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) Å, 100.37(3)°, V 936.4(2) Å3, I2/a, Z = 4 has been refined to an R1 value of 3.2% for 1175 observed (|Fo| ≥ 4 F) 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 Na0.77 K0.03 Pb0.01 Ca0.01 Mn0.01 Zn0.13 Mg2.18 Cu1.55 Fe0.26 Al0.06 (As2.96 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 1⁄4 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 Å, 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+.