Structure and disorder in basaltic glasses and melts: Insights from high-resolution solid-state NMR study of glasses in diopside–Ca-tschermakite join and diopside–anorthite eutectic composition

Abstract

Here, we report experimental results on the effects of composition on the structure of quaternary CaO–MgO–Al2O3–SiO2 (CMAS) glasses in diopside (CaMgSi2O6) and Ca-tschermakite (CaAl2SiO6) join and glass in the diopside–anorthite eutectic composition (Di64An36)—model systems for basaltic melts—using multi-nuclear solid-state NMR. The 27Al 3QMAS NMR spectra of CMAS glasses in diopside–Ca-tschermakite join show predominant [4]Al and a non-negligible fraction of [5]Al. The fraction of [5]Al species increases with increasing mole fraction of diopside (XDiopside). The structurally relevant quadrupolar coupling constant of [4]Al in the glasses decreases with increasing XDiopside, suggesting a decrease in network distortion around [4]Al. Approximately 3.3% of [5]Al is observed for Di64An36 glass, consistent with a previous study (Xue and Kanzaki, 2007). There are also non-negligible fraction of Al–O–Al and significant fractions of Si–O–Al in Di64An36 glass, indicating extensive mixing between Si and Al and violation of the Al-avoidance in basaltic glasses. The 17O 3QMAS NMR spectra of CMAS glasses show that three types of bridging oxygens (BO, Si–O–Si, Al–O–Al, and Si–O–Al) and two types of non-bridging oxygens (NBO, Ca–NBO, and mixed {Ca,Mg}–NBO) are partially resolved. The fraction of NBO in the basaltic glasses decreases with decreasing XDiopside. A presence of the prominent 3Ca–NBO peak (NBO surrounded by three Ca2+ cations) in the CMAS glass at an intermediate compositions (XDiopside=0.5) suggests non-random distributions of Ca2+ and Mg2+ around NBOs and BOs, characterized either by preferential partitioning of Ca2+ into NBOs and/or structural arrangement toward unmixing of Ca2+ and Mg2+ around NBO.The observed structural changes in the CMAS glasses can provide an improved understanding of their structure–property relationships. The predominance of [4]Al and its extensive mixing with [4]Si is consistent with the negative enthalpy of mixing for CMAS glasses obtained by solution calorimetry. The observed increase in NBO fraction (as also expected from the chemical composition) with increasing XDiopside indicates an obvious decrease in melt viscosity toward a diopside endmember. The partitioning of Ca2+ and Mg2+ and/or unmixing of these cations between NBOs and BOs may result in variations in the activity coefficients of CaO and MgO, thus the compositions of melts.