HR: 1340h
AN: MR13A-0079    [Abstracts]
TI: Effect of Network Modifying Cations on the Structural Disorder in Multi-Component Silicate Glasses
AU: * Sung, S
EM: sysung1@snu.ac.kr
AF: Seoul National University, San 56-1, Silim-dong, kwanak-gu, Seoul, 151-742 Korea, Republic of
AB: The structure of complex, multi-component silicate glasses and melts provides insight into the physiochemical properties (e.g. viscosity, diffusivity, and thermodynamic properties) of natural silicate melts. Particularly, quaternary aluminosilicate glasses are important as it covers compositions of slab driven melts at the convergent margin and, thus, yield constraints for the dynamics of magmas in the earth­_s interior. Despite this importance, the structure of CaO-Na₂O-Al₂O₃-SiO₂ (CNAS) glasses has not been well understood because of inherent disorder in glasses and melts (Lee et al., 2005, Am, Min. p 1393). Solid-state ¹⁷O triple quantum (3Q) MAS MNR has been effective in yielding unpresented resolutions among oxygen sites in the glasses over conventional 1D MAS NMR, allowing us to determine the structure of multi-component silicate glasses (Lee et al., 2005, Am, Min. p 1393, Stebbins and Xu, 1997, Nature, p 60). Here, we report the first ¹⁷O MAS and 3QMAS NMR spectra for CNAS glasses with varying the Ca/Na ([(CaO) _x (Na₂O) _1.5-x](Al₂O₃)_0.5 (SiO₂)₆) ratios at static magnetic fields of 14.1 Tesla (600 MHz). ¹⁷O MAS spectra show bridging oxygen (BO) peaks and partially resolved non-bridging oxygen (NBO) peaks (Ca-NBO and mixed {Ca, Na}-NBO). NBO in Ca-aluminosilicate glass moves toward lower frequency with increasing the Na/Ca ratios due to the formation of mixed-{Ca, Na}-NBO. This result suggests that the chemical shielding of mixed {Ca, Na}-NBO peak increases with increasing Ca contents and there is a mixing between Ca and Na around NBO in the Ca-Na aluminosilicate glasses. ¹⁷O 3QMAS NMR spectra exhibit well resolved BO peaks and partially resolved Ca-NBO and mixed-{Ca, Na}-NBO peaks. The fraction of Ca-NBO decrease with increasing Na/Ca ratios while the fraction of BO appears to be invariant with Na/Ca ratios. These results imply that NBO interacts more severely with network modifying cations than BO does. Another inherent disorder in silicate glasses and melts is topological disorder due to bond angle and bond length distribution. The FWHMs of Si-O-Si in CNAS glasses increase with Ca contents from 6.8 (Na-aluminosilicate) to 10.8 ppm (Ca-aluminosilicate), manifesting that topological disorder, and thus, topological entropy in the CNAS glasses increases with increasing Ca contents. Quadrupolar coupling constants (C_q) of Si-O-Si in the CNAS glasses, one of the structurally relevant NMR parameter, also vary with Ca/Na ratios, demonstrating a strong variation of bond angle with Ca/Na ratios. The results given here indicate that the network modifying cations have an influence on the framework topology in the CNAS glasses, provide information about the structural configurations and the degree of disorder in the complex, multi-component silicate glasses and melts.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005