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

Kahlenbergite, ideally KAl11O17, and Fe-rich hibonite, CaAl10Fe2O19, are higherature 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 (P 63=mmc; a D 5:6486.1/Å; b D 22:8970.3/Å; Z D 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: Fe3C-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.