X-ray photoelectron diffraction studies of lepidolite

Abstract

A B S T R A C T: X-ray photoelectron diffraction data for three cleavage surfaces from a crystal of a Norwegian lepidolite containing 2.3% Rb and 3% Mn are reported and interpreted. Rubidium is shown to occupy anhydrous interlayer sites equivalent to those of potassium, and to be distributed uniformly throughout the crystals. Cleavage occurs in Mn rich regions in which the manganese(II) is located in octahedral sites essentially equivalent to those of lithium. The octahedral A1 sites can readily be distinguished from the Li,Mn sites and it is concluded that AI occupies predominantly M(2), cis sites while Li and Mn(lI) prefer M(1), trans sites. Photoelectron diffraction data also indicate that 40 + 5% of the AI is tetrahedrally coordinated, compared with a figure of ~34% deduced independently from a surface analysis.

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

“…The new XPD data are interpreted as confirming the previous suggestion (Evans & Raftery, 1982a) that this is also the case for the present lepidolite.…”

“…The previous observation (Evans & Raftery, 1982a) that the Si/Mn, Li and Si/A1 patterns were noticeably different, while retaining a considerable degree of similarity, is repeated throughout the new data, and this also is indicative of the presence of octahedral cation ordering. In the absence of ordering, these patterns should be identical except in amplitude.…”

“…The only plausible way in which this could be achieved is to segregate ions of differing size and/or scattering power between the cis M(2,3) and the trans M(1) octahedral sites: Li + is both larger and (because of its small number of electrons) of lower scattering power than A13+. However, it has been shown previously (Evans & Raftery, 1982a)--by demonstrating the identity of their XPD patterns--that Mn z+ and Li + occupy essentially equivalent sites in this lepidolite and, because Mn may be expected to have a larger scattering power than A1, this will to some extent reduce the scattering power difference between the cis and trans sites. The larger size of the Mn z § and Li + ions is thus probably the more important factor.…”

“…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 crystal is shown aligned for the collection of data for rotation about an axis parallel to a. (Evans & Raftery, 1982a). These summations improved the reproducibility of the patterns considerably.…”