A Raman Spectroscopic Study of Humite Minerals

Raman spectroscopy has been used to study the structure of the humite mineral group ((A2SiO4)n - A(OH, F)2 where n represents the number of olivine and brucite layers in the structure and is 1,2,3 or 4 and A2+ is Mg, Mn, Fe or some mix of these cations). The humite group of minerals forms a morphotropic series with the minerals olivine and brucite. The members of the humite group contain layers of the olivine structure that alternate with layers of the brucite-like sheets. The minerals are characterized by a complex set of bands in the 800 to 1000 cm-1 region attributed to the stretching vibrations of the olivine (SiO4)4- units. The number of bands in this region is influenced by the number of olivine layers. Characteristic bending modes of the (SiO4)4- units are observed in the 500 to 650 cm-1 region. The brucite sheets are characterized by the OH stretching vibrations in the 3475 to 3625 cm-1. The position of the OH stretching vibrations is determined by the strength of the hydrogen bond formed between the brucite-like OH units and the olivine silica layer. The number of olivine sheets and not the chemical composition determines the strength of the hydrogen bonds.