ELECTRONIC ABSORPTION-SPECTRA OF CHROMIUM-BEARING AMESITE

The crystal field parameters of Cr3+-ions in amesite (Mg2Al)[SiAlO5(OH )(4)], calculated from polarized optical absorption spectra (Dq = 1782 cm(-1), B = 694 cm(-1), beta = 0.756, CFSE = 61.2 kcal mol(-1)) are c lose to those of Cr3+-ions in Cr-clinochlore (kammererite). This can b e explained by the similarities of the arrangement of the Al- and Mg-o ctahedra in the aluminum-bearing octahedral layers of both mineral str uctures. The high Dq value of Cr3+-ions, incorporated in relatively la rge octahedral positions (R(Cr-O) = 2.03 Angstrom in amesite and 1.97 Angstrom in clinochlore) obviously result from the considerable influe nce of the next-nearest cations (Mg2+) on the effective charge of the ligands in the Cr3+-centred octahedral sites.