SiO2
Many terrestrial rocks have higher concentrations of
SiO2 (silica) than any meteorite. Many of these contain
quartz. If SiO2 is greater than 60%, the rock is not
a meteorite. The low-SiO2 chondrites are all carbonaceous
chondrites. Carbonaceous chondrites have a substantial proportion
of volatile elements that, in terms of this plot, as a diluants
that drag the compositions toward the origin. The low-SiO2
terrestrial rocks are mainly limestones [low Fe2O3(T) + MgO] and
iron ores [high Fe2O3(T) + MgO].
See Notes, Caveats, and References
