American Mineralogist, Volume 96, pages 841–853, 2011
0003-004X/11/0506–841$05.00/DOI: 10.2138/am.2011.3680 841
Non-bridging oxygen and high-coordinated aluminum in metaluminous and peraluminous
calcium and potassium aluminosilicate glasses: High-resolution
17
O and
27
Al MAS NMR results
Linda M. ThoMpson* and JonaThan F. sTebbins
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.
absTracT
Change in configuration and structure with composition are important components of thermody-
namic and transport properties of most aluminosilicate melts, but the complex interactions, particu-
larly in metaluminous and peraluminous compositions, are not yet well understood. In this paper, we
present high-resolution
27
Al and
17
O MAS and 3QMAS NMR data on several calcium and potassium
aluminosilicate glasses prepared at several SiO
2
isopleths, ranging from peralkaline to peraluminous
compositions. In all calcium aluminosilicate glasses, the
V
Al content increases with increasing Al
content, while the NBO content decreases (in one series, below the limit of detection), consistent with
other recent NMR studies of calcium aluminosilicate glasses. An increase in
V
Al content per total Al is
also observed as the glass approaches the calcia-silica binary. In the potassium aluminosilicate glasses,
NBO is directly quantified for the first time in metaluminous and peraluminous compositions;
V
Al is
below the detection limit. We discuss possible mechanisms for the incorporation of alumina into the
melt structure and show how changes in the
V
Al content as a function of composition may be used
to eliminate mechanisms that do not fit the observed data. We also explore reactions, which show
the difficulty of explaining the NBO present on the metaluminous join with only the observed
V
Al,
implying the necessity of multiple reactions producing NBO in such compositions.
Keywords: Aluminosilicate glasses, non-bridging oxygen, high-coordinated aluminum, NMR
spectroscopy, oxygen-17, aluminum-27
inTroducTion
Aluminosilicate glasses and melts are an important class of
materials in both geological processes, where they include almost
all magmas, and in technology, where they are used in flat-panel
computer display screens, glass fibers for composites, and many
other applications. As such, their properties have been the subject
of much research, recently reviewed in detail (Mysen and Richet
2005; Mysen and Toplis 2007; Neuville et al. 2006; Stebbins et al.
1995). It is known that the structure of silicate melts and glasses
has an impact on a wide range of macroscopic properties such as
viscosity, silica activity, diffusivity, and density (Giordano and
Dingwell 2003; Lange and Carmichael 1987; Mysen et al. 1982).
In particular, the network connectivity is a significant factor in
thermodynamic and transport properties (Lee et al. 2004; Lee
and Stebbins 2006). The amount of non-bridging oxygen atoms
(NBO, an O atom bound to a single network former, requiring
network modifiers to compensate the remaining valence charge)
present in the melt is commonly used as a measure of the net-
work connectivity, described as the fraction of NBO (X
NBO
=
non-bridging oxygen/total O atoms) or as the mean number of
NBO per tetrahedral cations (NBO/T).
The common method for determination of the NBO content
in glasses has relied on composition alone, in which calculations
are based on assumptions about the glass structure. Beginning
with concepts derived from crystal structures, the most common
model (referred to here as the “standard” model) assumes that
all Al
3+
cations are four-coordinated (
IV
Al) in ambient-pressure
melts with modifier oxides equal to or in excess of the alumina
content of the melt, e.g., mol% Al
2
O
3
≤ CaO or K
2
O. Along
metaluminous aluminate-silica joins (e.g., CaAl
2
O
4
-SiO
2
), it is
generally assumed that all O atoms are bridging (BO), with the
amount of NBO then increasing as the ratio of Al
2
O
3
to modifier
oxide decreases. In peralkaline or peralkaline earth compositions
(e.g., mol% Al
2
O
3
< CaO or K
2
O), the concentration of NBO are
assumed to be dictated by stoichiometry, with the formation of
2 mol of NBO for each “excess” mole of modifier oxide. This
model offers no suggestions in regard to the structural behavior
in the peraluminous region (e.g., mol% Al
2
O
3
> CaO or K
2
O).
Nonlinear effects on melt density and viscosity at higher
alumina contents suggested a potential shift to higher alumi-
num coordination (Bottinga et al. 1982; Riebling 1966). Direct
observations of this phenomenon are from high-resolution
27
Al
NMR studies (Neuville et al. 2004; Stebbins et al. 2000), which
have recently demonstrated that most glasses in the “percalcic”
region of the CaO-Al
2
O
3
-SiO
2
system contain about 4 to 7%
V
Al (Neuville et al. 2006). Exceptions include glasses with a
high-silica content or glasses with a low-silica and high-calcia
content. High-coordinated aluminum has also been detected in
ambient-pressure Mg- and alkali aluminosilicate compositions
(Allwardt et al. 2005b; McMillan and Kirkpatrick 1992; Neuville
et al. 2008; Toplis et al. 2000) and is predicted in large quantities
by some molecular dynamics simulations of aluminosilicates,
although this depends on model potentials and generally rep-
resents the melt at very high temperatures (Morgan and Spera
2001; Poe et al. 1992; Poole et al. 1995; Scamehorn and Angell
* E-mail: lison@stanford.edu