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

• Published in: Mineralogy and petrology Vol. 117; no. 2; pp. 359 - 372
• Main Authors: Talla, Dominik, Beran, Anton, Škoda, Radek
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 01.06.2023; Springer Nature B.V
• Subjects: Absorption; Absorption spectra; Anions; Brucite; Calcium fluoride; Defects; Earth and Environmental Science; Earth Sciences; Emission analysis; Fluorides; Fluorine; Fluorite; Geochemistry; Halides; Heat treatment; High temperature; Hydroxides; Inorganic Chemistry; Magnesium fluorides; Magnesium hydroxide; Melting point; Melting points; Mineralogy; Minerals; Periclase; Phases; Polyhedra; Rutile; Temperature; Thermal decomposition; Thermal degradation; Titanium dioxide; Sellaite; Fluorite; Deuteration; IR spectroscopy; Villiaumite; OH defects
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-023-00824-3

The long-known presence of a sharp OH absorption band in the tetragonal fluoride mineral sellaite, MgF 2 , 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 (CaF 2 ). 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 MgF 6 polyhedra, in analogy with the isostructural mineral rutile, TiO 2 . 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 MgF 6 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 (CaF 2 ) 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.