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 ( R 1 = 0.022) confirm the tetragonal symmetry, ω = 1.590(2), e = 1.586(2), space group I 4 1 / acd , a 19.2080(3), c 27.2158(7) A, V 10041.2(6) A 3 , and Z = 8. The results of the chemical analysis, in combination with the Mossbauer data and the crystal-structure refinement, give the formula (K 1.98 Na 0.01 ) ∑1.99 M 2 (Fe 2+ 1.13 Mg 0.49 Mn 2+ 0.27 Fe 3+ 0.10 Zn 0.01 ) ∑2.00 M 3 (Mg 2.52 Fe 3+ 1.49 ) ∑4.01 M 1 Fe 3+ 2.00 Al 1.00 (SO 4 ) 12.00 ·18 H 2 O, which can be simplified to an ideal formula: K 2 M 2 (Fe 2+ ,Mg) 2 M 3 (Mg,Fe 3+ ) 4 M 1 Fe 3+ 2 Al (SO 4 ) 12 ·18 H 2 O. The strongest eight lines of the powder pattern [ d in A( 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 (Cu K α radiation) are a 19.2167(2), c 27.1976(4) A. The tetragonal structure of pertlikite results from Mg,Fe order at the Fe 2 site. Relative to the isometric voltaite and zincovoltaite, the Fe 2 site splits to create a site that is predominantly Mg ( Mg 3) and another that is dominantly Fe ( Fe 2) in pertlikite. For the three metal sites, the results of the refinement are Fe 1: . Fe; Fe 2: Fe >> Mg; Mg 3: Mg >> Fe. The H 2 O 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/cm 3 (by pycnometer), and the calculated density is 2.56(1) g/cm 3 . 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.