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

Here, we report experimental results on the effects of composition on the structure of quaternary CaO-MgO-Al ₂O ₃-SiO ₂ (CMAS) glasses in diopside (CaMgSi ₂O ₆) and Ca-tschermakite (CaAl ₂SiO ₆) join and glass in the diopside-anorthite eutectic composition (Di ₆₄An ₃₆)-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 (X _Diopside). The structurally relevant quadrupolar coupling constant of ^[4]Al in the glasses decreases with increasing X _Diopside, suggesting a decrease in network distortion around ^[4]Al. Approximately 3.3% of ^[5]Al is observed for Di ₆₄An ₃₆ 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 Di ₆₄An ₃₆ 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 X _Diopside. A presence of the prominent ³Ca-NBO peak (NBO surrounded by three Ca ²⁺ cations) in the CMAS glass at an intermediate compositions (X _Diopside=0.5) suggests non-random distributions of Ca ²⁺ and Mg ²⁺ around NBOs and BOs, characterized either by preferential partitioning of Ca ²⁺ into NBOs and/or structural arrangement toward unmixing of Ca ²⁺ and Mg ²⁺ 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 X _Diopside indicates an obvious decrease in melt viscosity toward a diopside endmember. The partitioning of Ca ²⁺ and Mg ²⁺ 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.