Active volcanic fumaroles are one of the most spectacular natural objects in terms of mineral diversity. The Great Tolbachik Fissure Eruption (GTFE) (Kamchatka) fumaroles are renowned for its exceptional number of mineral species. The total number of mineral species that have been reliably identified from this particular locality exceeds 600, which is approximately 10 % of all known minerals to date. In this study, we employ a comprehensive approach (bulk chemistry, microprobe analysis, powder X-ray diffraction, HR X-ray computed tomography, and 34S, 18O, and 65Cu isotope measurements) to study the distribution of primary exhalation and secondary mineral assemblages and to reveal the driving factors responsible for the unique mineral diversity in the Yadovitaya fumarole. High oxygen fugacity, interaction of minerals with atmospheric oxygen and water from seasonal precipitation (leading to abundant hydrated mineral associations), temperature conditions controlling the spatial distribution of mineral-forming components, gas-rock interactions, and basaltic scoria morphology perfect for the crystallization of various minerals are some of the factors revealed. The Yadovitaya fumarole cross-section consists of 12 zones, each of which has characteristic mineral assemblages. The temperature ranges from 400 °C at the bottom to 30 °C at the surface. The zonal cross-section and sequential formation of exhalative fumarolic minerals are associated with the gradual distribution of a number of key mineral-forming elements, primarily S, Cu, Pb, Zn, and As. Furthermore, characteristic patterns for Rb and Cs are observed. The significance of atmospheric oxygen (δ18O = 23 ‰) and the impact of δ18O fractionation versus δ65Cu fractionation on the formation of exotic fumarolic minerals is discussed.
Abstract A new mineral glikinite, ideally Zn 3 O(SO 4 ) 2 , was found in high-temperature exhalative mineral assemblages in the Arsenatnaya fumarole, Second scoria cone of the Great Tolbachik Fissure Eruption (1975–1976), Tolbachik volcano, Kamchatka Peninsula, Russia. Glikinite is associated closely with langbeinite, lammerite-β, bradaczekite, euchlorine, anhydrite, chalcocyanite and tenorite. It is monoclinic, P 2 1 /m , a = 7.298(18), b = 6.588(11), c = 7.840(12) Å, β = 117.15(3)°, V = 335.4(11) Å 3 and R 1 = 0.046. The eight strongest lines of the powder X-ray diffraction pattern [ d in Å ( I ) ( hkl )] are: 6.969(56)(00 $\bar{1}$ ), 3.942(52)(101), 3.483(100)(00 $\bar{2}$ ), 3.294(49)(020), 2.936(43)(120), 2.534(63)(201), 2.501(63)(20 $\bar{3}$ ) and 2.395(86)(02 $\bar{2}$ ). The chemical composition determined by electron-microprobe analysis is (wt.%): ZnO 42.47, CuO 19.50, SO 3 39.96, total 101.93. The empirical formula calculated on the basis of O = 9 apfu is Zn 2.07 Cu 0.97 S 1.98 O 9 and the simplified formula is Zn 3 O(SO 4 ) 2 . Glikinite is a Zn,Cu analogue of synthetic Zn 3 O(SO 4 ) 2 . The crystal structure of glikinite is based on OZn 4 tetrahedra sharing common corners, thus forming [Zn 3 O] 4+ chains. Sulfate groups interconnect [Zn 3 O] 4+ chains into a 3D framework.
Abstract Hydration processes of primary anhydrous minerals as well as dehydration of the hydrated phases are relevant not only for answering geochemical and petrological questions, but are also interesting in the context of the theory of the ‘Evolution of minerals’. Our study of the evolution of anhydrous exhalative sulfates in hydration and dehydration processes has demonstrated the complexity of the processes for a number of minerals from the active high-temperature fumaroles of Tolbachik volcano (chalcocyanite Cu(SO 4 ), dolerophanite Cu 2 O(SO 4 ), alumoklyuchevskite K 3 Cu 3 AlO 2 (SO 4 ) 4 and itelmenite Na 2 CuMg 2 (SO 4 ) 4 ). Hydration and dehydration experiments were carried out for all four minerals using powder X-ray diffraction. A typical structural characteristic of several anhydrous copper sulfate minerals of fumarolic origin is the presence of oxygen-centred OCu 4 tetrahedra. These are absent in the structures of all known hydrated minerals or synthetic compounds of the class under consideration. Hydration of minerals initially containing O 2– anions as part of oxocomplexes, proceeds with sequential formation of a large series of hydroxysalts. On the contrary, hydration of itelmenite with its relatively complex ‘initial’ structure, but without additional oxygen atoms that are strong Lewis bases, results in formation of simpler hydrates. The lower the temperature and the larger the excess of water, the stronger the tendency of the cations to adopt higher hydration numbers thus outcompeting the sulfate anions as ligands. Ultimately, the water molecules completely expel the bridging sulfate anions from the metal coordination sphere yielding relatively simple fully hydrated structures.
