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

Two low temperature (15 K) neutron powder diffraction data sets from 2M1 and 3T phengite samples were collected on the D2B beamline at ILL (Grenoble, France), to investigate the cation ordering and the occurence of fractional occupants at the M1-site. Low temperature has been used in order to reduce the thermal motion. The complexity of this study poses problems to properly quantify our results, but on the basis of the issues from several refinement schemes we obtained indications in favour of occupancy factors other than zero for the formally empty in dioctahedral micas M1-sites (roughly ≈ 0.12 and ≈ 0.18 atom per M1-site, in case of 2M1 and 3T polytype, respectively). The atomic displacement parameters, and the large uncertainties upon the M1-O bond lenghts, suggests the large M1 site 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 TOT layers might mimic a fictitious M1 site occupancy.