Understanding the structural origin of crystalline phase transformations in nepheline (NaAlSiO₄)‐based glass‐ceramics

Abstract

Nepheline (Na 6 K 2 Al 8 Si 8 O 32 ) is a rock-forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline-based glass-ceramics proceeds through several polymorphic transformationsmainly orthorhombic, hexagonal, cubicdepending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline-based glass-ceramics. Accordingly, two different sets of glasses (meta-aluminous and per-alkaline) have been designed in the system Na 2 O-CaO-Al 2 O 3 -SiO 2 in the crystallization field of nepheline and synthesized by the melt-quench technique. The detailed structural analysis of glasses has been performed by 29 Si, 27 Al, and 23 Na magic-angle spinningnuclear magnetic resonance (MAS NMR), and multiple-quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non-isothermal conditions using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass-ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na 2 O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO 2 -poor glass-ceramics with (Na 2 O+CaO)/Al 2 O 3 > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.

Structural dependence of crystallization in glasses along the nepheline (NaAlSiO4) ‐ eucryptite (LiAlSiO4) join

“…Shielding of theNa signal has been observed in aluminosilicate glasses where K → Na, in aluminosilicate and aluminoborosilicate glasses where Na → Ca, and in silicate glasses where Na → Li . This trend seems to indicate that substitution for a higher field strength cation tends to shield the 23 Na, explained as being due to the decreased concentration of negative charge on the coordinating oxygens, and as bridging oxygen to non‐bridging oxygen ratio increases in the coordination sphere of 23 Na …”

“…Electron probe microanalysis (EPMA) of nepheline dendrites in slow‐cooled HLW glasses revealed that their general formula could be written as (K,Na,Ca,Mg,vac) 2 Hex (Na,Ca,Mg,vac) 6 Oval (Si,Al,Fe) 16 O 32 where “Hex” refers to the large hexagonal rings comprising Fe‐O, Al‐O, or Si‐O tetrahedra, “Oval” refers to the so‐called “squashed” six‐membered rings of the same tetrahedral building blocks, and “vac” refers to a vacancy . Our recent studies, directed toward understanding the compositional dependence of nepheline crystallization in simplified HLW glasses, have focused on understanding the impact of various framework and non‐framework cations on the crystallization behavior of simplified HLW glasses …”

Crystallization of iron‐containing sodium aluminosilicate glasses in the NaAlSiO4‐NaFeSiO4 join

“…Carnegieite is the high‐temperature polymorph of nepheline with an orthorhombic crystal structure with ABCABC stacking [ Withers and Thompson , ], whereas nepheline is hexagonal with ABAB layer stacking of Al‐SiO 4 [ Nayak and Kutty , ]. Crystallization of nepheline is sometimes preceded by formation of orthorhombic carnegieite [ Deshkar et al ., ].…”

Crystallization behavior of iron‐ and boron‐containing nepheline (Na2O·Al2O3·2SiO2) based model high‐level nuclear waste glasses

“…However, extensive data pertaining to nepheline crystallization in glasses over a broader compositional space is required to strengthen this submixture model to design advanced glass formulations with increased waste loadings. The results from this study along with our previous studies will play a crucial role in further strengthening these predictive models.…”

“…On further heating to 1400°C, nepheline transforms into the high‐temperature (high‐T) cubic carnegieite, the stable polymorph of NaAlSiO 4 at that temperature . However, crystallization in SiO 2 ‐deficient (or Al 2 O 3 ‐rich) nepheline‐derived glasses has been shown to initiate from cubic carnegieite which may or may not transform into hexagonal nepheline depending on compositional complexity …”