An updated model of Rietveld structure refinement of Na/K–feldspar

Abstract

Alkali feldspar is highly abundant on earth, and studies of its structure are very important in geology. When a single crystal is unavailable for alkali feldspar in some cases, X-ray powder diffraction (XRPD) is essential. However, some problems on extracting high-quality structural data of Na/K–feldspar from XRPD exist, such as deviation of Al/Si–O bond length from expected value, difficulty in identifying Al and Si elements with similar scattering power because of their same crystallographic site, and severe overlapping of diffraction peaks caused by a low symmetry of crystal structure. To extract high-quality structural data, two equations related to Si/Al–O bond length and Si/Al occupancy in terms of the crystal structure database were respectively calculated for albite and microcline. A better model was established by tactically combining the two equations and soft distance constraints for Al/Si–O bond lengths. Application of the updated model successfully obtained high-quality structural data from two different samples using Bragg–Brentano XRPD via the Rietveld method with the general structure analysis system. The reliability of these structural data for low-symmetry albite and microcline was discussed by comparing the data with the results of previous studies.