Crystal chemistry of natural and synthetic trioctahedral micas: Exploring the limits of geometric crystal chemical models
Creators: name: Mercier, Patrick H. J
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2003
Subjects: subject: Mineralogy.
subject: Chemistry, Physical.
subject: Physics, General.

description: Seventy-five synthetic powder trioctahedral mica samples (between Mg, Co, Ni, and Fe end members, with different degrees of oxidation, vacancy and Al/Si contents, and including an OH/F substitution series) were studied by room-temperature powder X-ray diffraction. The iron-bearing samples were studied by 57Fe Mossbauer spectroscopy. Subsets of the samples were also characterized by scanning electron microscopy combined with energy dispersive spectroscopy, optical microscopy, X-ray fluorescence spectroscopy, and gas chromatography. Lattice parameters (refined under the 1M stacking polytype, space group C2/m) were determined for all powder samples and iron site populations ([4]Fe 3+, [6]Fe2+, and [6]Fe 2+) were obtained from Mossbauer spectroscopy. The relation (c/a)cosbeta* = 113 was found to hold exactly (within experimental error) for all synthetic powders whereas it does not hold in general for synthetic and natural 1M single-crystals. The above relation is predicted to hold for geometric home-octahedral sheets (having equal M1 and M2 site bond lengths) and not to hold for geometric meso-octahedral sheets (having unequal M1 and M2 site bond lengths). The counter-rotation of the M2 site of 1M single-crystals exactly (within experimental error) follows the geometric meso-octahedral sheet model, which, assuming a uniform octahedral sheet height and site-specific M1 and M2 bond lengths, predicts site-specific flattening angles and a counter-rotation angle for the M2 site which is uniquely determined by the bond length difference between the M1 and M2 sites. A geometric meso-octahedral 2:1 layer silicate was shown to require corrugated tetrahedral sheets composed of bond-distorted tetrahedra. Key geometric meso-octahedral distortions in 1M single-crystals were identified and elucidated: (i) intra-layer top-bottom displacements within a TOT layer; and (ii) a tetrahedral bending angle between the apical bond and the pyramidal base formed by the three basal bonds. Plots of lattice parameter b versus average-octahedral-bond-length allowed the following distinction to be made: Unoxidized divalent synthetic solid solution series tend to evolve along constant flattening-angle lines whereas trivalent octahedral cation and vacancy bearing natural single-crystals and synthetic powders follow trends with varying flattening angles. We found that the bond length of a given interlayer cationic species monotonously increases as the tetrahedral rotation angle alpha decreases in trioctahedral-1 M single-crystals. An upper limit of tetrahedral rotation of alpha = 9.5° was demonstrated to occur in trioctahedral-1M K-rich micas having an &lt;AlSi3&gt; sheet, for both synthetic powders and natural single-crystals. Other attempts at applying geometric crystal chemical models and at identifying structure-chemical relationships from structural refinement data will benefit from the perspective of our more complete and systematic approach based on pursuing simple geometrical models using 'regular' coordination polyhedrons and characteristic cation-specific bond lengths up to the limit beyond which such models are shown to necessarily breakdown because of unavoidable 'non-regular' polyhedral distortions. (Abstract shortened by UMI.)