Silicate And Aluminum-containing Silicate Minerals, Raman Spectroscopy Study

The structure of magma has attracted intense attention by the geologists recently.Since silicate melts and aluminosilicate melts are the main components of naturalmagma, the study of their structure is very important to exploring the properties ofmagma and the igneous processes. Raman spectroscopy was widely recognized as oneof the key methods to study the structure of silicate melts. In order to deconvolvingthe Raman envelopes of silicate melts and aluminosilicate melts accurately, it isnecessary to accumulate the Raman characters of different structure units through thestudy of Raman spectra of silicate and aluminosilicate mineral crystals.Simultaneously, finding an effective method to analyze and predict the symmetricstretching vibrational wavenumber of non-bridging oxygen (NBO) in thehigh-wavenumber range (800-1200cm-1 range) has very important practicalsignificance to unscramble the Raman spectra of silicate minerals (especially thecomplex minerals with more than one types of Qi species in their structure).This paper tested the Raman spectra of tremolite (which has a typical chainstructure) and aluminosilicate minerals such as kyanite, andalusite, etc. With using themethods of "SIT" calculation and self-consistent (SCF) molecular orbital ab-initiocalculation it can predict that the characteristic Raman bands of tremolite are1033cm-1 and 1059cm-1 which are relevant to the symmetric stretching vibration ofNBOs of the Q2 and Q3 species, respectively. Moreover, the Raman spectra of silicateminerals with aluminum included and (Si, Al)-O tetrahedra model clusters areanalyzed by Density Functional Theory (DFT) calculation and ab-initio calculation.On the basis of these calculated results, and also comparing with the Raman spectra ofNa2O-Al2O3-SiO2 glasses, it can conclude that the influences of Al3+ to the Ramanspectra of silicates are as follows: ① The symmetric stretching vibrationalwavenumber of AlⅣ-O is in 700-800cm-1 range and 800-1200cm-1 range has noRaman bands which are relevant to the vibration of AlⅣ-O. However, with AlⅣ in thestructure, the symmetric stretching vibrational wavenumbers of NBOs in silicates willmove to low-frequency range. ②The presence of AlⅥ can cause the symmetricstretching vibrational wavenumbers of NBOs moving to the high-frequency range, butother influences of Al Ⅵ in the Raman spectra of silicates still need furtherinvestigation.All above researches provided new methods to study the Raman spectra and thestructure of silicates (both crystals and melts) and also acquired some primaryinformation of how Al3+ influenced the Raman spectra of silicates.