IMPERIAL TOPAZ FORMATION AND COLOR CONSTRAINT

Topaz crystallization and color generation is explained in the Ouro Preto deposits furnishing subsides to clear up the problem. Ouro Preto topaz deposits occur in a partly altered rock defined as BT (Brown Terrain or Terrena) composed by topaz-quartz-muscovite-kaolinite-pyrophyllite-hematite-goethite. Extensive fluid (HF, H₂O and CO₂) migration occurred during deformation, facilitated by the aperture of open spaces and producing topaz crystallization and goethite after hematite and their relative spreading. Weakly stained muscovite flakes by Fe³⁺-rich fluids are barely oriented aligned to the fold axial surface together patchy hematite and pyrophyllite-kaolinite pair. Fluid migration and implanting must have also open space to crystallize topaz. Whereas shearing transects a muscovite-hematite-quartz rock accompanied by HF-rich fluid percolation topaz must form, and the existing hematite altered by fluids generates goethite furnishes Fe³⁺ to the fluids to form imperial topaz. 2 Fe₂O₃ + 2H₂O + HF = 4FeOOH + H⁺ + F^- (1) Fe₂O₃ + 3 H⁺  = FeOOH + H₂O + Fe³⁺ (2) Fe₂O₃ + 2 HF + Al₂Si₂O₅(OH)₄ = 2 FeOOH + Al₂SiO₄F₂ + SiO₂ + 2H₂O (3) Fe₂O₃ + 2 HF + Al₂Si₂O₅(OH)₄ + 3H⁺ = FeOOH + Al₂SiO₄F₂ + SiO₂ + 4H₂O + Fe³⁺  (4) FeOOH + 2 HF + Al₂Si₂O₅(OH)₄ + 3H⁺ = Al₂SiO₄F₂ + SiO₂ + 5H₂O + Fe³⁺ (5) It is obvious that tinging and incorporation of Fe³⁺ in the concordant and sub-concordant to main foliation topaz crystals results in the yellowish peach color, perhaps during metamorphism, initial metasomatism and hydrothermal process. Imperial topaz shows twinning, characteristic lozenge crystal shape, bluish internal reflections and zoning marked by goethite and reddish color. Contrasting with topaz from ongonites, topaz rhyolites, topazites derived directly from pegmatoidal and volcanic origin the Ouro Preto imperial topaz formed under lower temperatures (300-350⁰C) and Fe₂O₃ contents in the range 0.32% and 0.67%. Topaz is stable in the open system Al-Si-O-H-F which may be exhibited in a chemical potential diagram µH₂O and µH⁺, applied to test the relative stability in relation to topaz is prepared in function of µH₂O-µH⁺, defined by the reactions.