VAPOR PRESSURE AND VAPOR FRACTIONATION OF TEKTITE MELTS

摘要

The total vapor pressure of philippinite tektite melts of approxi¬mately 70 percent silica has been determined for temperatures ranging from 1500 to 2100°C. It is 190 ± 40 mm Hg at 1500°C, 450 = 50 mm at 1800°C, and 850 ± 70 mm at 2100°C.nThe data were obtained by visually observing the temperature at which bubbles began to form for a constant low ambient pressure. By varying the ambient pressure a boiling point curve was constructed. This curve differs from the equilibrium vapor pressure curve because of surface tension effects. This difference was evaluated by determining the equilibrium bubble size in the melt and calculating the pressure due to surface tension. The latter was assumed to be 380 dynes/cm.nThe relative volatility from tektite melts of the oxides of Na, K, Fe, Al, and Si has been determined as a function of temperature, total pres-sure, and, roughly, oxygen fugacity. The volatility of Si02 is decreased and that of Na20 and K20 is increased in an oxygen-poor environment. Preliminary results indicate that volatilization at 2100°C under atmos-pheric pressure causes little or no change in the percentage of Na20 and K20. The Fe3/Fe2 ratio of the tektite is increased in ambient air at a pressure of 9 x 10-4 mm Hg (106.5 atm 02, partial pressure) at 2000°C. This suggests that tektites either formed at lower oxygen pressures or were a product of the incomplete oxidization of a parent material with a still lower ferric-ferrous ratio.