Programmed cooling experiments with compositions in the system MgO-SiO 2 ranging from 59 wt.% SiO 2 (with olivine on the liquidus) to 63 wt.% SiO 2 (with protoenstatite on the liquidus) cooled at rates of from 0.4 to 25% min crystallize forsterite but not pyroxene. Because pyroxene does not nucleate, the forsterite plus melt reaction does not take place, allowing forsterite to crystallize metastably at sub-solidus temperatures. A 65.1 wt.% SiO 2 composition (with cristobalite on the liquidus) crystallizes a number of different phases depending on cooling rate and initial superheating. The ease of forsterite nucleation relative to pyroxene offers an explanation for the occurrence of olivine instead of low-Ca pyroxene in the groundmass of tholeiitic basalts.--Modified journal abstract.

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