Local structural study of mantle minerals by NMR spectroscopy

为了利用核磁共振波谱技术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。从地幔过渡带到下地幔顶部，Si和Al的氧原子配位数由4变为6。因此，定量研究配位变化引起的化学位移对了解这些深度的相结构具有重要意义。本研究采用从头算分子轨道法研究了化学位移与局部结构的关系。在这项研究中,使用集群:Si (OH) _4, Si (OH) _5 ^ - Si (OH) _6 ^ < 2 - >, Al (OH) _4 ^ - Al (OH) _5 ^ < 2 - >, Al (OH) _6 ^ < 3 - > Si_2O(哦)_6 Si_3O_3(哦)_6 Si_3O_2 _8(哦)。从这些计算中得到以下结果。(1)硅和铝配位变化引起的核磁共振化学位移；(2)桥氧数量变化引起的硅化学位移；(3)硅-硅-硅角度变化引起的桥氧O化学位移和NQCC。这些计算结果与现有的实验数据相当一致，可用于从观测到的核磁共振光谱中提取局部结构信息。在本研究的下一阶段，我们将测量深地幔矿物如尖晶石、镁钛矿和钙钛矿的核磁共振光谱。利用本研究获得的化学位移与局部结构之间的关系，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，我们可以评价矿物的构型熵及其对相关系的影响。

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.