Raman spectra of silicate garnets |
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Authors: | B A Kolesov C A Geiger |
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Institution: | Institute of Inorganic Chemistry, Lavrentiev prosp. 3, Novosibirsk 630090, Russia, RU Mineralogisch-Petrographisches Institut, Universit?t Kiel, Olshausenstr. 40, D-24098 Kiel, Germany Fax: +49?04?31?8?80-44?57; e-mail: NMP4b@rz.uni-kiel.de, DE
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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.
Received: 3 December 1996 / Revised, accepted: 13 April 1997 |
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