File:Ordinary chondrite (Park Forest Meteorite) 3 (14780739165).jpg

Ordinary chondrite - Park Forest Meteorite. (cut surface, showing reflectance of the iron-nickel metal; public display, Field Museum of Natural History, Chicago, Illinois, USA)

Chondrites are the most common type of meteorites that fall to Earth. Chondrite classification is moderately complicated, and considers isotopic, chemical, mineralogical, textural, metamorphic, and weathering factors. Chondrites are derived from bodies in the Asteroid Belt that never underwent differentiation. That is, the parent bodies never experienced a heating event sufficient to produce a core, mantle, and crust.

All chondrites contain spherical to subspherical to somewhat irregularly shaped structures called chondrules. Chondrules are composed principally of mafic minerals (olivine and pyroxene). Chondrules are nearly the oldest materials in the entire solar system. Chondrites subjected to significant thermal metamorphism some time in their history have chondrules that are partially to almost completely recrystallized.

The Park Forest Meteorite is the only rock from space to impact in a modern urban area. It fell on 26 March 2003 in the suburbs of Chicago in northeastern Illinois, USA. The original rock is estimated to have been the size of a small car before fragmentation. Its entry into Earth’s atmosphere was marked by a noticeable fireball seen by many people in Ohio, Indiana, Wisconsin, and Illinois. The fireball and impact was captured by police cruiser videos. Meteorite collectors descended en masse to the suburb of Park Forest and surrounding areas. Many large pieces ended up in the Field Museum of Natural History’s collections (downtown Chicago), where several are on display.

The Park Forest Meteorite is a brecciated L5 chondrite. “L” means low total iron content; “5” refers to a recrystallized chondritic rock, such that most of the chondrules are indiscernible. It consists of light-colored clasts in a dark matrix. The rock has been strongly shock metamorphosed some time in its history. Mineralogical analysis shows it is composed of olivine, pyroxene, troilite, maskelynite (= plagioclase feldspar glass), iron-nickel alloy, chromite, etc.

Dating of shock-metamorphosed L5 chondrites indicates that the parent body in the asteroid belt broke apart during the Middle Ordovician, about 465 million years ago. Fragments of the busted-up L5 chondrite parent body are now represented by the Flora family of stony asteroids.

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