کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
4701184 | 1637746 | 2007 | 11 صفحه PDF | دانلود رایگان |
عنوان انگلیسی مقاله ISI
Titanium substitution mechanisms in forsterite
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کلمات کلیدی
موضوعات مرتبط
مهندسی و علوم پایه
علوم زمین و سیارات
ژئوشیمی و پترولوژی
پیش نمایش صفحه اول مقاله
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چکیده انگلیسی
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 â 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 Ã
. This value is in excellent agreement with that of 1.8Â Ã
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 2.0Â Ã
. 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]TiO4 and Ti-clinohumite-like Mg[6]TiO2(OH)2 substitutions.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Chemical Geology - Volume 242, Issues 1â2, 30 July 2007, Pages 176-186
Journal: Chemical Geology - Volume 242, Issues 1â2, 30 July 2007, Pages 176-186
نویسندگان
Andrew J. Berry, Andrew M. Walker, Jörg Hermann, Hugh St.C. O'Neill, Garry J. Foran, Julian D. Gale,