Refinement of the crystal structures of two "protolithionites"

The crystals of 'protolithionites' 1M and 3T were refined in the space groups C2/m and P3 (sub 1) 12 with R 3.8 and 3.0%, respectively. The 1M mica has a meso-octahedral cation ordering (two octahedra identical), similar to that of muscovite but with less pronounced differences in size between M1 and M2; Li and Fe (super 2+) prefer M1 to M2 whereas Al does the opposite. Like all 3T structures refined so far, the 'protolithionite' 3T has a hetero-octahedral ordering. The two large (though not identical) octahedra M3, M1 contain all Li and most Fe, while the small (M2) octahedron is occupied by Al. This type of ordering is apparently suitable for a relatively wide range of compositions of Li-Fe micas, excluding the lepidolites and Fe-rich varieties.