| Research Abstract |
In order to study local structure of mantle minerals by means of NMR spectroscopy, it is inevitable to know the relationship between NMR chemical shift and local structure. The coordination number of Si and Al by oxygen atoms changes from four to six at mantle transition zone to top of lower mantle. Therefore, the quantitative study of chemical shift due to the coordination change is very important to understand the structure of phases at those depths. In this study, the relationship between chemical shift and local structure is studied by means of ab initio molecular orbital method. For this study, following clusters are used : Si(OH)_4, Si(OH)_5^-, Si(OH)_6^<2->, Al(OH)_4^-, Al(OH)_5^<2->, Al(OH)_6^<3->Si_2O(OH)_6, Si_3O_3(OH)_6, Si_3O_2(OH)_8. From these calculations following results are obtained.
(1) NMR chemical shift due to coordination change of Si and Al,
(2) Si Chemical shift due to number of bridging oxygens,
(3)O chemical shift and NQCC of bridging oxygen due to Si-O-Si angle change.
These calculations are quite consistent with available experimental data, and can be used to extract local structural information from observed NMR spectra.
In next stage of present study, we will measure the NMR spectra of deep mantle minerals such as spinel, majorite and perovskite. Using the relationship between chemical shift and local structure obtained in this study, we will investigate the distribution of Mg, Si, and Al in octahedral sites of these minerals. From this kind of study, we can evaluate the configurational entropies of minerals, and its effect to phase relations.
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