Surface tension of natural silicate melts from 1,200°–1,500° C and implications for melt structure

Abstract

The surface tension between silicate liquid and gas has been measured for four lava compositions (limburgite to andesite) from 1,200° to 1,500° C. The magnitude of surface tension (γ) is in the range 350–370 dynes/cm. Variations found in γ as a function of liquid composition were small and had no obvious relation to liquid composition. γ was also found to vary little with furnace atmosphere — air, Ar, CO₂, H₂, CO and their mixtures. A relaxation time of hours to days, depending on temperature, is required before reproducible results can be obtained from originally crystalline starting material.

The reproducible temperature dependence of γ for complex silicate liquid solutions was found to be small, positive, and a relatively simple function of liquid composition. To a first approximation, the dependence of dγ/dT on composition found by King (1953) for simple silicate binaries appears to extend to the complex solutions measured here. The extent of progressive dissociation of liquid constituents implied by the increase of γ with T appears to be principally determined by the average field strength (Z/R) of the network-modifying cations in the liquid and abundance of network-forming constituents $(X_{SiO_2 } )$. The actual amount of structural dissociation implied by these results reduces the molar volume of the average structural unit by at most 5% for a 100° C increase in temperature. Evidently the polymerization of coordination polyhedra is fairly stable over this temperature and composition range.