During fractional crystallization, when a mineral crystallizing from a melt has a different Mg:Si ratio than the melt itself, its removal shifts the residual melt composition away from the crystallizing mineral's composition on a compositional diagram; forsterite (Mg2SiO4) removes proportionally more MgO than SiO2, driving the residual melt toward lower MgO and higher SiO2.
Forsterite (Mg2SiO4) has a Mg:Si molar ratio of 2:1, meaning it incorporates a higher proportion of MgO relative to SiO2 than is present in the...
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