HR: 1340h
AN: MR13A-0054    [Abstracts]
TI: Constraining Oxygen-17 NMR Spectra of High Pressure Crystals and Glasses: New Data for Jadeite, Pyrope, Grossular, and Mullite
AU: * Kelsey, K E
EM: kkelsey@stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences Building 320, Stanford, CA 94305-2115 United States
AU: Stebbins, J F
EM: stebbins@pangea.stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences Building 320, Stanford, CA 94305-2115 United States
AU: Du, L
EM: lsdu@pangea.stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences Building 320, Stanford, CA 94305-2115 United States
AU: Hankins, B
EM: bhankins@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: ¹⁷O NMR is a direct way of analyzing the immediate environment around oxygen atoms and can provide information on cation ordering, mixing, and network connectivity in glasses and disordered crystals. Due to overlapping peaks and lack of data on crystalline model compounds, ¹⁷O NMR spectra of high pressure glasses have been difficult to interpret. Additionally, data on crystalline model compounds are needed to test the validity of quantum chemical calculations. In this study, ¹⁷O NMR spectra were collected for crystalline jadeite, pyrope, grossular, and mullite in order to determine the parameters for oxygen bonded to ^[6]Al in a variety of environments. Jadeite contains three oxygen sites: oxygen bonded to ^[4]Si, Na, and two ^[6]Al atoms (O1), oxygen bonded to ^[4]Si, Na, and ^[6]Al atoms (O2), and oxygen bonded to two ^[4]Si and two Na atoms (O3). The NMR parameters for O1 are C_Q = 3.3 MHz, δ = 64 ppm, and ν = 0.9; for O2 are C_Q = 4.1 MHz, δ = 59 ppm, and ν = 0.15; and for O3 are C_Q = 5.0 MHz, δ = 60 ppm, and ν = 0.15. The parameters for O2 are similar to interpretations of recent data for this kind of site in high pressure sodium aluminosilicate glasses (δ = 59 ppm) and to quantum chemical calculations (Lee et al., 2004, J. Phys. Chem., 108, 5897). Pyrope and grossular each contain one oxygen site, oxygen bonded to ^[4]Si, ^[6]Al, and two M²⁺ cations. The ¹⁷O NMR parameters for pyrope are C_Q = 3.4 MHz, δ = 84 ppm, and ν = 0.3 and for grossular are C_Q = 4.1 MHz, δ = 102 ppm, and ν = 0.4. In grossular, the NMR peak for oxygens bonded to ^[4]Si, Ca, and high coordinated Al seems to fall between those for "normal" bridging and non bridging oxygens, as reported for high pressure CAS glasses by Allwardt et al. (2005). These data will also be useful to help understand Ca-Mg ordering in the pyrope-grossular solid solutions. Mullite contains four oxygen environments: oxygen bonded to three tetrahedral Al or Si (Oc*), oxygen bonded to two tetrahedral Al or Si (Oc), and two sites with oxygen bonded to one ^[6]Al and two tetrahedral Al or Si (Oab and Od). Due to the fact that most tetrahedral sites can be occupied by either Al or Si, mullite has significantly more disorder and thus has broader NMR peaks. The site Oc* is a tetrahedral tricluster, and the NMR parameters are C_Q = 2.0 MHz, δ = 40.5 ppm, and ν = 0.4. The NMR parameters for Oc* are similar to those determined for triclusters in grossite (CaAl₄O₇) as well as previously suspected triclusters in glasses. The NMR parameters for Oc are C_Q = 3.3 MHz, δ = 76 ppm, and ν = 0.1. The sites Oab and Od are indistinguishable, and the NMR parameters for the combined peak are C_Q = 3.3 MHz, δ = 59 ppm, and ν = 0.3.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005