Chromium-containing muscovite: crystal chemistry and XANES spectroscopy

To verify chromium enrichment of the muscovite layer, a crystal chemical and XANES study on chromium-containing muscovite crystals from the South Island of New Zealand was carried out. The crystals studied differ from those of end-member muscovite in that they display variable levels of octahedral substitutions and homovalent and heterovalent substitution of K in interlayer sites. Single-crystal X-ray diffraction data were collected for three crystals in the space group C2/c to an agreement factor (R-obs) from 0.025 to 0.033. Tetrahedral cation disorder was found for each sample and the values of <T-O> mean bond length for both tetrahedra do not depart significantly from that of the end-member muscovite-2M(1). Electron density at the M2 site is greater than that required for the ideal muscovite-2M(1) structure, and a small excess of electron density is found for two crystals in M1. As the octahedral substitution of larger cations for Al increases in the octahedral sites, the match between tetrahedral and octahedral sheets improves and tetrahedral rotation angle, alpha, decreases. XANES spectra at the Cr K-edge in these chromium containing muscovite samples exhibit octahedral symmetry. Moreover, a careful analysis of the pre-edge region shows at least two features. A qualitative fitting procedure of the pre-edge region indicates that no more than 0.5% of total Cr(III), if any, may occupy the tetrahedral site.