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Trattnerite, (Fe, Mg)₂(Mg, Fe)₃[Si₁₂O₃₀], a new mineral of the milarite group

¹ Department of Mineralogy, Landesmuseum Joanneum, Raubergasse 10, A-8010 Graz, Austria
² Institute of Mineralogy and Petrology, Karl-Franzens-University, Universitätsplatz 2, A-8010 Graz, Austria

^* Corresponding author, e-mail: Walter.Postl{at}stmk.gv.at

The new mineral trattnerite, (Fe, Mg)₂(Mg, Fe)₃[Si₁₂O₃₀], a member of the milarite group, occurs in small cavities of a Sirich xenolith at the haüyne-nephelinite quarry Stradner Kogel, eastern Styria, Austria. Trattnerite is hexagonal and forms hypidiomorphic, short prismatic to tabular crystals up to 1 mm. Associated minerals are sanidine, tridymite, quartz, hematite, ortho-and clinopyroxene and clinoamphibole. The colour is pleocroitic deep blue () to yellowish-green (), the streak is white and the crystals have a vitreous luster. Optically, the mineral is uniaxial (–), = 1.589(1), = 1.586(1) at 589 nm. The cleavage parallel to (001) is good, poor parallel to (100), and the tenacity is brittle. Trattnerite is hexagonal, P6/mcc, with unit cell parameters a = 10.050(1) Å, c = 14.338(2) Å, V = 1254.1(1) ų, Z = 2 and D_calc. = 2.68 g/cm³. The strongest lines in the X-ray powder diffraction pattern (d_obs in Å, (hkl),I) are: 8.70, (100), 97; 7.17, (002), 100; 5.535, (102), 96; 5.026, (110), 61; 3.207, (211), 85. Two single-crystal structure refinements (R(F) = 0.028 and 0.025) of trattnerite yielded following site occupancies: the T1 tetrahedron of the double-ring unit is fully occupied by Si, the T2 tetrahedron and the A octahedron have a mixed (Mg, Fe) occupation with Fe predominantly in A and Mg in T2. The T2-O and A-O bond lengths indicate most of the iron to be in the trivalent state which is in accordance with the empirical formula: ^c(K_0.07Na_0.01)_0.08^{A, T2}(Mg_2.46Fe³⁺_1.99Fe²⁺_0.30Mn_0.08Zn_0.05A1_0.04Ti_0.01) _4.93^T1[Si₁₂O₃₀], obtained by a combination of electron microprobe data and LA-ICP-MS. The C-site located between consecutive double-ring units is occupied only by small amounts of K, the remaining 88–95% being vacancies, which lead to increased C-O distances of 3.123 and 3.118 Å. Trattnerite could be described as an end member in the series merrihueite-trattnerite with the chemical substitution Fe³⁺ + = Fe²⁺ + (K+Na)⁺.

Key-words: trattnerite, new mineral, milarite group, Mg-Fe-silicate, crystal structure, Stradner Kogel, Austria.

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