Labuntsovite: Solid solutions and features of the crystal structure

Our study of solid solution in labuntsovite and related monoclinic minerals is based on chemical and electron-microprobe data for 44 samples (19 publi shed and 25 new). The crystal structures of three samples with distinct com positions, recently refined, are here compared with the published structura l data. Correlations between the composition and infrared spectral paramete rs for labuntsovite and labuntsovite-type minerals were determined. The gen eral formula of minerals with labuntsovite structure (Z=1) is Al(4)A2(4)A3( 4-2x) [M2(x)(H2O)(2x)] [Ml(8)(O,OH)(8)] [Si4O12](4). nH(2)O, where 0 < x < 2, Al = Na, Ca, rectangle; A(2) = K, Na, H3O, rectangle, A3 = K, Ba, Sr, H3 O, rectangle, M1 = Ti, Nb; M2 = Mn2+, Fe2+, Mg, Zn, rectangle, usually, n a pproximate to 8. Correlations are observed among some components (large cat ions such as K, Ba, etc., R2+ octahedrally coordinated cations, total numbe r of octahedral cations). The A3 and M2 positions are linked and occupied a ccording to alternative mechanisms, e.g., A3 [(2K)] + M2 [rectangle] o A3 [ 2H(2)O] + M2 [Mn,Fe,Mg]. Four subgroups of monoclinic labuntsovite-type min erals are distinguished. There are those with the labuntsovite structure an d a predominance of vacancies in the M2 octahedra; more than 50% of the A3 sites are occupied by large cations. 2) There are minerals with the labunts ovite structure having a predominance of Fe, Mg, Mn, or Zn in the M2 positi ons (linking octahedra) and a predominance of H2O in the A3 position; their formula is: Na4K4(K,Ba...)(4-2x) [R-x(2+)(H2O)(2x)] [Ti-8(O,OH)(8)] [Si4O1 2](4). nH(2)O, where 1 < x < 2, and R = Mn, Fe, Mg, Zn. 3) There are minera ls with vacant M2 positions and more than three positions of extra-framewor k cations (vuoriyarvite and its analogues). An extensive solid-solution inv olving Ti and Nb, Na and K is typical for these phases. 4) Finally, there a re minerals with the kuzmenkoite structure, which show a predominance of Mn , Fe, Mg, or Zn in the M2 positions, and two positions of extra;framework c ations. A low content of Na is typical for minerals of this subgroup. Their formula (Z = 1) is: K(4)rectangle(4) [(Mn,Fe...)(2)(H2O)(4)] [(Ti,Nb)(8)(O H,O)(8)] [Si4O12](4). nH(2)O.