Iron and Oxygen

In the systems heretofore discussed, we have dealt mainly with the six components SiO2, Al2O3, MgO, CaO, Na2O, and K2O, which may be found the principal phases or mineral components of the basalt tetrahedron and of basalts themselves: nepheline, albite, orthoclase, quartz, anorthite, plagioclase in general, forsterite, diopside, enstatite, and Fe-free pigeonite. Missing from this list are the Fe-Ti oxides, which require consideration of FeO, Fe2O3, and TiO2 as components. FeO is required also for discussion of the ferrous silicate components of olivine and pyroxene. Phosphorus is also a constituent of importance in basalts, reporting either in groundmass glass or in apatite, which crystallizes during late fractionation stages when the concentration of phosphorus becomes high enough to saturate the melt in apatite component. Present generally in minor quantities, phosphorus can be ignored for present purposes, always bearing in mind that its abundance level in basaltic magmas may contain important information, which we may or may not be able to analyse correctly.