Spectroscopic and structural properties of synthetic micas on the annite-siderophyllite binary: Synthesis, crystal structure refinement, Mossbauer, and infrared spectroscopy

The effect of the incorporation of Al-Tschermak's molecule to the trioctahe dral potassium mica annite {K}[Fe-3]<AlSi3>O-10(OH)(2) on local and average structure has been investigated by hydrothermal synthesis, structure refin ement of X-ray powder diffraction data. Mossbauer and infrared spectroscopy . The various types of brackets indicate different structural sites. Sample s with compositions {K}[Fe3-xAlx]<Al1+xSi3-x>O-10(OH)(2) were prepared by h ydrothermal techniques. The maximum solubility of Al3+ is limited to x = 0. 92 at 500 degrees C and to x = 0.82 at 700 degrees C. The main factor contr olling the substitution limits is the ditrigonal distortion of the tetrahed ral rings. Lattice parameters decrease linearly with increasing Al3+ conten t of the mica. A considerable decrease of M2-O and nearly no change of M1-O bond lengths with increasing Al3+ contents is indicative of preferred occu pation of the M2 site by [Al3+]. Changes in K-O distances are also very pro nounced and reflect the ditrigonal distortion of the tetrahedral sheet. The bimodal ferrous quadrupole splitting distribution (QSD) in annite, extract ed from Mossbauer spectra, becomes narrower and more centered around 2.60 m m/s with increasing Al3+ contents, and its evolution suggests an increasing deviation from ideal octahedral coordination of Fe by O, illustrated by th e increasing octahedral flattening angle psi. The population of individual QSD components proves that it is impossible to resolve cis and trans M-site s in micas by Mossbauer spectroscopy. In the hydroxyl stretching region, up to 7 bands are observed in the infra-red spectra which correspond to OH gr oups adjacent to 3 Fe2+ (N-bands), to OH groups coordinated by Fe2+, Al3+, and Fe3+ (I-bands) and to configurations having one octahedral vacancy (V-b ands). N- and I-type bands are shifted toward lower wavenumbers with increa sing Al3+ content because of increasing OH ... O-tet interactions.