotka Autonomous Okrug, North-Eastern region, Russia. The new mineral forms anhedral grains up to 0.4 × 0.5 mm intergrown with pyrrhotite, sphalerite, galena, stannite, quartz, and Mn-Fe-bearing clinochlore. Other associated minerals include arsenopyrite, benavidesite, diaphorite, jamesonite, owyheeite, uchucchacuaite, cassiterite, and fluorapatite. Chukotkaite is lead-grey and has metallic luster and a grey streak. It is brittle and has an uneven fracture. Neither cleavage nor parting were observed. Mohs hardness is 2–2½. Dcalc. = 6.255 g/cm3. In reflected light, chukotkaite is white, moderately anisotropic with rotation tints varying from bluish-grey to brownish-grey. No pleochroism or internal reflections are observed. The chemical composition of chukotkaite is (wt.%; electron microprobe) Ag 3.83, Pb 53.67, Sb 24.30, S 18.46, total 100.26. The empirical formula based on the sum of all atoms = 28 pfu is Ag0.93Pb6.78Sb5.22S15.07. Chukotkaite is monoclinic, space group P21/c, a = 4.0575(3), b = 35.9502(11), c = 19.2215(19) Å, β = 90.525(8)°, V = 2803.7(4) Å3, and Z = 4. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 3.52 (100) (045), 3.38 (50) (055), 3.13 (50) (065), , 2.82 (25) (066), 1.91 (50) (0 1 10). The crystal structure of chukotkaite was refined from single-crystal X-ray diffraction data to R = 0.0712 for 3307 observed reflections with Iobs > 3σ(I). Chukotkaite belongs to the group of rod-based sulfosalts. The new mineral is named after the region of its type locality: Chukotka Autonomous Okrug, North-Eastern Region, Russia.

The new mineral eleomelanite, (K2Pb)Cu4O2(SO4)4, was found in the Arsenatnaya fumarole on the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik Volcano, Kamchatka, Russia. It is associated with euchlorine, fedotovite, wulffite, chalcocyanite, dolerophanite, dravertite, hermannjahnite, alumoklyuchevskite, klyuchevskite, piypite, cryptochalcite, cesiodymite, anglesite, langbeinite, calciolangbeinite, metathénardite, belomarinaite, aphthitalite, krasheninnikovite, steklite, anhydrite, hematite, tenorite, sanidine, sylvite, halite, lammerite, urusovite, and gold. Eleomelanite occurs as interrupted crusts up to 6 mm across and up to 0.3 mm thick consisting of equant, prismatic, or tabular crystals or grains up to 0.3 mm. It is translucent and black. The luster is oleaginous on crystal faces and vitreous on a cleavage surface. Dcalc is 3.790 g/cm3. Eleomelanite is optically biaxial (–), α 1.646(3), β 1.715(6), γ 1.734(6), 2Vmeas. = 60(15)°. The chemical composition (wt.%, electron-microprobe) is K2O 9.62, Rb2O 0.49, Cs2O 0.24, CaO 1.23, CuO 35.28, PbO 19.25, SO3 34.78, total 100.89. The empirical formula calculated based on 18 O apfu is (K1.88Pb0.79Ca0.20Rb0.05Cs0.02)Σ2.94Cu4.07S3.99O18. Eleomelanite is monoclinic, P21/n, a 9.3986(3), b 4.8911(1), c 18.2293(5) Å, β 104.409(3)°, V 811.63(4) Å3, and Z = 2. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 7.38(44)(101), 3.699(78)(112), , 3.173(40)(211), 2.915(35)(114), 2.838(35)(204), , and . The crystal structure was solved using single-crystal XRD data, R1 = 4.78%. It is based on heteropolyhedral Cu–S–O chains composed of Cu-centered polyhedra with [4+1+1] Cu2+ coordination and SO4 tetrahedra. Adjacent Cu–S–O chains are connected via chains of (K,Pb)O8 and KO10 polyhedra. Eleomelanite belongs to a novel structure type but has common structural features with klyuchevskite, alumoklyuchevskite, wulffite, parawulffite, and piypite. The name is derived from the Greek ελαιν (eleon), oil, and μλας (melas), black, due to its black color and oleaginous luster on crystal faces that are uncommon for sulfate minerals.

