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PERTLIKITE, A NEW TETRAGONAL Mg-RICH MEMBER OF THE VOLTAITE GROUP FROM MADENI ZAKH, IRAN

Andreas Ertl^1,, M. Darby Dyar², John M. Hughes³, Franz Brandstätter⁴, Mickey E. Gunter⁵, Markus Prem⁶ and Ronald C. Peterson⁷

¹ Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstrasse 14, A–1090 Vienna, Austria
² Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.
³ Office of the Provost, University of Vermont, 348B Waterman Building, Burlington, Vermont 05405, U.S.A.
⁴ Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Burgring 7, A–1010 Vienna, Austria
⁵ Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, U.S.A.
⁶ Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstrasse 14, A–1090 Vienna, Austria
⁷ Department of Geological Sciences & Geological Engineering, Miller Hall, Queen’s University, Kingston, Ontario K7L 3N6, Canada

E-mail address: andreas.ertl{at}a1.net

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, = 1.590(2), = 1.586(2), space group I4₁/acd, a 19.2080(3), c 27.2158(7) Å, V 10041.2(6) ų, and Z = 8. The results of the chemical analysis, in combination with the Mössbauer data and the crystal-structure refinement, give the formula (K_1.98Na_0.01)_1.99 ^M2(Fe²⁺_1.13Mg_0.49Mn²⁺_0.27Fe³⁺_0.10Zn_0.01)_2.00 ^M3(Mg_2.52Fe³⁺_1.49)_4.01 ^M1Fe³⁺_2.00 Al_1.00 (SO₄)_12.00·18 H₂O, which can be simplified to an ideal formula: K₂ ^M2(Fe²⁺,Mg)₂ ^M3(Mg,Fe³⁺)₄ ^M1Fe³⁺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 (CuK 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.

Keywords: pertlikite, voltaite group, crystal structure, chemical analyses, Madeni Zakh, Iran.