Cation ordering in lepidolite and biotite studied by X-ray photoelectron diffraction

Comprehensive X-ray photoelectron diffraction (XPD) studies have revealed octahedral cation ordering in a 1M lepidolite and in a 1M biotite. In the lepidolite (from Høydaler, Telemark, Norway), all three octahedral sites are differentiated, with Li and Mn concentrated in the trans M(1) site and Al in cis M(3); M(2) contains all three cations. In the biotite, the major cations do not order but Ti, present at only 1·2% concentration, preferentially occupies the two cis sites, information which is available uniquely from XPD. Long-range tetrahedral ordering should be detectable by XPD, but was not found in either mica.