Chemical Composition of Meteorites
Aluminum, Calcium, & Calcium
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.
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).
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.