Natural and artificial OH defect incorporation into fluoride minerals at elevated temperature—a case study of sellaite, villiaumite and fluorite

Abstract

AbstractThe long-known presence of a sharp OH absorption band in the tetragonal fluoride mineral sellaite, MgF2, inspired us to conduct a detailed study of the OH incorporation modes into this IR-transparent (where IR stands for Infrared) material as well as to search for hydrogen traces in two other IR-translucent halides—villiaumite (NaF) and fluorite (CaF2). Among these three phases, sellaite is the only one to incorporate ‘intrinsic’ OH groups, most commonly as O–H∙∙∙F defects oriented nearly perpendicular to the c-axis along the shortest edge of the constituent MgF6 polyhedra, in analogy with the isostructural mineral rutile, TiO2. Another defect type, seen only scarcely in untreated natural material, develops when subjecting sellaite to temperatures above 900 °C. It involves an O–H∙∙∙O cluster along the 2.802 Å edge of the original MgF6 dipyramid, as fluorine atoms are progressively expelled from the structure, being replaced by O2- anions. This is corroborated by the appearance of spectral absorption features typical for brucite (Mg(OH)2) and ultimately periclase (MgO), the presence of which could be proven via powder diffraction of the heat-treated material. Except for a ‘dubious’ peak most probably caused by included phases, neither villiaumite (NaF) nor fluorite (CaF2) showed any presence of ‘intrinsic’ OH defects. They do however decompose along a similar route into the respective oxide and hydroxide phases at high temperature. This thermal decomposition of the studied halide phases is accompanied by the emission of gaseous (HF)n species at temperatures well below their established melting point - a subject which seems to be quite overlooked.