First structure determination of an MDO-2O mica polytype associated with a1M polytype

The structure refinement of associated phlogopite-2O and phlogopite-IM from the Khibiny massif (Kola Peninsula, Russia) is reported. Crystal data are: a = 5.2781(5), b = 9.141(1), c = 20.124(4) Angstrom, Ccmm (2O); a = 5.305( 2), b = 9.199(2), c = 10.232(4) Angstrom, beta = 100.03(2)degrees, C2/m (1M ). Least-squares refinement of single-crystal X-ray diffraction data conver ged to R1 = 0.034 (2O, 926 independent reflections) and 0.037 (1M, 677 inde pendent reflections). This is the first structure refinement of an MDO (sta ndard) mica-2O: two previous structure reports concern anandite-2O, which w as not a true polytype, having an oP (primitive orthorhombic) lattice not c ompatible with the C-centred cell common to all mica polytypes. The two phl ogopite polytypes show practically the same chemical composition (K0.95Na0. 01)(Mg2.16Fe0.34Ti0.04Mn0.04Li0.40)[Si3.40Al0.60O10][(OH)(1.35)F-0.65 ] and no cation ordering. Both polytypes are affected by stacking disorder, whic h broadens the non-family reflections (k not equal 3n). As a consequence, t he diffracted intensities of the two types are measured at different scales and large residues in the difference Fourier maps were observed at +/- b/3 along [010], [310] and [3 (1) over bar0]. These residues (Durovic effect) disappear by carrying out the refinement with separate scale factors for th e two types of reflections. On the basis of structural, morphology and zoni ng considerations the formation of the two associated polytypes is attribut ed to chemical oscillation in the crystallization mean.