Effect of alkalis, phosphorus, and water on the surface tension of haplogranite melt

The sessile drop method has been used for measurements of the surface tensi on of haplogranite (HPG) melts containing an excess of alkalis and phosphor ous (HPG8, HPG8 + 5 wt% Li2O, 5 wt% Na2O, 20 wt% Na2O, 5 wt% K2O, 5 wt% Rb2 O, 5 wt% Cs2O, 10 wt% P2O5) and of Armenian rhyolite in the temperature int er val, 650-1665 degrees C, and at 1 bar pressure. Sessile drops were place d on graphite substrates in a Pyrex tube furnace purged with Ar. Drop shape was monitored with a videocamera and stored in a videorecorder. The surfac e tension was calculated by measuring the two principal radii of curvature of the drop shape in vertical cross section. The precision of the method wa s checked against the surface tension of water. The surface tension of HPG and rhyolite melt is similar to 280-300 + 5 mN/m in the temperature interva l 1200-1400 degrees C. Temperature dependence of the surface tension of hap logranite melts and rhyolite is weak and positive (d sigma/dT = 0.06 to 0.0 9 mN/m/degrees C). Addition of 5 wt% of alkali oxides (except Li2O) results in a decrease of the surface tension of haplogranite melts. The HPG melts with 10 wt% P2O5 have 30% higher surface tension than haplogranite melts wi th excess alkalis, and a negative temperature derivative (d sigma/dT = -0.1 mN/m/degrees C). The HPG melts with 20 wt% Na2O and 5 wt% Li2O exhibit a d ecrease in surface tension with temperature (d sigma/dT = -0.02 and -0.10 m N/m/degrees C, respectively). The surface tension of HPG8 melt saturated with water at 1-4 kbar was measu red on sessile drops quenched at high pressure in an internally heated gas vessel at temperatures of 800-1200 degrees C. Water pressure significantly decreases the surface tension of melt from 270 mN/m at 1 bar (1000 degrees C) to 65 mN/m at 4 kbar. At 1 bar in "dry" conditions, d sigma/dT = +0.056 mN/m/degrees C and at 3 kbar of water pressure, d sigma/dT = +0.075 mN/m/de grees C. The decrease in the surface tension of HPG melt at a water pressur e of several kbars is from -10 to -30 mN/m/wt% H2O. The increase of water c ontent to more than 10 wt% in granite melts may not result in any significa nt decrease in the surface tension, which may be explained by formation of a surface sublayer having physical properties very distinct from those of t he bulk.