![]() ![]() The ratio of CaO to Al 2O 3 varies greatly in terrestrial rocks. Chondrites have low concentrations of CaO. The terrestrial rocks that plot with the martian meteorites are mostly basalts, which have similar mineralogy to the martian basalts (but see Na 2O and K 2O below). Terrestrial rocks with very high CaO are usually rich in calcite, like the limestones that plot around 50% CaO. Lunar meteorites also have high concentrations of calcium (CaO) because most of the plagioclase is the Ca-rich extreme, anorthite. ![]() Chondrites have low concentrations of Al 2O 3 because they contain little 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 (or clays derived therefrom) are the major carrier of aluminum in terrestrial rocks, too. Most meteorites, especially lunar meteorites, have higher concentrations of aluminum (Al 2O 3) than terrestrial rocks of similar Fe 2O 3+MgO. Terrestrial rocks with 45-55% SiO 2 are usually basalts. The low-SiO 2 terrestrial rocks are mainly limestones and iron ores. No granitic meteorites have been found yet. The only possible exception would be a lunar granite, which is a volumetrically insignificant component of the Apollo collection. If SiO 2 is greater than 60%, the rock is not a meteorite. Many terrestrial rocks have higher concentrations of SiO 2 (silica) than any meteorite because they contain quartz and meteorites do not contain any significant amount quartz or other silica minerals. I cannot emphasize this enough: Most (88%) stony meteorites are ordinary chondrites. In the legends, the percentages (%) represent the relative abundance of each meteorite type among stony meteorites. If you have Fe values reported as FeO, just multiply the FeO value by 1.111 to get Fe 2O 3. ![]() If you had an analysis done, just add the Fe 2O 3 and MgO values together for comparison. For convenience, however, I use Fe 2O 3 in the plots. There is little or no Fe(III) in freshly fallen meteorites, however it is all Fe(II), ferrous iron, and Fe(zero), metal. The horizontal axis of all the plots is “Fe 2O 3(T) + MgO.” Most labs that are in the business of analyzing rocks reports total iron (T) as Fe 2O 3 because in Earth rocks much or most of the iron occurs as Fe(III), that is, ferric iron. So that you can check your data yourself, I show plots here of concentrations and ratios of concentrations of several chemical elements in meteorites compared to rocks that persons have had analyzed by commercial labs. ![]()
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