First-principles simulations were made of CaAl2Si2O8 (anorthite) liquid at pressures up to 120GPa and 3000, 4000 or 6000K. At the lowest degrees of compression the liquid was seen to accommodate changes in density by decreasing the abundance of 3- and 4-membered rings, while increases in coordination of network-forming cations took effect at somewhat higher degrees of compression. The results were fitted to a fundamental thermodynamic relationship with 4th-order finite strain and 1st-order thermal variable expansions. The pressure and temperature dependences of self-diffusivities were found to be well represented by an Arrhenius relationship, except at 3000K and pressures below 5GPa, where the self-diffusivities of Si, Al and O increased with pressure. Analysis of the lifetimes of individual coordination species revealed that this phenomenon arose due to the disproportionately high stability of 4-fold coordinated Si, and to a lesser extent 4-fold coordinated Al. The results represented a marked improvement in accuracy and reliability in describing the physics of CaAl2Si2O8 liquid at deep mantle pressures.

Structure, Thermodynamics, and Diffusion in CaAl2Si2O8 Liquid from First-Principles Molecular Dynamics. De Koker, N.: Geochimica et Cosmochimica Acta, 2010, 74[19], 5657-71