METEORITE OR METEOR-WRONG: A COMPARISON OF METEORITES WITH TERRESTRIAL ROCKS

Discovery of a rare meteorite, such as the Milton pallasite found in northwest Missouri, results in public awareness of rocks with extraterrestrial origin. Most meteorites have distinct characteristics that differentiate them from rocks that formed on Earth. The purpose of this research is to compare the mineralogy and petrology of meteorites with terrestrial rocks that are commonly mistaken for meteorites and develop a set of criteria for identification. In order to recognize the characteristics of meteorites, it is necessary to have some basic knowledge of their mineralogy and petrology. Major divisions of meteorites include stony, iron, and stony-iron groups. Ordinary chondrites, carbonaceous chondrites, and achondrites are all subdivisions within the stony group based on differences in mineralogy, chemistry, and texture. The stony-iron group is subdivided into pallasites and mesosiderites based on texture and origin. One characteristic that is almost exclusively found in meteorites is the presence of native, metallic iron as the minerals taenite and kamacite. The occurrence of taenite and kamacite in a Widmanstätten textural pattern is produced only in meteorites. This characteristic pattern can be recognized fairly easily in iron and stony-iron meteorites with a simple etching process. Stony-iron meteorites are also readily identified with olivine or pyroxene mineral grains supported in an iron metal matrix. Cumulate magmatic rocks that formed on Earth in a mafic intrusion may have a similar appearance to a stony iron meteorite; however, native iron metal will not be present. Most stony meteorites contain chondrules giving rise to ordinary and carbonaceous chondrites, two of the three major subdivisions. Chondrules are typically between 0.5 and 2 mm in diameter with a spherical shape and can be identified on the surface, in a cut and polished face, or a thin section. Chondrules are not present in rocks formed on Earth and may be differentiated from igneous textures such as spherulites by petrographic observations. Many stony meteorites also contain some taenite and kamacite in small quantities that may be recognized in a polished or thin section sample. Common “meteor-wrongs” include, but are not limited to hematite, magnetite, goethite, manganese nodules, pyrite concretions, basalt, diabase, mafic cumulate rocks, and furnace or smelter slag.