Natural hydrous amorphous
silica: quantitation of network speciation and hydroxyl content by 29Si MAS NMR and
vibrational
spectroscopy
Chemtob SM, Rosman GR
Division of Geological and Planetary Sciences, California Institute of
Technology, Pasadena, CA
Stebbins JF
Department of Geological and Environmental Sciences, Stanford
University, Stanford, CA
Abstract
Natural
and synthetic hydrous
amorphous silicas were investigated with single-pulse 29Si
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. 29Si
NMR
spectra consisted of two broad, overlapping peaks at -111 and -102 ppm
and a
smaller peak at -92 ppm, corresponding to Q4, Q3
and Q2
sites, respectively. The Hawaiian silica coating and silicic acid
samples
displayed high Q3 and Q2
contents; in particular, the structural
Si-OH content of the coating was unusually high for a natural silica
(5.4±0.4
wt.% H2O). 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 (SiO3OH)
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.