Chemical Composition of Meteorites
I suggest to people who think they have found a meteorite to get a chemical analysis by Actlabs or some other lab that can provide good compositional data. There are several different ways to determine whether or not a rock is a meteorite. A chemical analysis is a good one because it's cheaper to do than most of the other tests and it's usually unambiguous (meaning, with a chemical analysis, I'm not likely to say, "I still don't know" or "maybe.")
So that you can
check your data yourself, I show plots here of concentrations or ratios of
concentrations of several chemical elements in meteorites compared to rocks
people have had analyzed by Actlabs or some other lab. The horizontal axis of
all the plots is "Fe2O3(T) + MgO." Actlabs
and most labs that analyze rocks reports total iron as Fe2O3
because in Earth rocks much or most of the iron occurs as Fe(III), that is,
ferric iron. There is little or no Fe(III) in freshly fallen meteorites; it's
all Fe(II), ferrous iron, and Fe(0), iron metal. For
convenience, however, I use Fe2O3 in the plots. If you
had an analysis done, just add the Fe2O3 and MgO values
together for comparison.
Here's an example of one of the plots. This one is for SiO2.
Notes, Caveats, and References
1) Terrestrial - Meteorwrong. All the “meteorwrongs” in the plots (large white circles) are for rocks that people have had analyzed by Actlabs or some other lab and for which people sent me the data. If you obtain a chemical analysis of your rock, please send me the numbers!
2) Terrestrial - Geostandard. Many countries have agencies that pulverize large quantities of rock for use as interlaboratory standards. Several hundred geostandards are available that represent all common, and many uncommon, rock types of the earth. For most of these, there are many analytical data available. I have selected from the compilation of Govindaraju (1994) and Korotev (1996) data for 156 such rocks. I have avoided data for soils and unconsolidated sediments, monominerallic rocks (except chert, sandstone, limestone, hornblendite, magnesite), ores (except for some iron ores, because these are sometimes mistaken for meteorites), and geostandards that don't have data for the elements that I plot here. In total there are data for 7 andesites, 5 anorthosites, 17 basalts, 2 carbonatites, 1 chert, 6 diabases, 2 diorite or diorite gneiss, 2 dolomites, 4 dunites, 15 gabbros, 21 granites and related rocks, 5 granodiorites, 1 hornblendite, 6 iron ore or iron formation rocks, 2 kimberlites, 1 quartz latite, 10 limestones, 2 lujavrites, 1 magnesite, 1 monzonite, 1 norite, 3 peridotites, 1 pyroxenite, 5 rhyolites including 1 obsidian, 1 sandstone, 5 schists, 3 serpentinites, 12 shales, 2 slates, 6 syenites, 2 tonalites, 3 trachytes, and 2 “ultrabasic rocks.” Data for some trace elements are missing for some of the GRS's.
3) Terrestrial - Tektite. I have plotted data of Koeberl (1986) for various types of tektites. Note that tektites have compositions like terrestrial rocks (because they are!), not like meteorites.
4) All the white points represent terrestrial rocks. All the black points and colored points are for meteorites.
5) All the meteorites plotted in the plots (all square symbols) are stony meteorites, not stony-irons or irons.
6) Most (~95%) stony meteorites are chondrites, and most chondrites are ordinary chondrites. If you have actually found a meteorite, it's probably some kind of chondrite. That's why I made the points for chondrites black and the ordinary chondrites BIG and black. Chondrites are most dissimilar to Earth rocks. Each black point represents the average composition of one of the chondrite groups: H, L, LL, EH, EL, CI, CM, CV, CO, CR, CO, R, Ac, & K. Data from Wasson & Kallemeyn (1988).
7) The lunar meteorite data are from my own database. Each point represents a different meteorite.
8) For the martian meteorites, eucrites, howardites, diogenites, and “other rare achondrites,” each point represents a meteorite or analysis. Data mostly from Jarosewich (1990), Lodders & Fegley (1998), and Mittlefehldt et al. (1998).
9) The plots presented here reasonably represent >99% of all meteorites.
10) It’s like lottery numbers - you don’t win unless the composition is consistent with ALL the chemical-composition parameters shown here, not just some of them!
11) I should show some plots here for chalcophile (sulfur-loving) elements - Cu, Zn, As, In, Sn, and Sb. The problem is that the concentrations of these elements are so low in achondrites (= meteorites that are not chondrites) that there are few data to plot. Chondrites have higher concentrations of chalcophile elements than achondrites:
So, for example, if you have a rock with >5 ppm As (arsenic), then the rock is not a meteorite. Many terrestrial sedimentary rocks, as well as metamorphic rocks that formed from sedimentary rocks, have concentrations of chalcophile elements much higher than those in the table above.
Govindaraju K. (1994) 1994 compilation of working values and sample description for 383 geostandards. Geostandards Newsletter 18, 1–158.
Jarosewich E. (1990) Chemical analysis of meteorites: A compilation of stony and iron meteorite analyses. Meteoritics 25, 323-327.
Koeberl C (2006) Geochemistry of tektites and impact glasses. Annual Review of Earth and Planetary Sciences 1986 14, 323-350.
Korotev R. L. (1996) A self-consistent compilation of elemental concentration data for 93 geochemical reference samples. Geostandards Newsletter 20, 217–245.
Lodders K. and Fegley B. Jr. (1998) The Planetary Scientist’s Companion, Oxford University Press, New York, 371 pp.
Mittlefehldt D. W., McCoy T. J., Goodrich C. A., and Kracher A. (1998) Chapter 4. Non-chondritic meteorites from asteroidal bodies. In Reviews in Mineralogy, Vol. 36, Planetary Materials (ed. J. J. Papike), pp. 4-1–4-195, Mineralogical Society of America, Washington.
Wasson J. T. and Kallemeyn G. W. (1988) Compositions of chondrites. Philosophical Transactions of the Royal Society of London, Series A 325, 535-544.