Titanium substitution mechanisms in forsterite
Titanium substitution mechanisms in forsterite
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DOI：
10.1016/j.chemgeo.2007.03.010
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发表时间：
2007-07
期刊：
影响因子：
3.9
通讯作者：
A. Berry;A. Walker;J. Hermann;H. O’Neill;G. Foran;J. Gale
中科院分区：
文献类型：
--
作者：
A. Berry;A. Walker;J. Hermann;H. O’Neill;G. Foran;J. Gale
Ti K边X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）光谱以及原子尺度计算表明Ti占据了无水镁橄榄石中的Si位点。 XANES 光谱中 1s→3d 跃迁的能量和强度可以诊断四坐标位点上的 Ti。 EXAFS 光谱确定 Ti-O 键长为 1.81±0.01 Å。该值与通过周期密度泛函理论和 Si 位点上的 Ti 嵌入簇方法计算确定的 1.8 Å 值非常一致；计算出 Mg 位点上 Ti 的键长为 2.0 Å。两种计算方法进一步发现，相对于用 Ti 替代 Mg，Ti 直接替代 Si 更受青睐，电荷通过 Mg 空位或 Si 位点上的 Mg 进行平衡。总之，这些结果为 Ti 占据无水镁橄榄石中的 Si 位点提供了明确的证据。然而，在上地幔条件下有水合成的含钛橄榄石表现出与六重配位相对应的前边缘特征。这与在上地幔尖晶石橄榄岩的橄榄石样品中鉴定出类钛斜镁石点缺陷是一致的。因此，Ti 位点的变化提供了橄榄石中水的结合机制。橄榄石的总 Ti 含量将包括 Mg2[4]TiO4 和 Ti-斜腐石类 Mg[6]TiO2(OH)2 替代物的不同贡献。
Ti K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra, together with atomic scale calculations, indicate that Ti occupies the Si site in anhydrous forsterite. The energy and intensity of the 1s→3d transition in the XANES spectrum is diagnostic of Ti on a four-coordinate site. The EXAFS spectra determine a Ti–O bond length of 1.81±0.01 Å. This value is in excellent agreement with that of 1.8 Å determined computationally by periodic density functional theory and an embedded cluster approach for Ti on the Si site; the calculated bond length for Ti on a Mg site is 2.0 Å. Both computational methods further find that the direct substitution of Ti for Si is energetically favoured relative to substitution of Ti for Mg, charge balanced by either a Mg vacancy or Mg on a Si site. Together the results provide unambiguous evidence for Ti occupying the Si site in anhydrous forsterite. Ti-bearing olivine synthesised at upper mantle conditions in the presence of water, however, exhibits a pre-edge feature corresponding to six-fold coordination. This is consistent with the identification of a Ti-clinohumite-like point defect in samples of olivine from upper-mantle spinel peridotite. The change in Ti site thus provides a mechanism for the incorporation of water in olivine. The total Ti content of olivine will comprise varying contributions from Mg2[4]TiO4and Ti-clinohumite-like Mg[6]TiO2(OH)2substitutions.