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

OBJECTIVES

Here we aim to provide a background on X-Ray Diffraction analysis of quartz-like crystal structures with varying amounts of Al 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 LiAlSiO derivates, LiAlSiO - SiO 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, Initial LiSi Block and Amber® Mill in the range of LiAlO - SiO 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 SiO molar fraction found in the literature.

RESULTS

Based on the lattice parameters available for low quartz, high quartz and its stuffed derivatives, including LiAlSiO and the mineral virgilite, a plot of the a- and c-parameters vs. the mol% SiO related to LiAlO was constructed with the literature data and the data found for the three dental lithium silicates. As per the definitions of virgilite as either LiAlSiO, with 0.5 < x < 1 or especially as members of the LiAlSiO - SiO solid-solution series with more than 50 mol% LiAlSiO, the crystal structures in CEREC Tessera, Initial LiSi Block and Amber® Mill failed to fall within the ranges of mol% SiO 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 LiAlSiO - SiO solid solution is ambiguous and can be considered as arbitrary.