Liquid immiscibility in basic alkaline magmas

On the assumption that very basic silicate magmas contain appreciable amounts of O^2? ions, non-ideal mixing of the melt species may be due to strongly preferred ionic associations. The anions in the melt, O^2? on the one hand, and SiO₄^4? and polymerized aluminosilicate ionic species on the other, discriminate between the cations according to their field strength and polarizing power. K⁺, Na⁺, Ba²⁺, Sr²⁺ and Ca²⁺ associate with singly bonded oxygen, and Mg²⁺, Fe²⁺ and Ni²⁺ attach themselves to O^2? rather than to silicate or aluminosilicate anions. In extreme cases these relations may lead to unmixing in the silicate melt, especially in the presence of water which may lower the liquidus relative to the solvus temperature. Liquid—liquid phase separation in ultrabasic magmas may be related to the present model.