Bulk moduli of (MgO,CaO,SrO,BaO)-Na2O-SiO2 melts.

The effect of composition and temperature on the compressibility of melts in the XO-Na2O-SiO2 (X = Mg, Ca, Sr, Ba) system has been investigated. The relaxed bulk moduli were measured using ultrasonic interferometric methods at frequencies of 3-7MHz in the temperature range of 1000 to 1600 degrees C. For these melts, the bulk modulus decreases with replacement of XO with Na2O as expected from previous observations that alkaline-earth silicate melts display anomalously high bulk moduli in comparison to alkali-silicate melts. It would appear that the bulk moduli of XO-Na2O-SiO2 melts follow a simple linear trend as a function of the number of cations per oxygen - similar to the observed behaviour for CaO-Al2O3-SiO2 and Na2O-SiO2 melts. The trend-line for the present xXO .(1-x)Na2O . SiO2 melts connects the separate trends for XO-SiO2 and Na2O-SiO2 at the XO . SiO2 and Na2O . SiO2 compositions. This simplification for estimating the compressibility of melts in the XO-Na2O-SiO2 system, however, does not apply to the XO-Na2O-Al2O3-SiO2 system as the addition of alumina significantly changes the structure of sodium-silicate melts and Na2O-Al2O3-SiO2 melts do not follow the same systematic bulk modulus-cations per anion behaviour.