Publication Date:
2011-08-01
Description:
The new mineral species klochite was found in a SiO2-rich xenolith from a nepheline basanite quarry at the Kloch 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. Klochite 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 {omega} 1.594(1) and {varepsilon} 1.590(1) at 589 nm; it is pleochroic with {omega} deep blue and {varepsilon} pale yellow. Electron-microprobe analyses gave the composition (K0.78Na0.22) ({square}1.67Na0.33) (Fe3+ 1.19Fe2+ 0.45 Mn0.27Ca0.04Co0.02Ni0.01Mg0.01Ti0.01) ({square}0.25Zn2.63Fe2+ 0.10Li0.02) [Si12.00O30.00]. Klochite belongs to the milarite group, space group P6/mcc, with a 10.120(1), c 14.298(3) A, V 1268.1(3) A3 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 A (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. Klochite 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) ({square}1.83Na0.17) (Fe1.74Mn0.26) ({square}0.21Zn2.79) [Si12O30] corresponds well to the ideal end-member formula of klochite, K1{square}2 (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.
Print ISSN:
0008-4476
Topics:
Geosciences
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