BIOTITE STUDIED BY X-RAY PHOTOELECTRON DIFFRACTION

octahedral cation ordering in a 1M lepidolite and in a 1M biotite. In the lepidolite (from H~ydaler, Telemark, Norway), all three octahedral sites are differentiated, with Li and Mn concentrated in the trans M(1) site and AI 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~o 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. A few years ago the first detailed X-ray photoelectron diffraction (XPD) studies of mica crystals were reported (Evans et al., 1979; Evans & Raftery, 1980a,b, 1982a,b); these revealed that XPD could provide information on details of their structure which were difficult or impossible to obtain by established techniques. In particular, information regarding the locations of substitutional ions present at low concentration was accessible (Mn and Rb in lepidolite--Evans & Raftery, 1982a; Ti and Na in biotite--Evans & Raftery, 1980a, 1982b), and the direct quantitative determination of the aluminium distribution between tetrahedral nd octahedral sites was demonstrated (Evans & Raftery, 1980b). It was suggested (Evans & Raftery, 1982a) that XPD might also be capable of characterizing catio