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Natural hydrous amorphous silica: Quantitation of network speciation and hydroxyl content by ^(29)Si MAS NMR and vibrational spectroscopy

Chemtob, Steven M. and Rossman, George R. and Stebbins, Jonathan F. (2012) Natural hydrous amorphous silica: Quantitation of network speciation and hydroxyl content by ^(29)Si MAS NMR and vibrational spectroscopy. American Mineralogist, 97 (1). pp. 203-211. ISSN 0003-004X.

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Natural and synthetic hydrous amorphous silicas were investigated with single-pulse ^(29)Si magic angle spinning (MAS) NMR and with vibrational spectroscopic methods. Samples included a volcanically derived silica coating on young basalt from Kilauea, Hawaii, as well as hyalite (opal-AN), silica sinters, and synthetic silica gels and silicic acid. Pulse delays of up to an hour were employed for silica samples with slow spin lattice relaxation rates, and nearly fully relaxed spectra (90–100%) were demonstrably achieved for all samples. ^(29)Si NMR spectra consisted of two broad, overlapping peaks at −111 and −102 ppm and a smaller peak at −92 ppm, corresponding to Q^4, Q^3, and Q^2 sites, respectively. The Hawaiian silica coating and silicic acid samples displayed high Q^3 and Q^2 contents; in particular, the structural Si-OH content of the coating was unusually high for a natural silica (5.4 ± 0.4 wt% H_(2)O). Saturation-recovery spectra of the Hawaiian silica with increasing delay times were consistent with “stretched exponential” relaxation behavior and three-dimensional distribution of paramagnetic centers. Attenuated total reflectance infrared (ATR-IR) and Raman spectra of the silica powders indicated fully amorphous structures, and displayed hydrous (SiO_(3)OH) and anhydrous silicate vibrational bands in positions consistent with previous work. Raman spectra of some samples indicated modest grain to grain heterogeneity. Inferred Si-OH contents from ATR-IR band ratios were strongly correlated with hydroxyl contents calculated from NMR spectra. The high Si-OH content of the Hawaiian silica coating suggests it is diagenetically immature and has not been exposed to elevated temperatures.

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Rossman, George R.0000-0002-4571-6884
Additional Information:© 2012 by the Mineralogical Society of America. Manuscript received June 21, 2011; manuscript accepted September 25, 2011; manuscript handled by Brian Phillips. We thank Don Lowe (Stanford University) for the Yellowstone silica samples, and two anonymous reviewers for helpful comments. This research was supported by the National Science Foundation, through grant EAR-1019596 (J.F. Stebbins), by the White Rose Foundation (G.R. Rossman), and by a National Aeronautics and Space Administration graduate student fellowship (NESSF).
Funding AgencyGrant Number
White Rose FoundationUNSPECIFIED
NASA graduate student fellowshipUNSPECIFIED
Subject Keywords:Amorphous silica; NMR spectroscopy; Raman spectroscopy; IR spectroscopy
Issue or Number:1
Record Number:CaltechAUTHORS:20120203-115857442
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:29128
Deposited By: Jason Perez
Deposited On:03 Feb 2012 21:06
Last Modified:03 Oct 2019 03:38

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