Experimental determination of the effect of the ratio of B/Al on glass dissolution along the nepheline (NaAlSiO 4)–malinkoite (NaBSiO 4) join

• Published in: Geochimica et cosmochimica acta Vol. 74; no. 9; pp. 2634 - 2654
• Main Authors: Pierce, E.M., Reed, L.R., Shaw, W.J., McGrail, B.P., Icenhower, J.P., Windisch, C.F., Cordova, E.A., Broady, J.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 01-05-2010; The Geochemical Society; The Meteoritical Society
• Subjects: ACTIVATION ENERGY; Aluminum; ATOMS; AVOIDANCE; CATIONS; CHEMICAL SHIFT; DISSOLUTION; DISTRIBUTION; Enrichment; Environmental Molecular Sciences Laboratory; GEOSCIENCES; GLASS; KINETICS; RAMAN SPECTROSCOPY; RUPTURES; SPECTRA; TEMPERATURE DEPENDENCE
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2009.09.006

The dissolution kinetics of five glasses along the NaAlSiO
4–NaBSiO
4 join were used to evaluate how the structural variations associated with boron–aluminum substitution affect the rate of dissolution. The composition of each glass varied inversely in mol% of Al
2O
3 (5–25
mol%) and B
2O
3 (20–0
mol%) with Na
2O (25
mol%) and SiO
2 (50
mol%) making up the remaining amount, in every case Na/(Al
+
B)
=
1.0. Single-pass flow-through experiments (SPFT) were conducted under dilute conditions as a function of solution pH (from 7.0 to 12.0) and temperature (from 23 to 90
°C). Analysis of unreacted glass samples by
27Al and
29Si MAS-NMR suggests Al (∼98%
[4]Al) and Si-atoms (∼100%
[4]Si) occupy a tetrahedral coordination whereas, B-atoms occupy both tetrahedral (
[4]B) and trigonal (
[3]B) coordination. The distribution of
[3]B fractionated between
[3]B(ring) and
[3]B(non-ring) moieties, with the
[3]B(ring)/
[3]B(non-ring) ratio increases with an increase in the B/Al ratio. The MAS-NMR results also indicated an increase in the fraction of
[4]B with an increase in the B/Al ratio. The
27Al peak maxima shift to lesser values with an increase in the B/Al ratio which suggests mixing between the
[4]Al and
[3]B sites, assuming avoidance between tetrahedral trivalent cations (
[4]Al–O–
[4]B avoidance). Unlike the
27Al and
11B spectra, the
29Si spectra illustrate a subtle shift to more negative chemical shift (chemical shift range between −88 and −84
ppm) and increases in the spectral widths as the B/Al ratio increases. Raman spectroscopy of unreacted glass samples was also used to cross-check the results collected from MAS-NMR and suggested that NeB4 (the glass sample with the highest B content) may consist of B–Na enriched and Al–Si enriched micro-domains, which affected the measured dissolution rates. Results from SPFT experiments suggest a forward rate of reaction and pH power-law coefficients,
η, that are independent of B/Al under these neutral to alkaline test conditions for all homogeneous glasses. The temperature dependence shows an order of magnitude increase in the dissolution rate with a 67
°C increase in temperature and suggests dissolution is controlled by a surface-mediated reaction, as indicated by the activation energy,
E
a
, being between 44
±
8 and 48
±
7
kJ/mol. Forward dissolution rates, based on Na and Si release, for homogeneous glasses are independent of the B/Al ratio, whereas dissolution rates based on Al and B release are not. Normalized dissolution rates, based on B release, increase with the molar fraction of
[3]B(ring). Finally in accord with previous studies, the data discussed in this manuscript suggest rupture of either the Al–O or Si–O bonds as the rate-limiting step controlling the dissolution of these glasses.