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Complex hydrogen bonding and thermal behaviour over a wide temperature range of kainite KMg(SO4)Cl⋅2.75H2O
- Artem S. Borisov, Oleg I. Siidra, Valery L. Ugolkov, Alexey N. Kuznetsov, Vera A. Firsova, Dmitri O. Charkin, Natalia V. Platonova, Igor V. Pekov
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- Journal:
- Mineralogical Magazine / Volume 86 / Issue 1 / February 2022
- Published online by Cambridge University Press:
- 18 February 2022, pp. 37-48
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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.
Anhydrous alkali copper sulfates – a promising playground for new Cu2+ oxide complexes: new Rb-analogues of fumarolic minerals.
- Oleg I. Siidra, Diana O. Nekrasova, Dmitry O. Charkin, Anatoly N. Zaitsev, Artem S. Borisov, Marie Colmont, Olivier Mentré, Darya V. Spiridonova
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- Mineralogical Magazine / Volume 85 / Issue 6 / December 2021
- Published online by Cambridge University Press:
- 29 September 2021, pp. 831-845
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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⋅(Cu0.83Rb0.17Cl) 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.
Evolution of fumarolic anhydrous copper sulfate minerals during successive hydration/dehydration
- Oleg I. Siidra, Artem S. Borisov, Dmitri O. Charkin, Wulf Depmeier, Natalia V. Platonova
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- Mineralogical Magazine / Volume 85 / Issue 2 / April 2021
- Published online by Cambridge University Press:
- 02 February 2021, pp. 262-277
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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 O2– 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.