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

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Abstract
IR spectra of antigorite and sepiolite show an absorption band at ∼ 1200 cm−1. Spectra of palygorskite show several small absorptions between 1140–1200 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 π bonding in the bridging Si-O-Si group is of a high order. Similar conclusions were drawn after 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 180° which can be stabilized by dπ-pπ 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.