American Mineralogist, Volume 91, pages 1922–1926, 2006

Depolymerization effect of water in aluminosilicate glasses: Direct evidence from 1H-27Al
heteronuclear correlation NMR
XIANYU XUE* AND MASAMI KANZAKI
Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori 682-0193 Japan

ABSTRACT
We have applied one-dimensional (1D) 1H MAS NMR, 27Al → 1H CP MAS NMR, as well as 2D 27Al
triple-quantum (3Q) MAS NMR, 27Al → 1H heteronuclear correlation (HETCOR) and high-resolution
3QMAS/HETCOR NMR techniques to KAlSi3O8 (Or), NaAlSi3O8 (Ab) and NaAlSiO4 (Ne) glasses
containing 0~2 wt% H2O to shed light on the dissolution mechanisms of water in aluminosilicate melts(glasses). An Al Q3-OH group, characterized by 1H chemical shifts of 1.3–1.9 ppm, was identiÞed for
all hydrous glasses. Its abundance increases with bulk Al/Si ratio. The 27Al chemical shifts (δiAl) of
this species are 64–68 ppm, larger than those of Al Q4 by 3–6 ppm. Despite this difference, it is only
through 27Al → 1H HETCOR and 3QMAS/HETCOR, but not 27Al MAS or 3QMAS NMR that thepeaks are resolved. This study has demonstrated that depolymerization and formation of AlOH/SiOH isa general water dissolution mechanism for polymerized aluminosilicate melts (glasses), and HETCORNMR experiments involving 1H are the key to its revelation.
Keywords: NMR, water, aluminosilicate glass, heteronuclear correlation, 1H, 27Al, depolymerization

INTRODUCTION
Solubility and dissolution mechanisms of water in silicate
melts and glasses are an issue of major interest, because water
is the most abundant volatile component in natural magmas andcan have dramatic effects on their phase relations and physicalproperties (cf. Mysen and Richet 2005). It is generally acceptedthat water is dissolved in silicate melts/glasses as both molecular
H2O and hydroxyls (OH), with the latter dominant at low watercontents (<2–3 wt%) and leveling off at high contents (cf. Mysenand Richet 2005). For Al-free silica and alkali silicate compositions, SiOH has been conÞrmed to be a major OH species by 29Si
magic-angle spinning (MAS) and 1H → 29Si cross-polarization(CP) MAS NMR studies (e.g., Farnan et al. 1987; Kümmerlenet al. 1992; Zotov and Keppler 1998; Xue and Kanzaki 2004;
Cody et al. 2005). The formation of SiOH is accompanied by
the rupture of Si-O-Si linkages that in turn may be responsible
for changes in many melt properties, such as the reduction ofviscosity (cf. Mysen and Richet 2005). However, for aluminosilicate compositions, debate continues as to whether similar depolymerization occurs. Models based on solubility and property
behavior or on vibrational spectroscopic data mostly involved
breakage of T-O-T bonds (T: Si or Al), although the details may
differ (cf. Mysen and Richet 2005). However, from multi-nuclear
NMR studies of hydrous NaAlSi3O8 (Ab), NaAlSiO4 (Ne), andCaAl2Si6.6O17.3 glasses, Kohn et al. (1989, 1992) found thatchanges in the 29Si MAS and 1H → 29Si CP MAS NMR spectra
are smaller than those of silica glasses and the 27Al MAS NMR
spectra become narrower with increasing water content. Theyinterpreted this as evidence for insigniÞcant abundances of Si
Q3-OH [SiO4 with three Si/Al and one H next nearest neighbors(NNN)], Si-NBO (nonbridging oxygen), and Al Q3-OH. Kohn et* E-mail: xianyu@misasa.okayama-u.ac.jp0003-004X/06/1112–1922$05.00/DOI: 10.2138/am.2006.2365

al. (1989, 1992) proposed that water dissolution does not disrupt
the aluminosilicate network, but involves Na-H exchange andformation of bridging OH [Si(OH)Al] and NaOH complexes.
Schmidt et al. (2000) examined glasses on the Qz (SiO2)–Ab
join, and suggested from similar NMR spectral changes between
Qz-rich and Qz glasses and between Ab-rich and Ab glasses thatthe water dissolution mechanism changes from formation ofSiOH for the former to one similar to that proposed by Kohn etal. (1989) for the latter. A recent 27Al triple-quantum (3Q) MAS
NMR study of hydrous Ne and NaAlSi7.7O17.4 glasses (Padro etal. 2003) revealed no resolvable additional peak and suggested
that the data support the model of Kohn et al. (1989). However,the 27Al and 29Si NMR characteristics of the different proposed
species are not well constrained. Others have argued that theNMR data are consistent with formation of SiOH/AlOH (Sykeset al. 1997; Zeng et al. 2000). Several 17O MAS and 3QMAS
NMR studies (Xu et al. 1998; Oglesby et al. 2001, 2002) havealso been reported, although spectral interpretations were often
inconclusive.
1H NMR is potentially a more sensitive probe of water
speciation because it detects all of the water species, but is notcomplicated by the presence of anhydrous species even at low
water contents. One-dimensional (1D) 1H MAS NMR often
yields unresolved peaks for hydrous aluminosilicate glasses
because of the small chemical shift (δiH) range (Kohn et al.
1989; Schmidt et al. 2000, 2001a). Nevertheless, it is possible
to increase the resolving power via double-resonance NMR.
A pioneering study of this type was the application of 27Al →1
H and 23Na → 1H TRAPDOR (Transfer of Population DoubleResonance) NMR to hydrous NaAlSi8O18-Ne glasses by Zeng etal. (1999). The data revealed a 1H peak near 1.5 ppm with large
1
H-27Al dipolar coupling, which was attributed to AlOH (Zeng etal. 1999). Additional insight may also be obtained from ab initio

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