Partial Molar Volumes in Highly Siliceous Melts and the Relationship to Liquid Immiscibility

Partial molar volumes ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mover><mml:mi>V</mml:mi><mml:mo accent="true">¯</mml:mo></mml:mover></mml:math>) of SiO 2 , K 2 O, Na 2 O, Li 2 O and BaO have been re-evaluated in binary silicate melts at 1673 K. Volumetrically, the SiO 2 component mixes ideally in K 2 O-SiO 2 melts but mixes non-ideally in Li, Na and Ba melts, with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mover><mml:mi>V</mml:mi><mml:mo accent="true">¯</mml:mo></mml:mover></mml:math> SiO2 displaying maxima between ~80–95 mole% SiO 2 . K 2 O partial molar volumes ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mover><mml:mi>V</mml:mi><mml:mo accent="true">¯</mml:mo></mml:mover></mml:math> K2O ) display weak, non-ideal behaviour in K 2 O-SiO 2 melts due to electrostriction, where tetrahedra collapse around the modifier cation, K + , in response to K-O Coulombic attraction. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mover><mml:mi>V</mml:mi><mml:mo accent="true">¯</mml:mo></mml:mover></mml:math> Na2O , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mover><mml:mi>V</mml:mi><mml:mo accent="true">¯</mml:mo></mml:mover></mml:math> Li2O and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mover><mml:mi>V</mml:mi><mml:mo accent="true">¯</mml:mo></mml:mover></mml:math> BaO also behave non-ideally in their respective binary melts due to electrostriction. The combined effects of non-ideal mixing of SiO 2 and electrostriction associated with the modifier cations result in molar volumes of the four melts being less than expected for ideal mixing. The extent of non-ideal volumetric mixing in the binary melts increases in the order K&lt;Na&lt;Li&lt;Ba, which is the same as the order of the consolute temperatures of the miscibility gaps for these binary melts. The similar order leads us to suggest that non-ideal volumetric mixing results from the same chemical interactions that give rise to melt immiscibility and that these interactions are due primarily to non-ideal behaviour of the SiO 2 component. The non-ideal volumetric mixing behaviour required use of quadratic expressions to fit molar volume-compositional trends of the four melt systems studied. Although mixing is non-ideal, the partial molar volumes of SiO 2 and modifier oxides are remarkably similar to values obtained from linear mixing models for melts containing ~45–70 wt.% SiO 2 . Pronounced effects of non-ideal mixing are mostly restricted to highly siliceous melts (X SiO2 &gt;0.75) where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mover><mml:mi>V</mml:mi><mml:mo accent="true">¯</mml:mo></mml:mover></mml:math> SiO2 values are appreciably greater than the molar volume of liquid SiO 2 ( V° SiO2 ), which is ~26.75 cm 3 /mole at 1673 K. The findings are consistent with volumetric (density) studies of highly siliceous (haplogranitic) melts.