Infrared evidence for the occurrence of SiO groups with double- bond character in antigorite, sepiolite and palygorskite.

IR spectra of antigorite and sepiolite show an absorption band at approx 1200 cm^-1. Spectra of palygorskite show several small absorptions between 1140 and 1200 cm^-1. Since these three minerals are the only layer-silicates which show this absorption, this band was assigned to Si-O-Si groups which serve as bridges between alternating alumino-Mg-silicate ribbons. By comparing the location of this band to bands observed in the spectra of P=O groups it was concluded that the contribution of pi bonding in the bridging Si-O-Si group is of a high order. Similar conclusions were obtained by comparing the spectra of antigorite, sepiolite and palygorskite with those of various pyrosilicates. An inversion of the tetrahedral sheet requires an Si-O-Si angle of 180o which can be stabilized by d_pi -p_pi bonding in the Si-O-Si group. Due to the inductive effect of the octahedral sheet this type of bonding may occur in trioctahedral clay silicates, but is not to be expected in dioctahedral clay silicates. (Author's abstract) -D.J.M.