Two new isostructural minerals radekškodaite-(La) (CaLa5)(Al4Fe2+)[Si2O7][SiO4]5O(OH)3 and radekškodaite-(Ce) (CaCe5)(Al4Fe2+)[Si2O7][SiO4]5O(OH)3 were discovered in polymineralic nodules from the Mochalin Log REE deposit, South Urals, Russia. Radekškodaite-(La) is associated with allanite-(Ce), allanite-(La), bastnäsite-(Ce), bastnäsite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperbøeite-(La), fluorbritholite-(Ce), törnebohmite-(Ce) and törnebohmite-(La). Radekškodaite-(Ce) is associated with ancylite-(Ce), bastnäsite-(Ce), bastnäsite-(La), lanthanite-(La), perbøeite-(Ce) and törnebohmite-(Ce). The new minerals form isolated anhedral grains up to 0.35 × 0.75 mm [radekškodaite-(La)] and 1 mm × 2 mm [radekškodaite-(Ce)]. Both minerals are greenish-brown with vitreous lustre. Dcalc = 4.644 [radekškodaite-(La)] and 4.651 [radekškodaite-(Ce)] g cm–3. Both minerals are optically biaxial (+); radekškodaite-(La): α = 1.790(7), β = 1.798(5), γ = 1.825(8) and 2Vmeas = 60(10)°; radekškodaite-(Ce): α = 1.798(6), β = 1.806(6), γ = 1.833(8) and 2Vmeas = 65(10)°. Chemical data [wt.%, electron-microprobe; FeO:Fe2O3 by charge balance; H2O by stochiometry; radekškodaite-(La)/radekškodaite-(Ce)] are: CaO 3.40/2.74, La2O3 27.68/22.23, Ce2O3 20.39/24.30, Pr2O3 0.94/1.48, Nd2O3 1.71/3.18, ThO2 0.23/0.24, MgO 0.85/1.04, Al2O3 10.35/10.84, MnO 0.64/0.69, FeO 2.55/2.76, Fe2O3 3.12/2.57, TiO2 0.13/0.04, SiO2 26.03/26.10, F 0.10/0.09, H2O 1.62/1.63, –O=F –0.04/–0.04, total 99.70/99.89. The empirical formulae based on O28(OH,F)3 are: radekškodaite-(La): (Ca0.98Th0.01La2.75Ce2.01Nd0.16Pr0.09)Σ6.00(Al3.28Fe3+0.63Fe2+0.57Mg0.34Mn0.15Ti0.03)Σ5.00Si7.00O28[(OH)2.91F0.09]; radekškodaite-(Ce): (Ca0.79Mn0.16Th0.01Ce2.39La2.20Nd0.30Pr0.14)Σ5.99(Al3.43Fe2+0.62Fe3+0.52Mg0.42Ti0.01)Σ5.00Si7.00O28[(OH)2.92F0.08]. Both minerals are monoclinic, P21/m; the unit-cell parameters [radekškodaite-(La)/radekškodaite-(Ce)] are: a = 8.9604(3)/8.9702(4), b = 5.7268(2)/5.7044(2), c = 25.1128(10)/25.1642(13) Å, β = 116.627(5)/116.766(6)°, V = 1151.98(7)/1149.68(11) Å3 and Z = 2/2. The crystal structures are solved based on single-crystal X-ray diffraction data; R = 0.0554 [radekškodaite-(La)] and 0.0769 [radekškodaite-(Ce)]. Both minerals belong to the epidote–törnebohmite polysomatic series and represent first members of ET2-type: their structure consists of regular alternating modules, one slab of the epidote (E) structure and two slabs of törnebohmite (T). The rootname radekškodaite is given in honor of the Czech mineralogist Radek Škoda (born 1979), Associate Professor at Masaryk University, Brno, Czech Republic. The suffix-modifier -(La) or -(Ce) indicates the predominance of La or Ce among REE in the mineral.