Kahlenbergite KAl₁₁O₁₇, a new <i>β</i>-alumina mineral and Fe-rich hibonite from the Hatrurim Basin, the Negev desert, Israel

Abstract

Kahlenbergite, ideally KAl11O17, and Fe-rich hibonite, CaAl10Fe2O19, are high-temperature minerals found in “olive” subunits of pyrometamorphic rocks, in the Hatrurim Basin, the Negev desert, Israel. The crystal structures of both minerals are refined using synchrotron radiation single-crystal diffraction data. The structure of kahlenbergite (P63/mmc; a=5.6486(1) Å; b=22.8970(3) Å; Z=2) exhibits triple spinel blocks and so-called R blocks. The spinel blocks show mixed layers with AlO6 octahedra and (Al0.56Fe0.44)O4 tetrahedra and kagome layers with (Al0.92Fe0.08)O6 octahedra. One-dimensional diffuse scattering observed parallel to c* implies stacking faults in the structure. Also, in one of the investigated kahlenbergite crystals additional reflections can be identified, which obviously belong to a second phase with a smaller lattice parameter c: Fe3+-rich hibonite. The structure of hibonite contains the same spinel blocks as kahlenbergite. The R blocks in hibonite contain Ca atoms, AlO5 bipyramids, and AlO6 octahedra, whereas the R blocks in kahlenbergite contain potassium atoms and AlO4 tetrahedra.