Biljana Krueger (University of Innsbruck, Austria)

Kahlenbergite (IMA 2018-158) is a natural potassium β-alumina, with an empirical formula of (K0.87Mg0.09Ca0.03Ba0.01)S1(Al9.46Fe3+1.36Mg0.14Cr3+0.02Si0.02)S11O17.  It occurs in small hematite segregations within wollastonite-gehlenite rocks. The mineral association suggests formation temperature between 1000 and 1200 °C (Sharygin, 2019). Kahlenbergite forms platy, light-brown crystals, epitaxially replaced and overgrown by hibonite. The unit cell dimensions (a=5.64860(6), b=22.8970(3) Å) and space group P63/mmc of kahlenbergite corresponds to that of synthetic K β-alumina. The crystal structure was refined using synchrotron diffraction data (beamline X06DA, SLS, PSI). Compared to synthetic K β-alumina, which often shows considerable amounts of positional and occupational cation disorder, the structure of kahlenbergite is fairly simple. It exhibits a fully occupied position of the K atom at (⅔, ⅓, ¼). The structure of kahlenbergite is made of spinel blocks, divided along c into mixed (M) layers with AlO6 octahedra and (Al0.56Fe0.44)O4 tetrahedra, Kagome (K) layers with (Al0.92Fe0.08)O6, and pillar (P) layers with two AlO4 tetrahedra and K-atoms. The presented structure model of kahlenbergite describes an idealised ordered structure. All investigated crystals exhibit one-dimensional diffuse scattering. In one crystal additional reflections can be identified, which obviously belong to the Fe3+-analog of hibonite.

The structure of kahlenbergite and the Fe3+-analog of hibonite contain identical blocks, which are connected by P-layers in kahlenbergite and so-called R-layers in the Fe3+-analog of hibonite. The R-layers contain Ca atoms, AlO5-bipyramids, and further AlO6 octahedra. Therefore, the connecting layers are most likely the source of the disorder.

Sharygin, (2019) Mineralogical Magazine  83, 123–135