The new mineral species klöchite was found in a SiO2-rich xenolith from a nepheline basanite quarry at the Klöch volcano, Bad Radkersburg district, Styria, Austria. It forms hexagonal tabular crystals flattened on {001}, with a diameter up to 0.3 mm and a maximal thickness of 0.1 mm. It is associated with sanidine, quartz, diopside, fluoro-richterite, titanite, enstatite, forsterite, ilmenite and mottramite. Klöchite is blue, has a vitreous luster, is devoid of cleavage, and is translucent and non-fluorescent; the calculated density is 3.007 g cm−3. Optically, it is uniaxial negative with ω 1.594(1) and ɛ 1.590(1) at 589 nm; it is pleochroic with ω deep blue and ɛ pale yellow. Electron-microprobe analyses gave the composition (K0.78Na0.22) (□1.67Na0.33) (Fe3+1.19Fe2+0.45 Mn0.27Ca0.04Co0.02Ni0.01Mg0.01Ti0.01) (□0.25Zn2.63Fe2+0.10Li0.02) [Si12.00O30.00]. Klöchite belongs to the milarite group, space group P6/mcc, with a 10.120(1), c 14.298(3) Å, V 1268.1(3) Å3 from single-crystal XRD data at 100 K, and Z = 2. The strongest eight lines in the calculated X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 7.149(100)(002), 5.540(43)(102), 4.130(40) (112), 3.736(70)(202), 3.227(67)(211), 2.919(40)(114), 2.770(68)(204) and 2.530(43)(220). From single-crystal X-ray data, the structure has been refined to R = 0.023. Klöchite has the typical structural units of the double-ring silicates of the milarite structure-type. Zinc dominates at the T2 position with a small vacancy defect at this site, and Fe dominates at the A site, which also contains Mn. The C site is fully occupied by K and Na, whereas the B site (Na) is nearly empty owing to electroneutrality of the chemical formula and the presence of Fe3+ at the A site, which is indicated from bond-valence calculations. From the structure refinement, the formula (K0.81Na0.19) (□1.83Na0.17) (Fe1.74Mn0.26) (□0.21Zn2.79) [Si12O30] corresponds well to the ideal end-member formula of klöchite, K12 (Fe2+1Fe3+1) Zn3 [Si12O30], with vacancy dominant at the B site. Angular distortions at T2 and A of selected minerals of the milarite group show two separate correlations, for the Be-, B-dominant and Li-, Zn-, and Mg-dominant minerals of this group.

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