Insights on the antigorite structure from Mossbauer and FTIR spectroscopies

Pure, selected samples of antigorite (#7 and #18, from Elba Island veins, Italy, with superperiodicities of 38 and 49 Angstrom, respectively) have been analyzed by Mossbauer and infrared spectroscopies. Mossbauer data indicate that most of the iron is present as ferrous iron (88.6% Fe2+ in #7 and 83.2% Fe2+ in #18). Both ferrous and ferric iron occur in octahedral coordination; ferric iron in tetrahedral coordination has not been detected. The infrared spectra of antigorites #7 and #18 are similar, with minor shifts in peak positions. More in general, the comparison with other vein antigorites from the Elba suite (#2, #4, #11, #16) rules out any relation between modulation wavelength and IR behaviour. Evident differences arise from the comparison between antigorite and lizardite spectra. The absorption bands corresponding to stretching of the basal Si-O bonds are systematically shifted towards higher wavenumbers in antigorite with respect to lizardite (from 951 to 979-991 cm(-1)), suggesting higher energy of the bridging bonds. In contrast, antigorite and lizardite show the same IR patterns in the apical Si-O stretching vibrations (1073-1084 cm(-1)). The OH stretching region (3700-3400 cm(-1)) indicates similar structural arrangement for the inner O-H in antigorite and lizardite, whereas the absence of the broad band at similar to3440 cm(-1) in antigorite indicates the lack of important hydrogen bonding in the interlayer. Other IR differences (e.g., absence of Si-O bending and of external OH bending in lizardite) are explained as due to different symmetries (monoclinic antigorite vs. trigonal lizardite). We conclude that antigorite and lizardite share common features (similar iron coordination and disordered distribution within the magnesium octahedra), but differ in the oxidation state (more reduced antigorite), in the tetrahedral sheet size (basal Si-O bond shrinked by 0.009 Angstrom in antigorite) and in the interlayer connections mechanism (absence of hydrogen bond in antigorite).