Classifying a new meteorite

This final lesson brings the course together, following a new meteorite from arrival at the laboratory to an approved name in the Meteoritical Bulletin.

Documentation and first look

Classification begins with provenance: where, when and by whom the meteorite was found or seen to fall, the total recovered mass, the number of pieces and the chain of custody. For dense collection areas, precise coordinates are required, and for purchased material the best available information on origin must be recorded.

The classifier then examines the hand specimen, describing fusion crust, weathering and any visible chondrules, metal or clasts. Magnetic susceptibility can be measured non-destructively and helps distinguish ordinary chondrite groups and weathering levels.

A small piece, ideally including an interior surface, is cut with a low-contamination saw. Much of the remaining material may be preserved uncut for the type specimen and for future research requiring pristine samples. Clean handling at this stage protects the sample for later work.

Petrography and mineral chemistry

A polished thin section or thick section is studied with transmitted and reflected light microscopy. This establishes texture, the presence and condition of chondrules, metal and sulphide abundance, shock features and terrestrial weathering.

Backscattered electron imaging in a scanning electron microscope shows phases by mean atomic number, and energy-dispersive spectroscopy identifies minerals. Quantitative compositions are measured with an electron microprobe.

For equilibrated ordinary chondrites, olivine fayalite and low-calcium pyroxene ferrosilite contents usually determine the group, and textural features determine the petrologic type. For unequilibrated chondrites, the spread of compositions and the chromium content of ferroan olivine help assign subtypes. For achondrites, iron-to-manganese ratios in pyroxene and olivine help discriminate parent bodies, distinguishing, for example, HED, lunar and Martian materials. Shock and weathering grades are assigned at this stage.

Isotopes and bulk chemistry

For carbonaceous chondrites, achondrites, unusual or ungrouped meteorites and planetary meteorites, oxygen isotopes are usually essential. Measured by laser fluorination, they place the sample on the three-isotope plot and confirm or exclude group membership.

Bulk chemistry, measured by techniques such as inductively coupled plasma mass spectrometry, helps distinguish groups of carbonaceous chondrites and achondrites, and for irons trace elements, especially nickel, gallium, germanium, iridium and gold, define the chemical group.

For exceptional samples, further work may include nucleosynthetic anomalies such as chromium-54 and titanium-50 to determine NC or CC affinity, noble gases for exposure ages, and radiometric dating. These go beyond routine classification but often turn a classification into a research paper. Each extra measurement must be weighed against the material it consumes, especially for small or rare meteorites.

Submission and approval

The classifier compiles the data into a submission to the Nomenclature Committee, including the proposed name, location, mass, classification, shock and weathering grades, and the repository holding the type specimen. The committee checks the data, may request more, considers possible pairing with known meteorites, and votes.

Once approved, the meteorite is listed in the Meteoritical Bulletin database and later in a published Meteoritical Bulletin.

As you have seen through this course, a classification is a compact summary of a long story: the reservoir from which the material came, the time its parent body formed, whether it melted, how it was altered by water, heat and shock, when it was launched, how long it travelled, and what Earth did to it after landing. Being able to read that story from a few letters and numbers is the mark of an expert.

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