Trattnerite, (Fe,Mg)2(Mg,Fe)3Si12O30, a new mineral of the milarite group: mineral data and crystal structure

Abstract

The new mineral trattnerite, (Fe, Mg) 2 (Mg, Fe) 3 [Si 12 O 30 ], a member of the milarite group, occurs in small cavities of a Sirich xenolith at the hauyne-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) A, c = 14.338(2) A, V = 1254.1(1) A 3 , Z = 2 and D calc. = 2.68 g/cm 3 . The strongest lines in the X-ray powder diffraction pattern ( d obs in A, ( 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 0.07 Na 0.01 ) 0.08 A, T2 (Mg 2.46 Fe 3+ 1.99 Fe 2+ 0.30 Mn 0.08 Zn 0.05 A1 0.04 Ti 0.01 ) 4.93 T1 [Si 12 O 30 ], 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 A. Trattnerite could be described as an end member in the series merrihueite-trattnerite with the chemical substitution Fe 3+ + □ = Fe 2+ + (K+Na) + .