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

Author(s): Dominik Talla, Anton Beran, Radek Škoda
Abstract: The 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 O^2- 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.
Organisation(s): Department of Mineralogy and Crystallography
External organisation(s): Masaryk University
Journal: Mineralogy and Petrology
Volume: 117
Pages: 359-372
No. of pages: 14
ISSN: 0930-0708
DOI: https://doi.org/10.1007/s00710-023-00824-3
Publication date: 04-2023
Peer reviewed: Yes
Austrian Fields of Science 2012: 105116 Mineralogy, 105113 Crystallography
Keywords
ASJC Scopus subject areas: Geophysics, Geochemistry and Petrology
Portal url: https://ucrisportal.univie.ac.at/en/publications/7f474827-fe84-44e7-bd7c-2dc7683be088

Back