Diffusivity and thermodynamic properties of diopside and jadeite melts by computer simulation studies

We report the results of ion dynamics computer experiments on two liquid silicates of geochemical interest in which the charge characteristics of the cations other than Si cause differences in the extent to which the integrity of the open network structure of liquid SiO₂ is maintained. The two cases are jadeite, the transport behavior of which was described in an earlier paper, and diopside. The focus of the study is on the pressure-density relations. In diopside, Mg is found to exist in four-fold coordination up to pressures above 50 kbar at 4000 K while Ca, like Na in jadeite, has a large and poorly defined coordination. Likewise, Ca in diopside is the most mobile element at low pressures but becomes jammed at high pressures. The activation volume corresponds to the ion volume. Both melts show highly similar pressure-density relations, the compressibility of jadeite becoming larger than that of diopside only below 30 kbar. The calculated values fall between the measured value for diopside and the estimated value of jadeite, but show the same form of pressure dependence. By taking advantage of the low probability of cavitation in simulated systems, we carry the simulations into the expanded, negative pressure region, to show how the increasing compressibility at low pressures is a reflection of the impending mechanical stability limit which is found at weakly negative pressures at the temperatures of these simulations.