Feodosiyite is a very rare chloride mineral, just recently approved, with the formula Cu11Mg2Cl18(OH)8•16H2O. Its structure is unique.Pekov, I.V., Zubkova, N.V., Yapaskurt, V.O., Belakovskiy, D.I., Lykova, I.S., Vigasina, M.F., Britvin, S.N., Sidorov, E.G., and Pushcharovsky, D.Y. ,2015. Feodosiyite, IMA 2015-063. CNMNC Newsletter No. 28, December 2015, 1860; Mineralogical Magazine 79, 1859–1864 Feodosiyite comes from the Tolbachik volcano, famous for many rare fumarolic minerals. Chemically similar minerals, chlorides containing both copper and magnesium, include haydeeite, paratacamite-(Mg) and tondiite.
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