『Abstract
For two suites of volcanic aluminosilicate glasses, the accessible
and reactive sites for covalent attachment of the fluorine-containing
(3,3,3-trifluoropropyl) dimethylchlorosilane (TFS) probe molecule
were measured by quantitative 19F nuclear magnetic
resonance (NMR) spectroscopy. The first set of samples consists
of six rhyolitic and dacitic glasses originating from volcanic
activity in Iceland and one rhyolitic glass from the Bishop Tuff,
CA. Due to differences in the reactive species present on the
surfaces of these glasses, variations in the rate of acid-mediated
dissolution (pH 4) for samples in this suite cannot be explained
by variations in geometric or BET-measured surface area. In contrast,
the rates scale directly with the surface density of TFS-reactive
sites as measured by solid-state NMR. These data are consistent
with the inference that the TFS-reactive M-OH species on the glass
surface, which are known to be non-hydrogen-bonded Q3
groups, represent loci accessible to and affected by proton-mediated
dissolution. The second suite of samples, originating from a chronosequence
in Kozushima, Japan, is comprised of four rhyolites that have
been weathered for 1.1, 1.8, 26, and 52 ka. The number of TFS-reactive
sites per gram increases with duration of weathering in the laboratory
for the “Icelandic” samples and with duration of field weathering
for both “Icelandic” and Japanese samples. One hypothesis is consistent
with these and published modeling, laboratory, and field observations:
over short timescales, dissolution is controlled by fast-dissolving
sites, but over long timescales, dissolution is controlled by
slow-dissolving sites, the surface density of which is proportional
to the number of TFS-reactive Q3 sites. These latter
sites are not part of a hydrogen-bonded network on the surface
of the glasses, and measurement of their surface site density
allows predictions of trends in reactive surface area. The TFS
treatment method, which is easily monitored by quantitative 19F
solid-state NMR, therefore provides a chemically specific and
quantifiable proxy to understand the nature of how sites on dissolving
silicates control dissolution. Furthermore, 27Al NMR
techniques are shown here to be useful in identifying clays on
the glass surfaces, and these methods are therefore effective
for quantifying concentrations of weathering impurities. Our interpretations
offer a testable hypothesis for the mechanism of proton-promoted
dissolution for low-iron aluminosilicate minerals and glasses
and suggest that future investigations of reactive surfaces with
high-sensitivity NMR techniques are warranted.』
1. Introduction
2. Methods
2.1. Samples
2.2. Sample preparation
2.3. Surface modification
2.4. Nuclear magnetic resonance
3. Results
3.1. Observations from the “Icelandic” suite (including Bishhop
Tuff)
3.2. Observations from the Japanese chronosequence suite
4. Discussion
5. Conclusions
Acknowledgments
Appendix I. Clay reactivity to silanization
References