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The effect of CaO and MgO, with or without Ti[O.sub.2] and [P.sub.2][O.sub.5], on the two-melt field in the simplified system [Fe.sub.2]Si[O.sub.4]-KAl[Si.sub.3][O.sub.8]-Si[O.sub.2] has been experimentally determined at 1,050[degrees]-1,240[degrees]C, 400 MPa. Despite the suppressing effect of MgO, CaO, and pressure on silicate melt immiscibility, our experiments show that this process is still viable at mid-crustal pressures when small amounts (0.6-2.0 wt%) of [P.sub.2][O.sub.5] and Ti[O.sub.2] are present. Our data stress that the major element partition coefficients between the two melts are highly correlated with the degree of polymerisation (nbo/t) of the Si[O.sub.2]-rich melt, whatever temperature, pressure, or exact composition. Experimental immiscible melt compositions in natural systems at 0.1 MPa from the literature (lunar and tholeiitic basalts) plot on similar but distinct curves compared to the simplified system. These relations between melt polymerisation and partition coefficients, which hold for a large range of compositions and f[O.sub.2], are extended to various volcanic and plutonic rocks. This analysis strengthens the proposal that silicate melt immiscibility can be important in volcanic rocks of various compositions (from tholeiitic basalts to lamprophyres). However, the majority of proposed immiscible compositions in plutonic rocks are at least not coexisting melts, but may have suffered accumulation of early crystallized minerals.