Roméite Solubility in the Fluid Immiscibility Region of the NaF–H₂O System at 800°C, 200 MPa

New data on roméite (CaNa)Sb₂O₆F solubility in the NaF–H₂O system of P–Q type have been obtained within a wide range of sodium fluoride concentrations (from 0 to 25 wt % NaF). The concentration of antimony, in equilibrium with roméite and fluorite, in the range of NaF concentrations from 1 to 8 mol kg^–1 H₂O (25 wt % NaF), is in the range of 0.02–0.2 mol kg^–1 H₂O. According to the data, the concentration of antimony in phases L₁ and L₂ in the region of fluid immiscibility of the NaF–H₂O system at 800°C, 200 MPa and fO₂ = 50 Pa, specified by the Cu₂O–CuO buffer, is 0.4 and 2.1 wt % Sb, respectively. Our experiments were the first ever to produce skeletal fluorite crystals and the intermetallic compound Pt₅Sb, which belongs the hexagonal crystal system and has the following lattice parameters (LP): a = b = 4.56(4) Å, c = 4.229(2) Å, and α = β = 90°, γ = 120°. Pentaplatinum antimonide was formed on the inner surface of the Pt capsules at 800°C, Р = 200 MPa, and fO₂ ≤ 10^–3.47 Pa (Cu–Cu₂O buffer) in experiments on the incongruent dissolution of roméite, which causes a sharp decrease (more than 1000 times) in antimony concentration in the solution.