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 has enabled the characterization of the thermal decomposition of krohnkite and provided an insight into the high-temperature behaviour of other krohnkite-type materials. Saranchinaite is monoclinic, P2(1), a = 9.0109(5). b = 15.6355(8), c = 10.1507(5) , beta= 107.079(2)degrees, V= 1367.06(12) (3), Z = 8 and R-1 = 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 similar to 2.9-3.0, resulting in [4+1+2] CuO7 polyhedra. Each of the Cu-centred polyhedra shares common comers with SO4 tetrahedra resulting in a [Cu-4(SO4)O-8 framework with a complex channel system occupied by Na atoms. Saranchinaite is sensitive to moisture and transforms into krohnkite within one week when exposed to open air at 87% relative humidity and 25 degrees C. High-temperature X-ray diffraction studies were performed for both krohnkite (from La Vendida mine. Antofagasta Region, Chile) and saranchinaite. During thermal expansion krohnkite retains its strongly anisotropic character up to its full dehydration and the formation of saranchinaite at similar to 200 degrees C, which then transforms back into krohnkite after exposure to open air. The thermal expansion of saranchinaite is more complex than that of krohnkite. Saranchinaite is stable up to 475 degrees C with subsequent decomposition into tenorite CuO, thenardite Na2SO4 and unidentified phases.