HR: 0830h
AN: T11C-0421    [PDF]
TI: Single Crystal Raman Spectroscopy and Thermodynamics of Garnet Solid Solutions II: Pyrope - Almandine Binary
AU: * Savage, F B
EM: fsavage@u.washington.edu
AF: Department of Chemistry, University of Washington, Box 35-1700, Seattle, WA 98195-1700 United States
AU: Chopelas, A
EM: chopelas@phys.washington.edu
AF: Department of Physics, Univeristy of Washington, Box 35-1560, Seattle, WA 98195-1560 United States
AB: Garnets are ubiquitous in the crust and mantle and are key minerals in geothermometry and geobarometry. Macroscopic properties, such as enthalpy and volume, exhibit non-linear behavior in solid solutions making it difficult to model their behavior. Insights into the cause of the non-linearity of such properties may be found by investigating the effects of compositional variation using a microscopic technique such as vibrational spectroscopy. A further advantage of using vibrational spectroscopy is the ability to probe the effect of small changes in the frequencies on the thermodynamic properties using vibrational modeling. Investigating the pyrope-almandine (substitution of ferric iron for magnesium) binary allows the examination of the effect of doubling the mass on the dodecahedral cation without significantly changing the cell parameter (11.45 to 11.52 A, compare to approx. 12 for Ca bearing garnets). The garnet samples here have low moisture content, eliminating a possible source of non-linear behavior. Although previous infrared data* show linear behavior across this series, our data show this not to be the case. Raman modes vary from linearity but all modes do not vary in the same way. Generally the trend is to have lower frequencies in the solid solution than expected for linear behavior, as in the andradite-grossular binary. However, a few modes at low frequencies have higher frequencies than the linear trend. Since the lowest frequency modes are most strongly affected by the divalent cation, this would suggest that the electronic and magnetic effects of iron play a role in this unexpected variation. Substitution of ferric iron into minerals raises the heat capacity above the values that can be accounted for by vibrational modeling. The effect of adjusting vibrational models for this non-linear frequency shift will be further discussed. Preliminary thermodynamic models will also be presented. *Hofmeister et al., Am. Min. 1996, 81: 418-428.
DE: 3655 Major element composition
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3939 Physical thermodynamics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting