The structural evolution of
andesitic melts with varying compositions remains one of the unsolved questions
in high-temperature geochemistry and petrology. In this article, we report the
structural details of model andesitic glasses [CaO–MgO–Na2O–Al2O3–SiO2
(CMNAS)] in the diopside (CaMgSi2O6) and jadeite (NaAlSi2O6)
join using high-resolution, multi-nuclear, solid-state nuclear magnetic
resonance (NMR). The 27Al NMR spectra of CMNAS glasses confirm that [4]Al
is dominant. While a minor fraction of [5]Al is observed, its
presence is only prevalent in the glasses with higher Ca-Mg content.
Topological disorder in the glass network also tends to increase with Ca–Mg
content as evidenced by the increase in the quadrupolar coupling constant (Cq) of [4]Al for
glasses with increasing diopside contents (XDiopside).
Despite the complex nature of the glasses studied here (with five oxide
components), the 17O 3QMAS NMR spectra resolve diverse bridging
oxygens (BOs) and non-bridging oxygens (NBOs), from which the degree of Al
avoidance among framework cations (Si and Al) and preferential proximity among
non-network cations (Ca2+, Mg2+, and Na+) and
each oxygen site can be estimated: presence of Al–O–Al in jadeite glass implies
a violation of the Al-avoidance rule in the glasses and the decrease in the
fraction of NBOs with increasing XDiopside is consistent with a
decrease in their viscosity. Analysis of the peak position of {Ca, Mg}-mixed
NBOs, along with the absence of Na-NBO peak, and
the peak shape of Si–O–Al reveals preferential partitioning of Ca2+ and Mg2 into
NBOs and the proximity of Na+ to
BOs. The fraction of highly
coordinated Al has been linked to thermodynamic and transport properties of the
melts. Considering all the experimental Al coordination environments available
in the literature, together with the current experimental studies, we attempt
to establish the relationship between the fractions of highly coordinated Al
and composition, particularly average cationic potential of non-network forming cations (ave,
defined as cationic
potential normalized by the mole
fraction of each non-network cation). The fraction of highly coordinated Al
increases nonlinearly with increasing ave. The fraction
of [5,6]Al is negligible up to ave =~ 1.7, then it increases above ave
> ~1.7 regardless of changes in other compositional
variables (e.g., Si/Al, NBO content). This indicates the presence of a
threshold ave value for
the formation of [5,6]Al. The current experimental results with the
changes in network polymerization, coordination environments, and the degree of
disorder in the CMNAS glasses can improve understanding of the
structure–property (particularly, configurational thermodynamic properties)
relationships of multi-component natural silicate melts, including andesitic
melts and glasses.
 Figure | The predicted population of [5]Al in natural volcanic glass that includes basalt, rhyolite, andesite, phonolite, and trachyte with varying ave. |