Ordering in synthetic aluminous serpentines; infrared spectra and cell dimensions

Infrared and X-ray diffraction studies were made on synthetic serpentines (Si₄ to Si₂Al₂ tetrahedral compositions). Changes in cell dimensions and variations in infrared spectra indicate that ordering can occur in the octahedrally coordinated site and possibly the tetrahedral site. Octahedra order into an Mg₃ and/or Mg₂Al configuration depending upon bulk mineral composition. In the latter cases an Al ion preferentially fills only one of the large M₂ sites found in dioctahedral minerals and is thus characteristic of neither di-nor trioctahedral minerals. Ordering in octahedral and possibly tetrahedral sites does not appear to affect the basic higher frequency vibrations (OH stretch, Si-O stretch) by creating new modes but definite band splitting is seen for vibrations of lower frequency. The alumina-free composition was crystallized into monophase products which can be assimilated to natural chrysotile or antigorite forms. Addition of alumina produces lizardite-type serpentines which are very closely related to the chrysotile structure as indicated by cell-dimension and Si-O stretch bands in infrared spectra.