STACKING FAULTS IN CELADONITE MINERALS
F Muller1, A. Plançon1 and V. A. Drits2
1 Centre de Recherche sur la Matiere
Divisée, U.M.R. Université d'Orléans - C.N.R.S., B.P. 6759,
rue de Chartres, 45067 Orléans Cedex 2, France
2 Geological Institute, Russian Academy
of Sciences, Pijzevshy Street 7, 109017 Moscow, Russia
Celadonites are 2:1 phyllosilicates i.e. they are stacks of layers formed of two tetrahedra sheets sandwiching one octahedral sheet. They are also qualified as dioctahedral, i.e. only two thirds of the octahedra are occupied by a cation, which must be trivalent; the vacant octahedron can be in the trans or in one of the two cis positions. Natural celadonites are trans-vacant [1]. The nature of the octahedral cations differ in the 2:1 dioctahedral phyllosilicates; celadonites are Fe-rich.
It is well known that the stackings of the layers in phyllosilicates are not perfect, but stacking faults occur, classically described as n x 60° rotations (n-integer). This is in agreement with Selected Area Electron Diffraction (SAED) patterns which exhibit an hexagonal distribution of the reflexions (because i) b = a/ 31/2 and ii) the layer unit-cell is C-centered; consequently exist only reflexions with h + k even). But two models for stacks containing defects can be imagined differing by the range of the interaction between layers (the reichweite, R). If R = 1 the orientation of a layer depends on the orientation of the preceding one i.e. after a rotation stacking fault the layer stacks preferentially with the orientation of the rotated layer; on the contrary for R = 0 it stacks independantly of the orientation of this layer. In other words, for sufficiently thick crystals with a few tenths probability of stacking defects between adjacent layer, one orientation dominates if R = 0 while the six orientations have a equal proportion if R = 1. The calculation of theoretical X-ray powder diffraction patterns for these two structural models lead to almost similar patterns for a 0.15 probability of stacking faults between adjacent layer which yield a good fit between experimental and theoretical patterns of natural celadonites. These results are in accordance with those of Sakharov et al. (1990, [2]).
To remove the indetermination on reichweite, celadonite samples heated at 650°C have been studied by XRD and SAED. At this temperature, a deshydroxylation reaction accompanied by an octahedral cations migration from cis- into trans-sites transforms the C-centered layer unit-cell into a primitive one [1]. As for non-heated sample, the two structural stacking models (R = 0 and 1) lead to the same agreement between experimental and calculated XRD curves for heated samples. But the solution of this problem comes from the SAED patterns. In fact the heated samples exhibit hk0 reflections distributed according a single primitive unit-cell. This rejects the structural model with R = 1 because the coexistence within a single crystal of an equal amount of layers rotated by n x 60° should have led to appearance of six primitive cells in the reciprocal space rotated with respect to each other through 60°.