Molar volumes of mixing of almandine-pyrope and almandine-spessartine garnets and the crystal chemistry and thermodynamic-mixing properties of the aluminosilicate garnets

Abstract

Other| June 01, 1997 Molar volumes of mixing of almandine-pyrope and almandine-spessartine garnets and the crystal chemistry and thermodynamic-mixing properties of the aluminosilicate garnets Charles A. Geiger; Charles A. Geiger Christian-Albrechts-Universitaet zu Kiel, Mineralogisch-Petrographisches Institut, Kiel, Federal Republic of Germany Search for other works by this author on: GSW Google Scholar Anne Feenstra Anne Feenstra Universitaet Bern, Switzerland Search for other works by this author on: GSW Google Scholar American Mineralogist (1997) 82 (5-6): 571–581. https://doi.org/10.2138/am-1997-5-617 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Charles A. Geiger, Anne Feenstra; Molar volumes of mixing of almandine-pyrope and almandine-spessartine garnets and the crystal chemistry and thermodynamic-mixing properties of the aluminosilicate garnets. American Mineralogist 1997;; 82 (5-6): 571–581. doi: https://doi.org/10.2138/am-1997-5-617 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract The aluminosilicate garnet binaries almandine-pyrope and almandine-spessartine were studied by powder X-ray and 57Fe Mössbauer methods. Refinements of the unit-cell constants along the almandine-pyrope join show that the volumes of mixing are ideal. Those of the almandine-spessartine join show very small positive deviations from ideality, which can be fitted with a symmetric model having an interaction parameter of WV = 0.24 (±0.05) cm3/mol. Mössbauer spectra recorded at 298 and 77 K show the presence of small amounts of [6]Fe3+, which in the case of almandine-pyrope garnets is also measurable from microprobe analyses. The amount of Fe3+ is generally less than 3.5% of the total Fe for the almandine-pyrope garnets and 1–2% for almandine-spessartine garnets. The molar volumes of mixing of the aluminosilicate garnet binaries are interpreted using a crystal-chemical model involving rigid tetrahedral rotation. The degree of tetrahedral rotation is not linear with increasing size of the divalent X-site cation for the four common aluminosilicate garnet end-members or along the solid solution binary pyrope-grossular. The vibrational entropies of mixing should be positively correlated with the volumes of mixing in the case of garnet, but the masses of the X-site cations must also be considered. The phonon density of states at low energies should show the vibrations of the weakly bonded divalent cations and rigid-unit modes related to tetrahedral rotation. Positive excess vibrational entropies of mixing along a binary could result from increased amplitudes and lower frequencies of vibration of the smaller of the two X-site cations substituting within larger and more distorted dodecahedral sites, as compared to the X site in the smaller volume end-member. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.