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Free Content PREPUBLICATION: Saranchinaite, Na2Cu(SO4)2, A New Exhalative Mineral from Tolbachik Volcano, Kamchatka, Russia, and a product of the Reversible Dehydration of Kröhnkite, Na2Cu(SO4)2(H2O)2

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The new mineral saranchinaite, ideally Na2Cu(SO4)2, was found in sublimates of the Saranchinaitovaya fumarole, Naboko Scoria Cone, Tolbachik volcano, Kamchatka, Russia. Its discovery and study allowed to characterize the thermal decomposition of kröhnkite and provided an insight into the high-temperature behavior of other kröhnkite-type materials. Saranchinaite is monoclinic, P21, a = 9.0109(5), b = 15.6355(8), c = 10.1507(5) Å, β = 107.079(2)º, V = 1367.06(12) Å3, Z = 8, R1 = 0.03. Saranchinaite is a unique mineral in that two of its four independent Cu sites display a very unusual Cu2+ coordination environment with two weak Cu-O bonds of ~2.9-3.0 Å, resulting in [4+1+2] CuO7 polyhedra. Each the Cu-centered polyhedra shares common corners with SO4 tetrahedra resulting in a [Cu4(SO4)8]8- framework with a complex channel system occupied by Na atoms. Saranchinaite is sensitive to moisture and transforms into kröhnkite within one week when exposed to open air at 87% relative humidity and 25°C. High-temperature X-ray diffraction studies were performed for both kröhnkite (from La Vendida mine, Antofagasta Region, Chile) and saranchinaite.The thermal expansion of kröhnkite retains its strongly anisotropic character up to its full dehydration and formation of saranchinaite at about 200°C. Saranchinaite thus formed transforms back into kröhnkite after exposure to open air. The thermal expansion of saranchinaite is more complex than that of kröhnkite. Saranchinaite is stable up to 475°C with subsequent decomposition into tenorite, CuO, thénardite, Na2SO4, and unidentified phases.
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Keywords: Tolbachik Fissure eruption 2012-2013; copper coordination; dehydration; framework structures; high-temperature x-ray diffraction; hydration; kröhnkite; new mineral; saranchinaite; sulfates

Affiliations: 1: St. Petersburg State University 2: Institut für Geowissenschaften der Universität Kiel 3: Fersman Mineralogical Museum

Appeared or available online: Mon Jun 19 12:30:00 UTC 2017

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