Study of roméite solubility in the fluid immiscibility region of the NaF–H<sub>2</sub>O system at 800°C, 200 MPa

New data on roméite (CaNa)Sb2O6F solubility in the NaF–H2O system of P–Q type in a wide range of sodium fluoride concentrations (from 0 to 25 wt. % NaF) have been obtained. 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 interval of 0.02–0.2 mol kg−1 H2O. According to the data obtained, the concentration of antimony in the L1 and L2 phases in the fluid immiscibility region of the NaF–H2O system at 800°C, 200 MPa and f(O2) = 50 Pa, specified by the Cu2O–CuO buffer, is 0.4 and 2.1 wt. % Sb, respectively. For the first time, during these experiments, the formation of fluorite skeletal forms and an intermetallic compound Pt5Sb of a hexagonal crystal system with lattice parameters (LP): a = b = 4.56(4), c = 4.229(2) Å, α = β = 90°, γ = 120° was established. Pentaplatinum antimonide is formed on the surface of Pt ampoules at 800°C, P = 200 MPa and f(O2) ≤ 10−3.47 Pa (Cu–Cu2O buffer) in experiments on the incongruent dissolution of romeite, which causes a sharp decrease (more than 1000 times) the concentration of antimony in solution.