How Are Meteorites Classified?

The specimen is weighed, photographed from all angles, and examined under a low-power binocular microscope. The classifier notes external features: fusion crust presence and condition, regmaglypts, surface color, texture, and any visible metal or oxidation. This visual record establishes baseline condition before any sampling occurs.

Provenance information is recorded at this stage: recovery location, acquisition chain, and custody history. This documentation becomes part of the permanent Meteoritical Bulletin entry.

A small chip or slice is cut from the specimen, typically a few grams. This is the most consequential step: it permanently removes material from the meteorite. The cut piece is ground progressively finer, mounted in epoxy on a glass slide, and continued until the section reaches approximately 30 micrometers in thickness. At that thickness, most silicate minerals become transparent to transmitted light.

A polished thin section, which has one face polished optically flat rather than ground all the way through, is prepared separately for electron beam analysis. These two sections, one for optical microscopy and one for the microprobe, are the primary analytical substrates for the entire classification.

The thin section is examined under a petrographic microscope using transmitted and reflected polarized light. Different minerals display characteristic optical properties under polarized light, including distinctive colors, interference figures, and extinction angles, which allow the classifier to identify them and map their distribution across the section.

This stage documents the internal structure of the meteorite in detail: the presence and character of chondrules, the size and distribution of metal grains, the texture of the matrix, and any secondary minerals formed by aqueous alteration on the parent body or terrestrial weathering after landing. Shock features such as planar deformation in olivine crystals or conversion of plagioclase to maskelynite are recorded and used to assign a shock stage from S1 through S6.

The electron probe microanalyzer is the central instrument of meteorite classification. It directs a focused beam of electrons at individual mineral grains in the polished section. The interaction generates X-rays at wavelengths characteristic of each element present, which the instrument measures to determine precise chemical composition of a spot only a few micrometers across.

For chondrite classification, the critical measurements are the iron content of olivine and low-calcium pyroxene, expressed as mole percent fayalite (Fa) and ferrosilite (Fs) respectively. These values are tightly diagnostic. Different chondrite groups produce distinct, narrow ranges:

For achondrites, irons, and planetary meteorites, the microprobe measures additional parameters: FeO/MnO ratios in pyroxene that fingerprint specific parent bodies, nickel content in metal phases for iron meteorite group assignment, and plagioclase compositions that help constrain petrologic type.

Using the combined petrographic and microprobe data, the classifier assigns a petrologic type from 1 to 7. The key indicator for distinguishing type 3 from equilibrated types 4 through 6 is the standard deviation of olivine Fa values across multiple grains. In a type 3 chondrite, different olivine grains have measurably different iron contents because equilibration never occurred. In a type 5 or 6, repeated microprobe measurements return nearly identical Fa values across the entire section, indicating the rock reached chemical equilibrium during thermal metamorphism.

The classifier also assigns a weathering grade from W0 (fresh, no visible terrestrial oxidation) to W6 (heavily weathered, primary minerals largely replaced). This is assessed from the extent of rust staining on metal and troilite grains and the presence of secondary iron oxides. Weathering grade does not reflect authenticity, only how long and under what conditions the specimen was exposed to Earth's environment before recovery.

Specimens that do not fit known groups require additional techniques. Oxygen isotope analysis measures the ratios of oxygen-16, oxygen-17, and oxygen-18 by isotope ratio mass spectrometry. Because different parent bodies formed in different regions of the solar nebula, they carry distinct oxygen isotope signatures. This technique is one of the most powerful for identifying ungrouped meteorites and is essential for confirming lunar and Martian classifications.

Bulk chemistry by ICP-MS provides whole-rock elemental concentrations used to classify iron meteorites by their germanium, gallium, iridium, and nickel ratios, which cluster by chemical group. Cosmic ray exposure ages, derived from cosmogenic nuclides that accumulate during the rock's transit through space, independently constrain how long the specimen was exposed to cosmic radiation before landing.

With analysis complete, the classifier submits a write-up to the Meteoritical Society's Nomenclature Committee. The submission includes the proposed name and classification, provenance history, physical description, petrographic description, all geochemical data, weathering and shock grades, and type specimen information. A portion of the meteorite is retained by the classifying institution as the permanent type specimen.

The Nomenclature Committee reviews submissions for consistency with established standards. Once approved, the meteorite receives its official entry in the Meteoritical Bulletin and enters the permanent scientific record. The full classification, for a typical equilibrated ordinary chondrite, might read: L5, W2, S3. Each element, group, petrologic type, weathering grade, shock stage, is a measured quantity, not an estimate.