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Article Dans Une Revue Mineralogical Magazine Année : 2021

Anhydrous alkali copper sulfates-a promising playground for new Cu 2+ oxide complexes: new Rb-analogues of fumarolic minerals related compounds

Oleg I Siidra
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Diana O Nekrasova
Dmitry O Charkin
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Anatoly N Zaitsev
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Artem S Borisov
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Marie Colmont
Darya V Spiridonova
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Résumé

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.
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hal-03438932 , version 1 (22-11-2021)

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Oleg I Siidra, Diana O Nekrasova, Dmitry O Charkin, Anatoly N Zaitsev, Artem S Borisov, et al.. Anhydrous alkali copper sulfates-a promising playground for new Cu 2+ oxide complexes: new Rb-analogues of fumarolic minerals related compounds. Mineralogical Magazine, 2021, pp.1-15. ⟨10.1180/mgm.2021.73⟩. ⟨hal-03438932⟩
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