The study of melts in the ternary calcium oxide-magnesium oxide-silicon dioxide at high pressure and the nature of immiscibility in binary systems.

摘要

A review of immiscibility data in 62 silicate, borate and germanate binaries permits identification of four groups of cations displaying different immiscibility behaviour. The first group consists of network-modifier cations which have ionic radii ≳ 87.2 pm and coordination numbers equal to, or higher than, 5 (e.g. Ca2+, La3+, U4+). The second group involves cations with ionic radii ≲ 87.2 pm. They have at least two coordination numbers: the first one is always 4 and the second is ≥5; for this reason they are called amphoterics (e.g. Li1+, Mg2+, Al3+). The third group contains cations with variable crystal field stabilization energies (e.g. Fe2+, Ni2+, Cr3+), and the fourth group includes cations with a lone pair of electrons (e.g. Pb 2+, Bi3+, Te4+).; Immiscibility data suggest that the origin of phase separation is associated with coulombic repulsions between poorly screened cations. The larger the ionic potential of a cation, the greater the repulsions with its neighbours, and the larger the size of its immiscibility field. However, amphoterics do not obey this rule because network-formers like SiO2 exert a structural control upon immiscibility which creates a selective solution mechanism that affects cations with radii ≲ 87.2 pm. Such small cations appear to be capable of fitting in pentagonal-like cages where they adopt a 4-fold coordination. In tetrahedral coordination, the bonds have a greater covalent character and the oxygens are more polarized towards the amphoterics which efficiently shield their positive charges, reducing coulombic repulsions and thus immiscibility.; Experiments performed in the system CaO-MgO-SiO2 at 1.0 GPa show that pressure has little effect on miscibility gaps associated to network-modifiers such as Ca2+ and "weak" amphoterics like Mg 2+. However, it is shown that amphoterics with substantial fractions of cations in 4-fold coordination and cations with variable crystal field stabilization energies capable of high spin to low spin transitions are expected to enlarge their immiscibility fields at high pressure. Magmas rich in these cations are potential candidates to develop phase separation at depth.; Experiments were also performed in the systems CaO-SiO2 and MgO-SiO2 at 1.0 GPa and results were combined with all the currently available phase equilibria and thermodynamic data at 1 bar in the same systems to critically optimize the thermodynamic properties of the liquid phase at low and high pressures. Assessments were made with the modified quasi-chemical model of Blander & Pelton using a computer program that simultaneously optimized all reliable data to give a small set of excess Gibbs free energy parameters. Pressure was found to have little effect on the topology of the CaO-SiO2 system but a pronounced one on the MgO-SiO2 binary. These contrasting behaviours from two homovalent and isochemical cations are linked to the polymorphic transition of the magnesium metasilicate. The amphoteric nature of the Mg2+ cation in the liquid phase makes MgO-SiO2 melts compressible but this effect appears to be small.