Johillerite from Tolbachik, Kamchatka Peninsula, Russia: Crystal-structure refinement and chemical composition

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) Angstrom, beta 100.37(3), V 936.4(2) Angstrom(3), I2/a, Z = 4 has been refined to an R-1 value of 3.2% for 1175 observed (\F-o\ greater than or equal to 4sigmaF) reflections measured with a Bruker P4 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 Na-0.77 K-0.03 Pb-0.01 Ca-0.01 Mn-0.01(2+) Zn-0.13 Mg-2.18 Cu-1.55(2+) Fe-0.26(3+) Al-0.06 (As-2.96(5+) P-0.04) O-12. The unusual feature of the johillerite structure is the A(1)' site, a square-planar-coordinated site fully occupied by Cu2+, that is displaced (1)/(4) along x. The A (2) site is similar to that in other alluaudite-group minerals, surrounded by eight O-atoms with a <A(2)-O> distance of 2.70 Angstrom, and occupied by Na plus small amounts of other large cations. The refined site-scattering values and observed <M-O> 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+.