The crystal structure and redefinition of utahite

Abstract

Utahite was first described in 1997 based mainly on powder X-ray diffraction data and electron microprobe data. No crystal structure was reported. The re-examination of utahite using single-crystal X-ray diffraction and electron microprobe analysis has shown that utahite contains essential Mg, along with Cu, Zn, Te, O and H. The missing MgO was originally attributed to additional H2O. The redefinition of utahite to MgCu2+4Zn2Te6+3O14(OH)4·6H2O from Cu2+5Zn3(Te6+O4)4(OH)8·7H2O has been accepted by the IMA–CNMNC, Proposal 20-C. Utahite is triclinic, crystallising in P $$\overline{1}$$ with the unit-cell parameters a = 5.6831(4) A, b = 8.7793(6) A, c = 9.9818(9) A, α = 95.415(7)°, β = 104.129(7)°, γ = 90.098(6)° and V = 480.65(7) A3, in good agreement with the original study. Utahite features a new framework arrangement of Cuφ6 octahedra, Znφ4 tetrahedra and Teφ6 octahedra (where φ = O or OH), with Mg(H2O)6 octahedra occupying the channel space. Two-thirds of the Te sites form Te6 + 2O10 dimers and one third form [Te6+O4(OH)2]4− octahedra, spatially separated from other Te6+ sites. Although unique, the structural framework of utahite is similar to that of leisingite, with both minerals having layers composed of Cuφ6 and Teφ6 octahedra with Mg(H2O)6 octahedra in the interlayer space; however leisingite does not contain Zn. New Raman spectroscopic data is also reported for utahite.