Raman spectra of silicate garnets

Abstract The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine, grossular, andradite, and uvarovite, and two synthetic end-members spessartine and pyrope, were measured, along with the powder spectra of synthetic pyrope-grossular...

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Bibliographic Details
Published in:Physics and chemistry of minerals 1998-01, Vol.25 (2), p.142-151
Main Authors: Kolesov, B. A., Geiger, C. A.
Format: Article
Language:English
Subjects:
Bonding strength
Calcium ferrous silicates
Cations
Composition
Crystals
Garnets
Magnesium aluminum silicates
Raman spectra
Single crystals
Solid solutions
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Summary:Abstract The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine, grossular, andradite, and uvarovite, and two synthetic end-members spessartine and pyrope, were measured, along with the powder spectra of synthetic pyrope-grossular and almandine-spessartine solid solutions. Mode assignments were made based on a comparison of the different end-member garnet spectra and, in the case of pyrope, based on measurements made on additional crystals synthesized with 26Mg. A general order of mode frequencies, i.e. R(SiO4)>T(metal cation)>T(SiO4), is observed, which should also hold for most orthosilicates. The main factors controlling the changes in mode frequencies as a function of composition are intracrystalline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO4-vibrational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations reflect their weak bonding and dynamic disorder in the large dodecahedral site, especially in the case of pyrope. Two mode behavior is observed for X-site cation vibrations along the pyrope-grossular binary, but not along the almandine-spessartine join.
ISSN:0342-1791
1432-2021
DOI:10.1007/s002690050097