Cryptochalcite, K (sub 2) Cu (sub 5) O(SO (sub 4) ) (sub 5) , and cesiodymite, CsKCu (sub 5) O(SO (sub 4) ) (sub 5) , two new isotypic minerals and the K-Cs isomorphism in this solid-solution series

Two new isotypic minerals cryptochalcite, K (sub 2) Cu (sub 5) O(SO (sub 4) ) (sub 5) , and cesiodymite, CsKCu (sub 5) O(SO (sub 4) ) (sub 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: alpha 1.610(3), beta 1.632(4), gamma 1.643(4), 2Vmeas 65(5) degrees ; cesiodymite: alpha 1.61(1), beta 1.627(4), gamma 1.635(4), 2Vmeas 70(10) degrees . The empirical formulae, based on 21 O apfu, are: cryptochalcite, (K (sub 1.83) Na (sub 0.09) Rb (sub 0.09) Cs (sub 0.06) ) (sub Sigma 2.07) (Cu (sub 3.86) Zn (sub 1.02) Mg (sub 0.19) ) (sub Sigma 5.07) S (sub 4.97) O (sub 21) ; cesiodymite, (K (sub 1.14) Rb (sub 0.16) Cs (sub 0.73) ) (sub Sigma 2.03) (Cu (sub 3.69) Zn (sub 1.33) ) (sub Sigma 5.02) S (sub 4.99) O21. Both minerals are triclinic, P-1, Z = 4; cryptochalcite: a 10.0045(3), b 12.6663(4), c 14.4397(5) Aa, alpha 102.194(3), beta 101.372(3), gamma 90.008(3) degrees , V 1751.7(1) Aa (super 3) ; cesiodymite: a 10.0682(4), b 12.7860(7), c 14.5486(8) Aa, alpha 102.038(5), beta 100.847(4), gamma 89.956(4) degrees , V 1797.5(2) Aa (super 3) . 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 {Cu (sub 5) O(SO (sub 4) ) (sub 5) } (super 2) framework composed by two types of alternating Cu (super 2+) -S-O polyhedral layers {Cu (sub 2) (SO (sub 4) ) (sub 2) } (super 0) and {Cu (sub 3) O(SO (sub 4) )} (super 2+) connected via SO (sub 4) tetrahedra. K and Cs cations occupy sites in the tunnels of the framework. Cryptochalcite is named from Greek kappa rho upsilon pi tau zeta , concealed, and chi alpha lambda kappa ozeta , copper: it is associated with other green copper oxysulfates and is visually very similar to them. Cesiodymite is named from cesium and Greek delta idelta upsilon mu omicron zeta , a twin brother, being a Cs-K-ordered analogue of cryptochalcite.