The ammonium ion in a silicate under compression: infrared spectroscopy and powder X-ray diffraction of NH₄AlSi₃O₈—buddingtonite to 30 GPa

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• 作者：Cara E. Vennari ; Earl F. O’Bannon ; Quentin Williams
• 关键词：Buddingtonite ; NH4AlSi3O8 ; High ; pressure ; Infrared spectroscopy ; Powder X ; ray diffraction ; Nitrogen
• 刊名：Physics and Chemistry of Minerals
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：44
• 期：2
• 页码：149-161
• 全文大小：
• 刊物类别：Earth and Environmental Science
• 刊物主题：Mineralogy; Crystallography and Scattering Methods; Geochemistry; Mineral Resources;
• 出版者：Springer Berlin Heidelberg
• ISSN：1432-2021
• 卷排序：44

文摘

The behavior of the ammoniated feldspar buddingtonite, NH₄AlSi₃O₈, has been studied using infrared (IR) spectroscopy up to ~30 GPa and using synchrotron powder X-ray diffraction to 10 GPa at room temperature. We examine the bonding of the ammonium ion under pressure and in particular whether hydrogen bonding is enhanced by compaction, as well as probe how the ammonium ion affects the elasticity and behavior of the aluminosilicate framework at pressure. Powder diffraction data yield a bulk modulus of 49 GPa for a pressure derivative of 4, implying that the ammonium ion substitution may induce a modest softening of the feldspar lattice relative to the potassium ion. Under compression, the N–H vibrations are remarkably insensitive to pressure throughout the pressure range of these experiments. However, the vibrations of the aluminosilicate framework of buddingtonite undergo changes in their slope at ~13 GPa, implying that a change in compressional mechanism occurs near this pressure, but the vibrational modes of the ammonium molecule show little response to this change. These results show that (1) there is little, if any, enhancement of hydrogen bonding between the ammonium ion and the oxygen ions of the silica and aluminum tetrahedral framework under pressure, as manifested by the slight (and mostly positive) shifts in the N–H stretching vibrations of the ammonium ion; (2) ordering of the ammonium ion is not observed under compression, as no changes in peak width or in the general appearance of the spectra are observed under compression; and (3) structural changes induced by pressure in the aluminosilicate framework do not produce significant changes in the bonding of the ammonium ion. Hence, it appears that the ammonium ion interacts minimally with its surrounding lattice, even at high pressures: Its behavior is compatible with it being, aside from Coulombic attraction to the oxygen-dominated matrix, a largely non-interactive guest molecule within the silicate framework. This lack of interaction with the surrounding oxide lattice under compression may impact the stability of ammoniated minerals at high pressures and temperatures and ultimately likely favors nitrides or fluid phases as the dominant nitrogen carriers within the deeper mantle.