On the assignment of quartz-like LiAlSi2O6 - SiO2 solid solutions in dental lithium silicate glass-ceramics: Virgilite, high quartz, low quartz or stuffed quartz derivatives?

Abstract

Objectives: Here we aim to provide a background on X-Ray Diffraction analysis of quartz-like crystal structures with varying amounts of Al³⁺ and Li⁺ substitution, existing confusions on their nomenclature and its implications for novel lithium silicate glass-ceramics.

Methods: We reviewed the literature dealing with modifications of the quartz crystal structure and their stuffed LiAlSi₂O₆ derivates, LiAlSi₂O₆ - SiO₂ solid solutions, the terminology of such phases and criteria used to define the structure known as virgilite. Based on this information, we attempted to allocate the quartz-like phases found in CEREC Tessera^TM, Initial^TM LiSi Block and Amber® Mill in the range of LiAlO₂ - SiO₂ solid solutions. For this purpose, their lattice parameters obtained from Rietveld refinement were compared with the lattice parameters of members of the corresponding solid solutions with defined SiO₂ molar fraction found in the literature.

Results: Based on the lattice parameters available for low quartz, high quartz and its stuffed derivatives, including LiAlSi₂O₆ and the mineral virgilite, a plot of the a- and c-parameters vs. the mol% SiO₂ related to LiAlO₂ was constructed with the literature data and the data found for the three dental lithium silicates. As per the definitions of virgilite as either Li_xAl_xSi_3-xO₆, with 0.5 < x < 1 or especially as members of the LiAlSi₂O₆ - SiO₂ solid-solution series with more than 50 mol% LiAlSi₂O₆, the crystal structures in CEREC Tessera^TM, Initial^TM LiSi Block and Amber® Mill failed to fall within the ranges of mol% SiO₂ confined for virgilite.

Significance: Based on available literature and definitions, the quartz-like phases found in the three dental lithium silicates should be addressed as stuffed (probably low) quartz solid solutions instead of "virgilite". However determined by mineralogical practices, the term "virgilite" for parts of the LiAlSi₂O₆ - SiO₂ solid solution is ambiguous and can be considered as arbitrary.

Keywords: Crystal; Glass-ceramics; Lithium silicate; Quartz; Virgilite; X-ray diffraction.