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(SO₄), dolerophanite Cu₂O(SO₄), alumoklyuchevskite K₃Cu₃AlO₂(SO₄)₄ and itelmenite Na₂CuMg₂(SO₄)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 OCu₄ 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.

A new mineral glikinite, ideally Zn₃O(SO₄)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, P2₁/m, a = 7.298(18), b = 6.588(11), c = 7.840(12) Å, β = 117.15(3)°, V = 335.4(11) ų and R₁ = 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, SO₃ 39.96, total 101.93. The empirical formula calculated on the basis of O = 9 apfu is Zn_2.07Cu_0.97S_1.98O₉ and the simplified formula is Zn₃O(SO₄)₂. Glikinite is a Zn,Cu analogue of synthetic Zn₃O(SO₄)₂. The crystal structure of glikinite is based on OZn₄ tetrahedra sharing common corners, thus forming [Zn₃O]⁴⁺ chains. Sulfate groups interconnect [Zn₃O]⁴⁺ chains into a 3D framework.

We report the crystal structures of eight new synthetic multinary Rb–Cu sulfates representing four new structure types: δ-Rb₂Cu(SO₄)₂, γ-RbNaCu(SO₄)₂, γ-RbKCu(SO₄)₂, Rb₂Cu₂(SO₄)₃, Rb₂Cu₂(SO₄)₃(H₂O), β-Rb₂Cu(SO₄)Cl₂, β-Rb₄Cu₄O₂(SO₄)₄⋅(Cu_0.83Rb_0.17Cl) and Rb₂Cu₅O(SO₄)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. Rb₂Cu₅O(SO₄)₅ is isostructural to cesiodymite, CsKCu₅O(SO₄)₅ and cryptochalcite, K₂Cu₅O(SO₄)₅. Rb₂Cu₂(SO₄)₃ also shows an example of crystallisation in the already known structure type first observed for synthetic K₂Cu₂(SO₄)₃. ‘Hydrolangbeinite’, Rb₂Cu₂(SO₄)₃(H₂O), was formed as a result of a minor hydration of the initial mixture of reagents.The minerals and synthetic framework compounds of the A₂Cu(SO₄)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(SO₄)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 A₂Cu(SO₄)₂ was first encountered during the discovery and description of saranchinaite, Na₂Cu(SO₄)₂.The discovery of β-Rb₂Cu(SO₄)Cl₂, a new monoclinic polymorph of chlorothionite, seems to be of particular interest considering the recently discovered interesting magnetic properties of synthetic K₂Cu(SO₄)X₂ (X = Cl and Br) and Na₂Cu(SO₄)Cl₂.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 Cu²⁺-centred coordination polyhedron. Until recently, such coordination was known only for the chlorothionite structure. The second is formation of ‘high-coordinate’ CuO₇ 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 Cu²⁺ 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.