Crystal-chemical study by XANES of trioctahedral micas: the most characteristic layer silicates

A crystal-chemical study of trioctahedral micas previously characterized by single-crystal XRD has been performed by XANES spectroscopy at the Si and A1 K edges. XANES, being a local structural probe, can investigate distortion and modification of the tetrahedral sheet with increasing Fe for Mg substitution in the octahedral sheet. Comparison of XANES spectra allows determining the size of the tetrahedral site occupied by either Si or A1. The Si-O distance remains essentially unchanged whereas the A1-O distance appears to increase. The behavior may be interpreted as a tilt of the tetrahedra, initially rotated to match the ideal mica geometry, with increasing Fe substitution in the octahedral sheet.