• journal_title：The Canadian Mineralogist
• Contributor：Kimberly T. Tait ; Frank C. Hawthorne
• Publisher：Mineralogical Association of Canada
• Date：2004-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.42.3.717
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：42
• issue：3
• firstpage：717
• section：Articles

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, β 100.37(3)°, V 936.4(2) ų, I2/a, Z = 4 has been refined to an R₁ value of 3.2% for 1175 observed (|F_o| ⋄ 4σF) 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) 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 <A(2)–O> distance of 2.70 Å, 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⁺.