Chemical Composition of Meteorites

Al₂O₃

Most meteorites have low concentrations of aluminum (Al₂O₃) compared to terrestrial rocks. The exception is meteorites from the Moon because most lunar meteorites contain a high abundance of plagioclase feldspar. For an explanation of why lunar meteorites plot along the diagonal trend, see How Do We Know That It's a Rock From the Moon? Feldspars are common in terrestrial rocks, too.
  

CaO

Lunar meteorites also have high concentrations of calcium (CaO) compared to most terrestrial rocks (except, of course, those rocks rich in calcite, like the limestones that plot around 50% CaO). The terrestrial rocks that plot with the martian meteorites are probably basalts, which have similar mineralogy to the martian basalts (but, see Na₂O & K₂O).
  

CaO/Al₂O₃

In most meteorites, virtually all the Al₂O₃ and most of the CaO is carried by feldspar, so there is little variation in CaO/Al₂O₃. Terrestrial rocks tend to have lower CaO/Al₂O₃ than meteorites.

See Notes, Caveats, and References