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 only possible
exception would be a lunar granite, which is a volumetrically
insignificant component of the Apollo collection. 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
