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

Abstract

Although natural materials are the subject of most Earth science articles, fundamental studies on analogous synthetic materials, produced under laboratory‐controlled conditions, can provide significant insight into expected behavior of natural systems. Iron, a common element in natural aluminosilicates as well as high‐level nuclear wastes, plays a crucial role in crystallization behavior. In the present study, effects of Fe‐Al substitution in nepheline‐based aluminosilicate glasses (NaAl(1 − x)FexSiO4, x = 0.0–1.0) were investigated to assess the role of iron in crystallization, employing semiquantitative X‐ray diffraction (XRD), vibrating sample magnetometry (VSM), and electron probe microanalysis (EPMA). Fe promotes nepheline crystallization when substituted for Al in low additions (x < 0.3), yet suppresses it at higher additions (x > 0.5). Since effect of Fe is the subject of the present work and is the most common magnetic element, magnetic techniques were used to further analyze the phase assemblage. VSM measurements revealed that Fe oxides, i.e., hematite and magnetite, are present in cases even when their fractions are below the XRD detection limit, and backscattered electron micrographs confirm their presence. EPMA also shows that Fe incorporation in nepheline increases with increasing Fe‐Al substitution, up to a maximum of x = 0.37 for the nepheline crystals in the sample with starting glass of Na(Al0.3Fe0.7)SiO4. The residual glass, on the other hand, contains approximately constant Fe concentration x ~ 0.54–0.59 for all samples with starting Fe addition 0.4 ≤ x ≤ 0.8, and excess iron is expelled into Fe oxide phases. The significance of these results for geological processes and immobilization of high‐level nuclear waste is discussed.

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

“…The heat‐treatment atmosphere (air vs inert vs reducing) does not exhibit a significant impact on the crystalline phase assemblage of the resultant glass‐ceramics. When compared with the crystalline phase assemblage of the AF‐0 glass‐ceramic, it is evident that iron tends to promote crystallization of nepheline over carnegieite as has also been shown in our previous studies . The high amount of residual glassy phase (56%‐75%) in all the glass‐ceramics may be attributed to the presence of 10 mol.% B 2 O 3 in the studied glass system.…”

“…Magnetic measurements, owing to their nondestructive nature and high sensitivity toward iron‐containing phases, are a rapid characterization technique to obtain valuable information about distribution of Fe in different phases present in the sample . Therefore, magnetic measurements were performed on isothermally produced glass‐ceramics using a vibrating sample magnetometer (VSM, PMC3900; Lakeshore Cryotronics, Westerville, OH).…”

“…In the second scenario, the as‐formed spinel crystals tend to act as nucleation sites for the crystallization of nepheline during cooling of HLW glass in canisters, which results in a waste form with poor chemical durability . In our recent studies, we have shown that iron forms a solid solution with nepheline crystallized from NaAl (1− x ) Fe x SiO 4 glass, with a level of incorporation up to x = 0.37, and promotes the crystallization of nepheline over carnegieite. Given the strong interaction of iron with nepheline, it is imperative to understand the chemistry of iron in HLW glasses, and its implications for crystallization behavior.…”

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

“…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 …”

Magnetic analysis of commercial hematite, magnetite, and their mixtures

“…Magnetite is a ferrimagnetic phase with high magnetization (∼92 Am 2 /kg) and low coercivity (10-40 mT) while hematite is a canted antiferromagnetic phase having a small magnetization (∼0.4 Am 2 /kg) and high coercivity (∼100-400 mT). 1,[5][6][7][8][9] Since oxidation-reduction reactions often occur in iron oxides, mixtures of magnetite-hematite are common, and magnetic interactions between discrete particles in a dense compact introduce additional complexity. It is reported that magnetizations of different phases are linearly additive in non-interacting mixtures, while interacting assemblages can introduce nonlinearities.…”