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 (ω) to yellowish-green (ϵ), the streak is white and the crystals have a vitreous luster. Optically, the mineral is uniaxial (−), ω = 1.589(1), ϵ = 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) Å3, Z = 2 and Dcalc. = 2.68 g/cm3. The strongest lines in the X-ray powder diffraction pattern (dobs 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(K0.07Na0.01)0.08A, T2(Mg2.46Fe3+1.99Fe2+0.30Mn0.08Zn0.05A10.04Ti0.01) 4.93T1[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)+.