Deformation of coordination polyhedra and their sheets in phyllosilicates
Weiss, Zdenèk; Rieder, Milan; Chmielovà, Marta
European Journal of Mineralogy Volume 4 Number 4 (1992), p. 665 - 682
published: Aug 11, 1992
manuscript accepted: Jan 6, 1992
manuscript received: Jan 30, 1991
ArtNo. ESP147050404006, Price: 29.00 €
Abstract A statistical evaluation of crystal-structure data for 112 phyllosilicates (85 micas, 14 1:1 phyllosilicates, 13 chlorites) shows that the same principles govern the geometry of polyhedra and their sheets irrespective of the phyllosilicate subgroup. Regression relations for octahedral distortions and tetrahedral rotation as a function of interatomic distances are proposed. Multiple regressions were used to obtain generalized values for bond lengths and ionic radii for elements in different coordination. Using the relations derived here, it is possible to start with a chemical composition plus an anticipated cation ordering pattern and obtain estimates of the individual bond lengths and distortion characteristics of the crystal structure concerned. These can be translated into atomic coordinates and, ultimately, into peak intensities on diffraction patterns. The geometry of the coordination of interlayer cations in the micas shows considerable variation and the effective coordination number is seen to correlate with the tetrahedral rotation angle α. When the coordination drops to six, this angle may vary between 16° and 24°.