Crystallization of iron‐containing sodium aluminosilicate glasses in the NaAlSiO4‐NaFeSiO4 join
2017
Ahmadzadeh, Mostafa | Marcial, José | McCloy, John
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.
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