Experimental study of the system H 3BO 3–NaF–SiO 2-H 2O at 350–800 °C and 1–2 kbar by the method of synthetic fluid inclusions

Abstract

The phase state of fluid in the system H 3BO 3–NaF–SiO 2–H 2O was studied at 350–800 °C and 1–2 kbar by the method of synthetic fluid inclusions. The increase in the solubility of quartz and the high reciprocal solubility of H 3BO 3 and NaF in water fluid at high temperatures are due to the formation of complexes containing B, F, Si, and Na. At 800 °C and 2 kbar, both liquid and gas immiscible phases (viscous silicate-water-salt liquid and three water fluids with different contents of B and F) are dispersed within each other. The Raman spectra of aqueous solutions and viscous liquid show not only a peak of [B(OH) 3] 0 but also peaks of complexes [B(OH) 4] –, polyborates [B 4O 5(OH) 4] 2–, [B 3O 3(OH) 4] –, and [B 5O 6(OH) 4] –, and/or fluoroborates [B 3F 6O 3] 3–, [BF 2(OH) 2] –, [BF 3(OH)] –, and [BF 4] –. The high viscosity of nonfreezing fluid is due to the polymerization of complexes of polyborates and fluorine-substituted polyborates containing Si and Na. Solutions in fluid inclusions belong to P–Q type complicated by a metastable or stable immiscibility region. Metastable fluid equilibria transform into stable ones owing to the formation of new complexes at 800 ºC and 2 kbar as a result of the interaction of quartz with B-F-containing fluid. At high concentrations of F and B in natural fluids, complexes containing B, F, Si, and alkaline metals and silicate-water-salt dispersed phases might be produced and concentrate many elements, including ore-forming ones. Their transformation into vitreous masses or viscous liquids (gels, jellies) during cooling and the subsequent crystallization of these products at low temperatures (300–400 °C) should lead to the release of fluid enriched in the above elements.