Topaz solid solution in the F-rich granitic rocks from Blond (NW Massif Central, France)

Soufi, Mohamed; Remmal, Toufik; Hassani, Iz-Eddine El Amrani El; Makhoukhi, Samira

doi:10.1007/s12517-018-3722-0

Cited by 4 publications

(1 citation statement)

References 117 publications

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“…However, following the results obtained with synchrotron and neutron data and temperature-dependent structural modelling, which reveal that PadPar topaz maintain its crystallinity and symmetry up to 1170 K, we can argue that PadPar topaz started its nucleation at this temperature. This value is coherent with the topaz stability field in the system Na 63 , but it is also a rather high temperature for the formation of granitic pegmatites, (1075-625 K [68][69][70] ). However, liquidus temperatures of 1120 K was experimentally obtained for the initial crystallization of the topaz-albite granite assemblage from a supercritical fluid (~ 28 wt% H 2 O and ~ 45 wt% F completely miscible in all proportions at magmatic temperature and pressure) 71 .…”

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confidence: 71%

F/OH ratio in a rare fluorine-poor blue topaz from Padre Paraíso (Minas Gerais, Brazil) to unravel topaz’s ambient of formation

Precisvalle

Martucci

Gigli

et al. 2021

Sci Rep

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Topaz [Al2SiO4(F,OH)2] is one of the main fluorine-bearing silicates occurring in environments where variably acidic (F)/aqueous (OH) fluids saturate the silicate system. In this work we fully characterized blue topaz from Padre Paraíso (Minas Gerais, Brazil) by means of in situ synchrotron X-Ray and neutron powder diffraction measurements (temperature range 298–1273 K) combined with EDS microanalyses. Understanding the role of OH/F substitution in topaz is important in order to determine the hydrophilicity and the exchange reactions of fluorine by hydroxyl groups, and ultimately to characterize the environmental redox conditions (H2O/F) required for mineral formation. The fluorine content estimated from neutron diffraction data is ~ 1.03 a.f.u (10.34 wt%), in agreement with the chemical data (on average 10.0 wt%). The XOH [OH/(OH + F)] (0.484) is close to the maximum XOH value (0.5), and represents the OH- richest topaz composition so far analysed in the Minas Gerais district. Topaz crystallinity and fluorine content sharply decrease at 1170 K, while mullite phase starts growing. On the basis of this behaviour, we suggest that this temperature may represent the potential initial topaz’s crystallization temperature from supercritical fluids in a pegmatite system. The log(fH2O/fHF)fluid (1.27 (0.06)) is coherent with the fluorine activity calculated for hydrothermal fluids (pegmatitic stage) in equilibrium with the forming mineral (log(fH2O/fHF)fluid = 1.2–6.5) and clearly different from pure magmatic (granitic) residual melts [log(fH2O/fHF)fluid < 1]. The modelled H2O saturated fluids with the F content not exceeding 1 wt% may represent an anomalous water-dominant / fluorine-poor pegmatite lens of the Padre Paraíso Pegmatite Field.

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