Nepheline structural and chemical dependence on melt composition

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

“…Within HLW compositions, early property‐composition relationship measurements found that Na 2 O and Li 2 O had the strongest effect on the electrical conductivity, viscosity, and glass transition of a melt while Na 2 O, Li 2 O, and B 2 O 3 were found to have the strongest effect on increasing B release . In our previous work, we looked at the effect of high alkali in melt on the composition of nephelines, it was determined that melts with high Li incorporated almost no Ca but rather some Fe . However, as a low‐molecular weight atom, Li is difficult to analyze using standard electron probe microanalysis techniques and whether or not Li enters nepheline was unknown; however, in the HLW glasses measured in that study the fraction of Li would likely be small since the compositional totals neared 100%.…”

“…However, as a low‐molecular weight atom, Li is difficult to analyze using standard electron probe microanalysis techniques and whether or not Li enters nepheline was unknown; however, in the HLW glasses measured in that study the fraction of Li would likely be small since the compositional totals neared 100%. Despite this, the Li‐rich glass was found to crystallize the second highest fraction of nepheline (second to the K‐rich glass) . Therefore the questions arose: to what extent was Li enriched in the residual glass, how Li in the residual glass affects nepheline crystallization, and at which levels would lithium aluminosilicate phases crystallize from the residual glass.…”

“…Fundamental studies of this Al/B composition series synthesized from sol‐gels suggest that the lithium‐containing crystalline phase is β‐eucryptite (LiAlSiO 4 ) for 0 ≤ y ≤ 0.4 and β‐spodumene (LiAlSi 2 O 6 ) for 0.6 ≤ y ≤ 0.8 . In HLW glasses, lithium is also assumed to chemically partition away from nepheline crystals, leading to the formation of β‐eucryptite, particularly in highly undercooled glasses . The presence of lithium‐bearing phases was verified through X‐ray diffraction (XRD); however, compositional measurements of Li‐bearing phases are not possible through conventional techniques such as EPMA due to low energy X‐ray emission from Li.…”

“…The most common phase observed to form upon cooling of glass melts in the compositional space of interest is nepheline (nominally NaAlSiO 4 ), a tectosilicate feldspathoid . 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 …”

“…The HLW nepheline composition is comparable to that observed in geological samples, despite a significant presence of boron and lithium in the HLW melt, which is not in typical geological systems . Boron in crystallized nuclear glass is assumed to chemically partition into the residual glassy phase, and in simple glass systems (of the form NaB y Al 1‐ y SiO 4 ), replacing aluminum with boron has been shown to decrease the crystalline fraction of sodium‐bearing glasses as a function of increasing boron . In lithium‐bearing simple glasses (of the form LiB y Al 1‐ y SiO 4 ), boron addition has been shown to increase their tendency toward crystallization up to y = 0.15, then it decreases with further boron addition .…”