Pertlikite, a new tetragonal Mg-rich member of the voltaite group from Madeni Zakh, Iran

Abstract

Pertlikite, a new member of the voltaite group, is the tetragonal Mg-dominant analogue of the isometric phases voltaite and zincovoltaite. At its type locality, Madeni Zakh, Iran, the mineral occurs in a pyrite-bearing trachytic eruptive rock, and probably formed after the decomposition of pyrite. Pertlikite forms single crystals up to 1 cm; the phase is associated with metavoltine, botryogen, pyrite and alunite. In contrast to voltaite and zincovoltaite, the optical properties and the single-crystal study of the structure (R1 = 0.022) confirm the tetragonal symmetry, ω to = 1.590(2), ε = 1.586(2), space group 141/acd, a 19.2080(3), c 27.2158(7) Å, V 10041.2(6) Å3, and Z = 8. The results of the chemical analysis, in combination with the Mössbauer data and the crystal-structure refinement, give the formula (K1.98 Na0.01) ∑1.99M2(Fe2+1.13 Mg0.49 Mn2+0.27 Fe3+0.10 Zn0.0)∑2.00M3 (Mg2.52 Fe3+1.49) ∑4.01M1Fe3+2.00 Al1.00 (SO4)12.00 ·18 H2O, which can be simplified to ah ideal formula: K2M2(Fe2+, Mg)2M3(Mg,Fe3+)4M1Fe3+2 Al (SO4)12· 18 H2O. The strongest eight lines of the powder pattern [d in Å(I)(hkl)] are: 5.543(28)(132), 3.396(100)(440), 3.136(21) (345), 3.038(39)(444), 2.848(31)(361), 2.534(21)(273), 2.078(29)(567) and 1.601(21)(6104). The unit-cell parameters refined from the powder data (CuKα radiation) are a 19.2167(2), c 27.1976(4) Å. The tetragonal structure of pertlikite results from Mg,Fe order at the Fe2 site. Relative to the isometric voltaite and zincovoltaite, the Fe2 site splits to create a site that is predominantly Mg (Mg3) and another that is dominantly Fe (Fe2) in pertlikite. For the three metal sites, the results of the refinement are Fe1: ≈ Fe; Fe2: Fe ≫ Mg; Mg3: Mg ≫ Fe. The H2O molecules that bond to the Al atom are extensively disordered, as has been observed in the isometric phases. The measured density is 2.59(3) g/cm3 (by pycnometer), and the calculated density is 2.56(1) g/cm3. The name honors Franz Pertlik, Professor of Mineralogy and Crystallography, University of Vienna, Austria, in recognition of his extensive work on the crystal chemistry of minerals.