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

Abstract

A B S T RA C T : 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 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; 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, 1980aRaftery, , 1982b, and the direct quantitative determination of the aluminium distribution between tetrahedral and octahedral sites was demonstrated (Evans & Raftery, 1980b). It was suggested (Evans & Raftery, 1982a) that XPD might also be capable of characterizing cation ordering in micas--some of the data were only readily intelligible on the assumption that cation ordering could produce detectable effects in XPD--but much further work remained to be done.A far more comprehensive study of the same two micas using more sophisticated data collection techniques has now been completed; these new procedures are fully described elsewhere (Ash et al., 1987). In this paper only those aspects which relate to cation ordering in micas are discussed; XPD is now a powerful tool for the discovery and limited characterization of this phenomenon. The information accessible does not extend to bond lengths and angles, but, for example, differentiation of the three octahedral cation sites can now readily be made (as shown below). Moreover, although large crystals are still required, the degree of crystal perfection necessary is lower than for normal X-ray methods. EXPERIMENTALThe c-axis repeat distances were found by Cu-K~ oscillation photographs to be consistent with a single-layer repeat in both micas. Close approximations to a mirror plane were evident * Present address: English China

Angle-Resolved X-Ray Photoelectron Studies of Cleavage in Chlorites

“…The essential constancy of the Si2s/Si2p and Mg2s/Mg2p intensity ratios was first checked for each data set. Random variations of ---5% were typical, as in previous studies (Ash et al 1987(Ash et al , 1988Evans et al 1979a; 1 Derived from the mole ratios obtained from the XPS peak areas (see note 3) by: 1) normalizing so that the sum of the charges on the detected cations becomes 28, for charge balance against O10(OH)8; 2) assigning all the Si and as much of the A1 as required to fill all the tetrahedral sites; and 3) then assigning the balance of the cations to the octahedral sites. Since there are fewer than 10 cations for the 28 positive charges, there are octahedral vacancies, indicated by [~, in all the surface formulae.…”

“…A detailed interpretation of these XPD patterns is more difficult than it was for the micas (Ash et al 1987) because of the multiplicity of possible octahedral sites when all the possibilities for cation ordering are taken into account. However, some conclusions can be drawn simply by considering the symmetry properties of the patterns.…”

“…However, information on atomic identities is not so readily achieved, and the element-specific surface spectroscopies still have an important role to play. Over a number of years we have pioneered the use of angle-resolved X-ray photoelectron spectroscopy (XPS) in studies of the micas, developing new routes to the characterization of cation ordering patterns (Adams et al 1978;Evans et al 1979a;Ash et al 1987Ash et al , 1988. The information obtained from XPS strictly relates only to the outermost few repeat units of the crystals, although for many micas there has been evidence indicating that the surface and bulk compositions are closely similar.…”