Two new isotypic minerals cryptochalcite, K2Cu5O(SO4)5, and cesiodymite, CsKCu5O(SO4)5, were found in fumarole sublimates at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. They are associated with one another and with euchlorine, chalcocyanite, alumoklyuchevskite, anglesite, fedotovite, wulffite, langbeinite, aphthitalite, steklite, hematite. Both minerals are, visually indistinguishable from one another and form coarse tabular or prismatic crystals or grains up to 0.3 mm. They are brittle, transparent, light green to green, with vitreous lustre. Calculated densities for cryptochalcite and cesiodymite are 3.41 and 3.59 g cm−3, respectively. Both are optically biaxial (–); cryptochalcite: α 1.610(3), β 1.632(4), γ 1.643(4), 2Vmeas 65(5)°; cesiodymite: α 1.61(1), β 1.627(4), γ 1.635(4), 2Vmeas 70(10)°. The empirical formulae, based on 21 O apfu, are: cryptochalcite, (K1.83Na0.09Rb0.09Cs0.06)Σ2.07(Cu3.86Zn1.02Mg0.19)Σ5.07S4.97O21; cesiodymite, (K1.14Rb0.16Cs0.73)Σ2.03(Cu3.69Zn1.33)Σ5.02S4.99O21. Both minerals are triclinic, P-1, Z = 4; cryptochalcite: a 10.0045(3), b 12.6663(4), c 14.4397(5) Å, α 102.194(3), β 101.372(3), γ 90.008(3)°, V 1751.7(1) Å3; cesiodymite: a 10.0682(4), b 12.7860(7), c 14.5486(8) Å, α 102.038(5), β 100.847(4), γ 89.956(4)°, V 1797.5(2) Å3. Their crystal structures are topologically identical and have been refined from single-crystal X-ray diffraction data to final agreement indices R = 0.0503 for cryptochalcite and 0.0898 for cesiodymite. They are based upon the heteropolyhedral {Cu5O(SO4)5}2− framework composed by two types of alternating Cu2+-S-O polyhedral layers {Cu2(SO4)2}0 and {Cu3O(SO4)}2+ connected via SO4 tetrahedra. K and Cs cations occupy sites in the tunnels of the framework. Cryptochalcite is named from Greek κρυπτóς, concealed, and χαλκóς, copper: it is associated with other green copper oxysulfates and is visually very similar to them. Cesiodymite is named from cesium and Greek δίδυµος, a twin brother, being a Cs-K-ordered analogue of cryptochalcite.

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