If you think that you might have found a candidate meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and
, 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:
- Who will test it? The availability of specific meteorite testing labs changes over time, so you should conduct a search to findMeteorite not seen to fall, but recovered at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago. one suitable for conducting the test.
- 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.
- 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.
- 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.
- 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 sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope.
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 chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such
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 achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational
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 microprobeInstrument that analyzes the chemistry of very small spots by bombarding the sample with a focused electron beam and measuring the X-rays produced. The amount and energy of the x-rays indicate the chemical composition of the sample. The electron microprobe is an important tool used in determining the composition of
. For chondrites, one tries to pin down the average composition of olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion
and pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category.
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 plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite
, 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 oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only
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 chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such
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 elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of
analysis must be performed on the metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn
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 neutronCharge-neutral hadron with a mass of 1.6748 x 10-27 kg, equivalent to 939.573 MeV, and an intrinsic angular momentum, or spin, of ½ (in units of h/2π). The neutron is a nucleon, one of the two basic constituents of all atomic nuclei (apart from 1H, which consists of a single
activation analysis (INAA).
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 isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus.
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