New acoustic velocity measurements on CaO-MgO-Al2O3-SiO2 liquids: Reevaluation of the volume and compressibility of CaMgSi2O6-CaAl2Si2O8 liquids to 25 GPa



[1] Relaxed sound speed measurements on 12 liquids in the CaO-MgO-Al2O3-SiO2 (CMAS) system have been performed from 1410 to 1620°C at 1 bar with a frequency sweep acoustic interferometer. In all liquids, the sound speeds either decrease or remain constant with increasing temperature. These data are combined with those in the literature to calibrate models for βT and (∂V/∂P)T as a function of composition and temperature for CMAS liquids. CaO is the only oxide component that contributes to the temperature dependence of compressibility. The new compressibility models permit the bulk modulus (KT,0) of CaMgSi2O6 (Di), CaAl2Si2O8 (An), and the Di64-An36 eutectic liquid to be directly obtained. These results are used to uniquely constrain values for the pressure dependence of the bulk modulus (K0′ = dK0/dP) in a third-order Birch-Murnaghan equation of state (EOS) for these three liquids from shock wave data in the literature. The revised K0′ value is 6.8 (versus 6.9) for CaMgSi2O6 liquid, 4.7 (versus 5.3) for CaAl2Si2O8 liquid, and 5.6 (versus 4.85) for Di64-An36 liquid. Information on both KT,0 and K0′ allows the density and compressibility for each of these three liquids to be calculated as a function of pressure to 25 GPa. Both the molar volume and isothermal compressibility of CaMgSi2O6-CaAl2Si2O8 liquids mix ideally between 0 and 25 GPa. The dominant mechanism of compression at low pressure (0–5 GPa) for all three liquids (CaMgSi2O6, CaAl2Si2O8, and the Di64-An36 eutectic) is topological, whereas gradual Al/Si coordination change plays an increasingly important role at higher pressure as topological mechanisms of compression are diminished.