Abstract
A simple ionic model which describes the solution of the forsterite component of olivine in silicate liquids is reported. The melting relation is represented: ( Mg 2 SiO 4) ol = 2( Mg 2+) L + ( SiO 4 4− L and is extended to all silicate liquids by normalizing their compositions to 4 oxygens. At 1 bar, the temperature at which olivine is in equilibrium with any alkali-depleted basaltic composition can be calculated to within ±30° C. This error is increased considerably when applied to terrestrial basalts which contain several weight percent alkalis. Alkalis interfere with the equilibrium by generating strongly repulsive interionic forces which can be crudely modelled in a manner consistent with constraints imposed by regular solution theory. The model quantifies the reduced activity of SiO 4 4− monomers due to increasing SiO 2 concentrations in the melt. This is a consequence of polymerization which does not appear to operate gradually over the entire spectrum of mafic and ultramafic compositions. The coordination of alumina in melts which precipitate olivine only appears to be dominantly octahedral. Titanium acts as a polymerizing agent by interconnecting previously isolated SiO 4 4− monomers. Calcium associated with normative diopside tends to exhibit small but perceptible repulsive forces involving Mg 2+.
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