Have You Found a Meteorite?

If you think that you might have found a candidate meteorite, perhaps the first thing you should do is become familiar with the appearance of a variety of authenticated meteorite samples such as those pictured on this website and others. In addition, there are some good websites that can aid in differentiating between actual meteorites and terrestrial look-a-likes, or meteorwrongs—visit Tim Heitz’s Meteorwrong Collection and Washington University’s A Photo Gallery of Meteorwrongs. Although the photos and information found within Meteorite Studies and these meteorwrong websites should be helpful in identifying a meteorite, there is no way to authenticate a meteorite without ultimately having it tested by a professional laboratory specializing in this type of work. The procedure is relatively easy for the submitter but may be time-consuming for the lab. Some helpful tips for submitting a candidate sample follow:

  1. Who will test it? The availability of specific meteorite testing labs changes over time, so you should conduct a search to find one suitable for conducting the test.
  2. How much will the test cost? Costs will vary, and some labs might test a sample for free in return for the right to keep the sample for future reference if it should prove to be a meteorite; inquire about the policies of a particular lab.
  3. What size sample will they need? The Nomenclature Committee of The Meteoritical Society has established a minimum quantity for authentication and naming of a meteteorite; that is, 20% or 20 g, whichever is less. For classification services, a particular lab may require more.
  4. If the testing lab does authenticate a meteorite, what next? The testing lab should provide the written analysis and classification information necessary for acquiring a provisional name, and submit the description to the Nomenclature Committee of The Meteoritical Society. The specific directions for utilizing the official template required for submission of a new meteorite can be accessed on the submission website of the Meteoritical Bulletin.
  5. How long will this process take? Due to the current boom in meteorite recoveries from the deserts of northwest Africa, meteorite testing labs have a substantial backlog of work. Unless by appearance alone your sample is recognized or strongly suspected to be a meteorite of interest, expect the process to take months, and in some cases, years (see classification procedure description below). Be patient.

Keep in mind that qualified laboratories typically identify an actual meteorite in less than 1% of the submitted samples from the public. The following is a description of the lab procedures that may be required to classify a particular meteorite, quoted from the former editor of the Meteoritical Bulletin, Dr. Jeffrey N. Grossman: For stony meteorites, one always starts by making a polished thin section for petrographic and mineralogical analysis. (Hand-sample classifications are generally considered as only tentative, although some people are good at it.) The thin section is then used to assign the meteorite to a general class: H, L-LL, EH, EL, CI, CM, CO, CV, CK, CH, CR, and R chondrites can all be distinguished at the optical microscope, although there is often confusion between L and LL, and there are many transitional and ambiguous C chondrites out there. Most achondrite groups can be distinguished from each other by optical methods as well. Petrologic types of chondrites, shock stages, and weathering grades are all assigned at the microscope, although it is sometimes not possible to tell type 3 from 4 chondrites.

The second step for stones is to analyze key minerals with an electron microprobe. For chondrites, one tries to pin down the average composition of olivine and pyroxene to confirm the group assignment. The standard deviation of olivine comps is used to separate type 3’s from 4’s in borderline cases. For types 4-6 ordinary chondrites, the minimum work needed is to analyze just a few olivine grains. In fact, Brian Mason at the Smithsonian skipped the entire thin section process for types 4-6 OC’s from Antarctica. He did the group assignment by separating a few crystals of olivine, and measuring refractive indices under oils. For achondrites, various minerals might be analyzed in this phase of basic characterization, including plagioclase, olivine, pyroxene, oxides, etc., depending on what is critical for that type of meteorite.

For very special meteorites, one might need to obtain an analysis of oxygen isotopic composition to confirm the classification. This would most likely be done for a Martian and a lunar meteorite, as soon as one suspects that this is what it is. It is also nice to have this analysis for other odd achondrites, as well as some rare chondrite groups.

Irons require much more work to classify. A polished slab is prepared and etched, and the structure must be described (both macro- and microscopically). Then, trace element analysis must be performed on the metal to assign the meteorite to a class, usually on the basis of, at least, the Ni, Co, Ga and Ir contents. The most common analytical method used for this is instrumental neutron activation analysis (INAA).

Time-consuming steps:

1) Opening the box in the first place (we have busy schedules!).
2) Waiting for a thin section to be made (can take weeks at a commercial lab).
3) Getting microprobe time that you can devote to nonresearch tasks like this on a busy instrument.
4) Getting oxygen isotope or trace element analyses, if needed.

Generally speaking, the rarer the meteorite type, the longer it will take to do a good classification and description. Labs that do a lot of classification also save things up to run all at once, in order to be more efficient (especially with microprobe time, which comes in fixed blocks). This can result in a long delay in doing the work on any given specimen.

© 1997–2019 by David Weir

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