Infrared evidence of order-disorder in amesitesr

An IR investigation of natural and synthetic amesites has been made. The naiural amesite from Chester, Massachusetts, and certain synthetic samples show order in the Al for Si substitution in tetrahedral sheets. Their IR spectra are characterized by sharp absorption bands and vibrations of "isolated" Al tetrahedra. In contrast, the broadening in the absorption bands and the presence of two absorptions around 800 cm-1 suggest that amesite from Saranoskoye and some synthetic samples have a random Al for Si substitution. The IR absorption bands for the hydroxyl groups in amesites appear to arise from the order-disorder nature of the Al for Si substitution in tetrahedral sheets. In ordered amesites the oxygens of the tetrahedral sheets are all coordinated to Al-Si pairs, providing a uniform envir.onment for each of the interacting hydroxyls. By contrast, in amesites with disordered Al for Si substitution the outer hydroxyl groups of the octahedral sheets would experience two different interactions with O-Al and O-Si. The presence and distribution of Al in tetrahedral sheets appear to account for the hydroxyl vibrations in the trioctahedral series amesite-antisorite.