Abstract Kainite, KMg(SO 4 )Cl⋅2.75H 2 O, 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, C 2/ m , a = 19.6742(2), b = 16.18240(10), c = 9.49140(10) Å, β = 94.8840(10)°, V = 3010.86(5) Å 3 and Z = 16. The structure was refined to R 1 = 0.0230 for 3080 unique observed reflections with | F o | ≥ 4σ F . 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 Mg 2+ 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(SO 4 )Cl⋅2.75H 2 O. 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 K 2 Mg 2 (SO 4 ) 3 , KCl and K 2 SO 4 . The thermal expansion was calculated, which for kainite has a character typical for monoclinic crystals and similar to the compressibility tensor described earlier.
Abstract We report the crystal structures of eight new synthetic multinary Rb–Cu sulfates representing four new structure types: δ-Rb 2 Cu(SO 4 ) 2 , γ-RbNaCu(SO 4 ) 2 , γ-RbKCu(SO 4 ) 2 , Rb 2 Cu 2 (SO 4 ) 3 , Rb 2 Cu 2 (SO 4 ) 3 (H 2 O), β-Rb 2 Cu(SO 4 )Cl 2 , β-Rb 4 Cu 4 O 2 (SO 4 ) 4 ⋅(Cu 0.83 Rb 0.17 Cl) and Rb 2 Cu 5 O(SO 4 ) 5 . The determination of their crystal structures significantly expands the family of anhydrous copper sulfates. Some of the anhydrous rubidium copper sulfates obtained turned out to be isostructural to known compounds and minerals. Rb 2 Cu 5 O(SO 4 ) 5 is isostructural to cesiodymite, CsKCu 5 O(SO 4 ) 5 and cryptochalcite, K 2 Cu 5 O(SO 4 ) 5 . Rb 2 Cu 2 (SO 4 ) 3 also shows an example of crystallisation in the already known structure type first observed for synthetic K 2 Cu 2 (SO 4 ) 3 . ‘Hydrolangbeinite’, Rb 2 Cu 2 (SO 4 ) 3 (H 2 O), was formed as a result of a minor hydration of the initial mixture of reagents. The minerals and synthetic framework compounds of the A 2 Cu(SO 4 ) 2 series demonstrate a vivid example of morphotropism with the formation of structural types depending on the size of the cations residing in the cavities of the [Cu(SO 4 ) 2 ] 2– open framework. To date, five types (α, β, γ, δ and ε) can be distinguished. We propose to call this series of compounds ‘saranchinaite-type’, as the stoichiometry A 2 Cu(SO 4 ) 2 was first encountered during the discovery and description of saranchinaite, Na 2 Cu(SO 4 ) 2 . The discovery of β-Rb 2 Cu(SO 4 )Cl 2 , a new monoclinic polymorph of chlorothionite, seems to be of particular interest considering the recently discovered interesting magnetic properties of synthetic K 2 Cu(SO 4 ) X 2 ( X = Cl and Br) and Na 2 Cu(SO 4 )Cl 2 . In these new structural architectures, a number of features have been revealed that were seldom observed previously. The first is the bidentate coordination of the sulfate tetrahedron via edge-sharing with the Cu 2+ -centred coordination polyhedron. Until recently, such coordination was known only for the chlorothionite structure. The second is formation of ‘high-coordinate’ CuO 7 polyhedra. The structures of the new compounds suggest that such coordination is not in fact so uncommon, at least among anhydrous alkali copper sulfates. All of the described features clearly indicate the importance of further systematic studies of anhydrous copper-sulfate systems. Their exploration, particularly of the new copper-oxide substructures with new coordination environments, is highly likely to lead to new potentially interesting magnetic properties due to the unusual arrangements of magnetically active Cu 2+ cations. In addition to experimental details on the synthesis of rubidium analogues of anhydrous potassium and sodium sulfates, this work also provides an analysis and a brief review of the geochemistry of rubidium in volcanic environments.
Abstract Thermal behavior of vergasovaite, ideally Cu3O(SO4)(MoO4), and its synthetic analog has been studied by high-temperature single-crystal X-ray diffraction in the temperature range of 300–1100 K. According to EMPA results, the empirical formulas are (Cu2.36Zn0.61)Σ2.97O[(Mo0.91S0.08V0.04)Σ1.03O4](SO4) for vergasovaite and Cu2.97O[(Mo0.92S0.09)Σ1.01O4](SO4) for its synthetic analog. The mineral is stable up to 950 ± 15 K; at 975 K, the unit-cell parameters and volume increase abruptly due to topotactic transformation of vergasovaite to cupromolybdite, Cu3O(MoO4)2. The transformation is accompanied by loss of sulfur (and excess copper) without destruction of the crystal. The thermal expansion of the vergasovaite structure is strongly anisotropic, being minimal along the [O2Cu6]8+ chains comprised of vertex-sharing OCu4 tetrahedra. This peculiar thermal behavior can be explained by the anisotropy of bond-length evolution in the Cu1O6 and Cu3O6 octahedra and the flexibility of the S-O-Cu and Mo-O-Cu bond angles. Synthetic Zn- and V-free analogs demonstrate negative thermal expansion at 425–625 K and melt at as low temperature as 700 K with no indication of transformation or recrystallization at least below 1200 K. The topotactic transformation observed in vergasovaite may have important implications for the design of novel materials and for understanding the alteration processes of copper minerals.
