Local structural study of mantle minerals by NMR spectroscopy

为了利用核磁共振波谱研究地幔矿物的局部结构，必须了解核磁共振化学位移与局部结构的关系。在地幔过渡带至下地幔顶部，氧原子对硅、铝的配位数由4个变为6个。因此，定量研究配位变化引起的化学位移对于了解这些深度的相结构是非常重要的。本文用从头算分子轨道方法研究了化学位移与局域结构的关系。在本研究中，使用了如下簇合物：Si(OH)_4，Si(OH)_5^-，Si(OH)_6^&lt；2-&gt；，Al(OH)_4^-，Al(OH)_5^&lt；2-&gt；这些计算结果如下：(1)由于Si和Al配位的变化而引起的核磁共振化学位移；(2)由于桥氧数目的变化而引起的硅化学位移；(3)由于Si-O-Si角度的变化而引起的桥氧的O化学位移和NQCC。这些计算与已有的实验数据非常一致，并可用于从观察到的核磁共振谱中提取局部结构信息。在下一阶段的研究中，我们将测量尖晶石、镁铝榴石和钙钛矿等深部地幔矿物的核磁共振谱。利用本研究得到的化学位移与局部结构的关系，我们将研究镁、硅和铝在这些矿物的八面体位置上的分布。通过这种研究，我们可以评估矿物的构型熵，以及它对相关系的影响。

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.