Ammonium ion behaviour in feldspar: variable-temperature infrared and 2H NMR studies of synthetic buddingtonite, N(D,H)4AlSi3O8

Mookherjee, M. and Welch, M. D. and Le Pollès, L. and Redfern, S. A. T. and Harlov, D. E. (2005) Ammonium ion behaviour in feldspar: variable-temperature infrared and 2H NMR studies of synthetic buddingtonite, N(D,H)4AlSi3O8. Physics and Chemistry of Minerals, 32. pp. 126-131. DOI 10.1007/s00269-005-0455-x

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Abstract

The behaviour of the ammonium ion in synthetic buddingtonite, N(D,H)4AlSi3O8, has been studied by infrared (IR) spectroscopy from 20 K to 298 K and by 2H NMR spectroscopy from 120 K to 298 K. IR spectra were collected from 500 to 3500 cm–1. Static 2H NMR spectra collected at 298 K and 120 K are very similar, consisting of a single sharp isotropic resonance, indicating complete averaging of quadrupolar interactions and implying that at these temperatures the ammonium ion is in rapid (<>mgrs) randomised motion within the M-site cavity of the feldspar framework. NMR spectroscopy indicates that the splitting of the internal modes of the ammonium ion observed by IR spectroscopy is not due to ldquofreezing inrdquo of the ammonium ion. This observation rules out the formation of a preferred N–H...O hydrogen bond, with precession of the ion about it, as proposed by Kimball and Megaw (1978), because any N–H...O hydrogen bond must be very weak and transient in nature. Contraction of the cavity site upon cooling imposes a distortion upon the ammonium ion that affects vibrational modes. This distortion does not affect the motion of the ammonium ion as observed on the NMR time-scale.

Item Type:	Article
Additional Information:	GeoForschungsZentrum,cml
Uncontrolled Keywords:	2005 AREP 2005 P IA48
Subjects:	03 - Mineral Sciences
Divisions:	03 - Mineral Sciences
Journal or Publication Title:	Physics and Chemistry of Minerals
Volume:	32
Page Range:	pp. 126-131
Identification Number:	10.1007/s00269-005-0455-x
Depositing User:	Sarah Humbert
Date Deposited:	31 Jan 2011 17:03
Last Modified:	23 Jul 2013 09:59
URI:	http://eprints.esc.cam.ac.uk/id/eprint/1754

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