Hydrogen incorporation mechanisms in forsterite: New insights from 1H and 29Si NMR spectroscopy and first-principles calculation
Hydrogen incorporation mechanisms in forsterite: New insights from 1H and 29Si NMR spectroscopy and first-principles calculation
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DOI：
10.2138/am-2017-5878
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发表时间：
2017-03
影响因子：
3.1
通讯作者：
X. Xue;M. Kanzaki;D. Turner;D. Loroch
中科院分区：
文献类型：
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作者：
X. Xue;M. Kanzaki;D. Turner;D. Loroch
摘要：名义上无水地幔矿物中水（氢）的存在可能对其物理性质（例如电导率、扩散性、流变性）产生深远的影响，并且这些影响预计取决于水如何融入晶体结构。对于橄榄石这种最丰富的上地幔矿物，尽管进行了广泛的研究（主要使用振动光谱），但解释仍然没有得到很好的限制。为了更好地理解这个问题，我们对在 12 GPa 和 1200 °C 下合成的含有约 0.5 wt% H2O 的 Mg2SiO4 镁橄榄石样品进行了全面的 1H 和 29Si NMR 研究，并辅以拉曼测量和模型结构的几何形状、稳定性和 NMR 参数的第一性原理计算。拉曼光谱在 3612、3579 和 3567 cm−1 附近包含相对尖锐的 O-H 伸缩带，以及在 3547 cm−1 附近更宽的带，与之前的报道类似。 1H静态和MAS NMR数据显示，水合镁橄榄石结构中有两个主要的质子群，一种经历强的1H-1H同核偶极耦合，并在2.4 ppm附近产生宽峰，另一种具有较弱的偶极耦合，在30 kHz的MAS NMR谱中，在1.2 ppm附近产生较窄的峰。二维 1H CRAMPS-MAS NMR 测量证实，两个质子组分属于同一相，MAS NMR 峰宽度的对比很大程度上是由于 1H-1H 同核偶极耦合强度的差异。此外，在 7.3 ppm 附近还有一个非常弱、窄的 1H MAS NMR 峰（贡献为 3450 cm−1），并且 (2H)M1 缺陷在振动光谱中给出较低的频率（由于丰度低而未被检测到，但很可能在 3160-3220 cm−1 附近）。这些结果可以作为（重新）解释不同压力和二氧化硅活性条件下产生的水合橄榄石的红外和拉曼光谱数据的指南，并且需要基于对这些数据的不同解释重新考虑水对橄榄石物理性质影响的任何模型。这项研究还证明了固态核磁共振和第一性原理计算相结合的方法在揭示名义无水矿物中氢结合机制方面的有用性。
Abstract The presence of water (hydrogen) in nominally anhydrous mantle minerals may have profound effects on their physical properties (e.g., electrical conductivity, diffusivity, rheology), and these effects are expected to depend on how water is incorporated in the crystal structure. For olivine, the most abundant upper mantle mineral, despite extensive studies, mostly using vibrational spectroscopy, the interpretations are still not well constrained. To provide better understanding on this issue, we carried out a comprehensive 1H and 29Si NMR study on an Mg2SiO4 forsterite sample containing about 0.5 wt% H2O synthesized at 12 GPa and 1200 °C, complemented by Raman measurement and first-principles calculation of the geometry, stability, and NMR parameters of model structures. The Raman spectra contain relatively sharp O-H stretching bands near 3612, 3579, and 3567 cm−1 and a broader band near 3547 cm−1, similar to previous reports. The 1H static and MAS NMR data revealed that there are two main populations of protons in the hydrous forsterite structure, one experiencing strong 1H-1H homonuclear dipolar couplings and contributing to a broad peak near 2.4 ppm, and another with weaker dipolar couplings and contributing to a narrower peak near 1.2 ppm in the MAS NMR spectrum at 30 kHz. Two-dimensional 1H CRAMPS-MAS NMR measurements confirmed that the two proton components belong to the same phase and the contrast in MAS NMR peak width is largely due to difference in the strength of 1H-1H homonuclear dipolar couplings. In addition, there is also a very weak, narrow 1H MAS NMR peak near 7.3 ppm (contributing to 3450 cm−1), and the (2H)M1 defects give lower frequencies (undetected here due to low abundance, but most likely near 3160–3220 cm−1 as previously reported) in vibrational spectra. These results can serve as a guide for (re-)interpretation of infrared and Raman spectroscopic data on hydrous olivine produced under different pressure and silica activity conditions, and require reconsideration of any models for the effects of water on physical properties of olivine based on different interpretations of such data. This study also demonstrated the usefulness of the combined solid-state NMR and first-principles calculation approach in unraveling the hydrogen incorporation mechanisms in nominally anhydrous minerals.