Ionic network analysis of tectosilicates: the example of coesite at variable pressure.

Area of Science:

• Mineralogy
• Crystallography
• Materials Science

Background:

• The ionic network analysis method provides a framework for understanding crystal structures.
• Tectosilicates, such as coesite (a high-pressure polymorph of SiO2), exhibit complex structural arrangements.

Purpose of the Study:

• To extend the ionic network analysis method to tectosilicates.
• To predict unit-cell parameters and atomic coordinates of coesite at various pressure-temperature-composition (p-T-X) conditions.
• To develop a quantitative measure of tetrahedral distortion.

Main Methods:

• Utilizing structural refinements of coesite as a starting point.
• Defining a pseudocubic parameterization for the O4 cages within SiO4 tetrahedra.
• Developing parametric curves to describe the evolution of structural networks and pseudocubic parameters with pressure.
• Fitting parametric curves to crystallographic data.

Main Results:

• Successfully applied the extended ionic network analysis to coesite, feldspars, quartz, and cristobalite.
• Introduced a novel two-parameter quantification for tetrahedral distortion.
• Identified a pressure threshold (approx. 20.45 GPa) beyond which a self-consistent solution for coesite could not be found, suggesting a phase transition.

Conclusions:

• The extended ionic network analysis method is effective for predicting the behavior of tectosilicates under varying conditions.
• The findings suggest a phase transition in coesite at high pressures.
• The developed quantification of tetrahedral distortion offers new insights into the structural properties of tectosilicates.