Exploring the role of OH/F substitution in topaz: relationships between structural features and chemical environments
Topaz, Al2SiO4(OH, F)2, is one of the principal fluorine-bearing silicates whose composition usually ranges from a nearly OH-free end member, Al2SiO4F2, in acid igneous rocks, to Al2SiO4F1.4(OH)0.6, with XOH = OH/(OH+F) = 0.30, in hydrothermal deposits. The existence of an upper limit of OH content is rationalized in terms of mutual repulsion between neighboring protons of hydroxyl ions, which strongly limits the OH substitution for F to 50%. Fluorine vs hydroxyl components of the solid solution Al2SiO4F2-Al2SiO4(OH)2 are strictly related to the forming redox conditions with important implications on the knowledge of the environmental systems. The objective of this proposal is to deeply investigate the role of OH/F substitution in topazes differing in terms of major and trace element compositions in order to highlight the relationships between structural features and chemical environments. With this aim, we intend to test the feasibility on a topaz from Minas Gerais and to study the thermal expansion of the structure and the correlation as a function of temperature between the fluorine content and the cell parameters (by heating in situ up to 1000°C).
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Annalisa Martucci and Thomas C. Hansen. (2019). Exploring the role of OH/F substitution in topaz: relationships between structural features and chemical environments. Institut Laue-Langevin (ILL) doi:10.5291/ILL-DATA.EASY-452
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