Meteorite Identification


Over the last 35 years, we have examined hundreds of samples that people thought were meteorites. Only two of these were identified as such! In the Midwest a lot of unusual rocks occur as glacial debris brought down to the United States from Canada or were either slag or crude iron smelted in "backyard" furnaces during early pioneer colonization. In California, strange rocks turned up from discarded ship ballast, mine dumps, or rock dumps (tossed by people who had accumulated too many tourist rocks from other states). Here in Arizona, many rocks found in desert areas are dark and shiny on the outside and may have surface depressions. They look like meteorites, quack like meteorites, but aren't necessarily meteorites. These usually turn out to be products of desert weathering where the rock surface is stained dark by iron and manganese oxides brought from within the rock by water, bacterial action, and polishing/etching by wind-borne dust. This characteristic is called desert varnish or desert patina. Just think of the patina on your old wooden furniture.

When you find a rock or metal object that you think might be a meteorite, there are several simple tests that can be made before more difficult analyses are requested from "busy professionals."

  1. If the sample is stony material, is it heavier than the average rock? Most stony meteorites weigh more than equivalent-size terrestrial rocks because they may contain 5 to 50 % nickel-iron (Ni-Fe) metal.
  2. Is it magnetic? With few exceptions, nearly all meteorites are at least weakly magnetic, either from the presence of Ni-Fe metal, magnetite, or both. An iron meteorite will be strongly magnetic, enough so to make your compass go weird.
  3. If the sample is possibly an iron meteorite, is the surface pitted? As meteorites travel through the atmosphere at incredibly high velocities, heat generated by friction with the atmosphere causes the surface to melt and ablate. Iron meteorites are susceptible to irregular melting and deep depressions are formed that resemble "thumbprints," also called regmaglypts.
  4. Is a black, melt-like surface present? Stony meteorites melt more evenly on atmospheric ablation and the melt surface forms a fusion or melt crust. In addition, objects in space tumble and as they enter the atmosphere and begin to melt, their rotational velocity slows down and the nearly static leading edge may develop a featureless surface with the molten material streaming to the rear. Given sufficient flight time, the meteor develops an aerodynamically shaped cone, and is called an oriented meteorite. Fusion crust is very thin and commonly shows flow or ripple marks from aerodynamic pressures. In contrast, some melt surfaces lack pronounced flow features and may show a shiny, dull or sugary texture with several elongated melt droplets and polygonal to irregularly-shaped shrinkage cracks. After a period of time on the Earth's surface, weathering from water and wind may reduce the fresh quality to a dull, oxidized crust or even erode away the entire crust. Meteorites from deserts typically show an eroded and polished surface from wind. Examples of these fusion crust characteristics together with a "false crust" example are shown below. Images are enlarged 10 to 15 times.

Murchison meteorite ablation crust that shows thick globs of melt flow.

Allende fusion crust that shows a static development of fusion crust (even textured). Dull gray fusion crust is typical of carbonaceous chondrite.

Ripple development in the Camel Donga achondrite fusion crust with shrinkage cracks accentuated by oxidized desert soil. Shiny, black glass is typical of achondrites.

Fine-grained fusion crust on Holbrook ordinary chondrite meteorite.

Wind-eroded and polished fusion crust surface of the Lunar meteorite NWA 032. Portions of the underlying minerals are visible through the eroded crust.

A false fusion crust that is actually desert varnish. Buff to white material on the surface and in holes is caliche, a calcareous compound commonly formed in the surface/near surface of desert soils.

If you think that you have found a meteorite and it passes these tests, please contact your nearest university geology department, NASA Center, natural history museum, etc. for confirmation.

A good place to start learning about meteorites on the web is: http://epsc.wustl.edu/admin/resources/meteorites/more_information.htm. This site includes some photographs of things that are not meteorites in its MeteorWrong photo galley at: http://epsc.wustl.edu/admin/resources/meteorites/meteorwrongs/meteorwrongs.htm.

Other sites that discuss meteorite identification that may be useful include: