Thermodynamic Parameters of Uvarovite (Ca3Cr_2Si_3O12) from Single Crystal Raman Spectroscopy

Garnets are ubiquitous in the crust and mantle and are key minerals in geothermometry and geobarometry as well as in the transition zone of the mantle. Uvarovites are found in nature as nearly endmember compositions in some terraines, and are significant components in diamond inclusions. They have not been studied as extensively as pyropic garnets. Raman spectroscopic studies of uvarovite provide several advantages for understanding the behavior of garnets. Thermodynamic properties, such as heat capacity and entropy, can be calculated using vibrational modeling. Using vibrational spectroscopy with cation substitution provides insight into how the properties vary with composition. The nonlinear behavior of garnet solid solutions is well-known, details of this behavior can be probed with spectroscopy (see F. Savage, A. Chopelas abstract this volume). Uvarovite is a calcic garnet with chromium at its octahedral site. It is close in volume to grossular (Al at the octahedral site), while the mass of the trivalent cation is similar to andradite (ferric iron at the octahedral site). All three have lattice parameters around 12 Å, much larger than for pyralspite garnets (lattice parameters around 11.5 Å). The samples in this study were synthetic end member uvarovite, a binary nearly halfway between andradite and uvarovite (Uv_59}An{41), and 2 other uvarovites, one with a small pyropic content and the other with a small grossular content. Single crystal polarized spectra on the end-member revealed 23 of the 25 Raman modes expected by symmetry. The high energy modes of uvarovite closely matched those of grossular while the low energy modes more closely matched those for andradite. This is not a surprising result since the high energy modes dominated by silicate stretching and bending modes, depend nearly entirely on the lattice parameter, while the low energy modes, mainly cation translations, have a heavier dependence of the mass of the cations. Other silicate minerals exhibit this same behavior suggesting a simple treatment of the silicate modes upon substitution. Andradite's unusual intensity distribution with a very strong high energy Eg mode is repeated in end member uvarovite but not in any of the mixed uvarovites. The Uv_59}An{41 has only a very small volume change and the lattice parameter is much closer to uvarovite's than would be expected by composition. This is reflected in the behavior of the modes, which will be further discussed. Thermodynamic calculations will also be presented.