M1-site occupancy in 3T and 2M(1) phengites by low temperature neutron powder diffraction: reality or artefact?

Two low temperature (15 K) neutron powder diffraction data sets from 2M(1) and 3T phengite samples were collected on the D2B beamline at ILL (Grenoble , France), to investigate the cation ordering and the occurrence of fractio nal occupants at the M1-site. Low temperature has been used in order to red uce the thermal motion. The complexity of this study poses problems to prop erly quantify our results, but on the basis of the issues from several refi nement schemes we obtained indications in favour of occupancy factors other than zero for the formally empty in dioctahedral micas M1-sites (roughly a pproximate to 0.12 and approximate to 0.18 atom per M1-site, in case of 2M( 1) and 3T polytype, respectively). The atomic displacement parameters, and the large uncertainties upon the M1-O bond lengths, suggest the large M1 si te to be positionally disordered in both polytypes. These results are to be taken up with due care as stacking faults resulting in staggering of the T OT layers might mimic a fictitious M1 site occupancy.