Magmatic fractionation/differentiation is known today as Bowen's Reaction Series. The progression in the series explains why the first lavas from a volcanic vent are rich in iron, magnesium, and calcium, are low in quartz (mafic), and are dark green to black and why the later lavas are lighter colored and contain more quartz (felsic).
Two magmas of similar composition and temperature at different pressure may crystallize different minerals. An example is high-pressure fractional crystallizaion of granites to produce single-feldspar granite, and low-pressure conditions which produce two-feldspar granites.
