JOHILLERITE FROM TOLBACHIK, KAMCHATKA PENINSULA, RUSSIA: CRYSTAL-STRUCTURE REFINEMENT AND CHEMICAL COMPOSITION

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Johillerite from a fumarole in the North Breach of the Great Fissure Tolbachik eruption (GFTE), Kamchatka Peninsula, Russia, monoclinic, a 6.7520(14), b 12.739(3), c 11.068(2) A, β 100.37(3)°, V 936.4(2) A3, I 2/ a , Z = 4 has been refined to an R 1 value of 3.2% for 1175 observed (| F o| ⋄ 4σ F ) reflections measured with a Bruker P 4 diffractometer equipped with a CCD detector. The crystal used for the collection of the X-ray intensity data was subsequently analyzed with an electron microprobe; the resulting structural formula is Na0.77 K0.03 Pb0.01 Ca0.01 Mn0.01 2+ Zn0.13 Mg2.18 Cu1.55 2+ Fe0.26 3+ Al0.06 (As2.96 5+ P0.04) O12. The unusual feature of the johillerite structure is the A (1)’ site, a square-planar-coordinated site fully occupied by Cu2+, that is displaced ¼ along x. The A (2) site is similar to that in other alluaudite-group minerals, surrounded by eight O-atoms with a distance of 2.70 A, and occupied by Na plus small amounts of other large cations. The refined site-scattering values and observed distances at the M (1) and M (2) sites are in accord with occupancy primarily by Mg, with Cu2+ at M (1) and Cu2+ + Fe3+ + Al at M (2). Relative to the compositions of the arsenate minerals arseniopleite and caryinite, Cu2+ has replaced Ca in the formula of johillerite. In arseniopleite and caryinite, Ca (+ Na) at the A (1) site is [7]- or [8]-coordinated, and the resulting sites occur in chains of edge-sharing coordination polyhedra extending in the a direction. Rather than replacing Ca at the center of the A (1) polyhedron, Cu2+ in johillerite occurs close to the edge shared between adjacent polyhedra, adopting square-planar coordination that is common for Cu2+.