ORDINARY CHONDRITES
Chondrites are the most common meteorites that fall to Earth. The vast majority of meteorite falls & finds are chondrites. Chondrite classification is moderately complicated, and considers isotopic, chemical, mineralogical, textural, metamorphic, and weathering factors. Chondrites are derived from asteroid belt bodies that never underwent differentiation. That is, the parent bodies never experienced a heating event sufficient to produce a core, mantle, and crust.
Samples of ordinary chondrites are shown below. All chondrites contain spherical to subspherical to somewhat irregularly shaped structures called chondrules. Chondrules are composed principally of mafic minerals (olivine & pyroxene). Chondrules are practically the oldest materials in the entire solar system.
Ordinary chondrite (above & below) (above: field of view ~2.2 cm across; below: field of view ~2.5 cm across) - NWA 3189 Meteorite, recently found in northwestern Africa. It has been classified as an LL3.2-3.4 ordinary chondrite ("LL" means very low total iron content; "3" refers to well-preserved chondrules - the rock has not been subjected to metamorphism intense enough to disrupt the chondritic texture). This chondrite has a nice, multicolored mix of chondrules of varying size and shape. Not all chondrites have obvious chondrules as this specimen does. Chondrites subjected to significant thermal metamorphism some time in their history have chondrules that are partially to almost completely recrystallized.
Ordinary chondrite (above & below) (above: 7.7 cm across; below: field of view ~1.7 cm across) - NWA 869 Meteorite from northwestern Africa. Published analysis indicates that this is an L5 ordinary chondrite ("L" means low total iron content; "6" refers to an intensely recrystallized chondritic rock, such that most of the chondrules are indiscernible). Its chondrules are not as strikingly obvious as in the NWA 3189 Meteorite shown above. NWA 869 has a fair amount of iron-nickel metal in it (the dark gray specks - better seen in the close-up below).
Park Forest Meteorite (above & below) - the only rock from space to impact in a modern urban area is the Park Forest Meteorite. It arrived on 26 March 2003 in the suburbs of Chicago, 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 & impact was captured by police cruiser videos. Meteorite collectors descended en masse to the suburb of Park Forest & surrounding areas. Many large pieces ended up in the Field Museum of Natural History's collections (downtown Chicago), where several are on public display.
The Park Forest Meteorite has been classified as 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.
Above: large individual of the Park Forest Meteorite with dark fusion crust (FMNH public display).
Below: cut surface of the Park Forest Meteorite showing its brecciated nature (FMNH public display).
Park Forest Meteorite (L5 chondrite) - cut surface showing presence of Fe-Ni metal (= shiny specks). FMNH public display.
New Concord Meteorite - fusion crusted individual (OSU public display, Orton Geology Museum, Ohio State University, Columbus, Ohio, USA). The New Concord Meteorite is one of only a few confirmed meteorites ever reported from Ohio, USA. The rock impacted mid-day on 1 May 1860 in the vicinity of the eastern Ohio town of New Concord. Historical records indicate that one of the fragments killed a young cow. New Concord is an L6 chondrite ("L" meaning low total iron content; "6" refers to an intensely recrystallized chondritic rock, such that almost all of the chondrules are indiscernible).
Thuathe Meteorite (above & below) (above: fusion-crusted individual, 3.5 cm across; below: broken surface is 2.1 cm across) - the fall of the Thuathe Meteorite in southern Africa was initially misinterpreted by some locals as an outbreak of ghost activity. The fall was accompanied by a loud detonation heard over an area of ~100-km radius. The pre-atmospheric, pre-fragmentation size of Thuathe has been estimated at 35 to 40 cm. Thuathe impacted at 3:47 PM (local time) on 21 July 2002 on the western Thuathe Plateau in northwestern Lesotho, southern Africa. The east-west trending strewn field is about 2 x 7.5 km in size, and occurs in the vicinity of the towns of Baruting, Boqate Ha Majara, Boqate Ha Sofonia, and Ha Thibakhoali.
The sample shown here displays a well-preserved, dark fusion crust. The Thuathe Meteorite has been classified as an H4/5 ordinary chondrite. The "H" refers to a chondrite having a high iron content. The "4/5" refers to the chondrite having been metamorphosed in its history, such that many of its chondrules are not well preserved. Published analysis shows that Thuathe is composed principally of olivine, pyroxene, cryptocrystalline plagioclase feldspar, metallic Fe-Ni, and troilite (FeS).
Isotopic dating shows that Thuathe is 4.4 billion years old and was ejected from its parent body in the asteroid belt ~5 million years ago.
Thuathe info. mostly synthesized from Reimold et al. (2004 - Meteoritics & Planetary Science 39(8): 1321-1341) & David Ambrose (BBC Focus on Africa magazine).
(More info. on the Thuathe Meteorite)
Benld Meteorite (above & below) - in 1938, a car in a garage was hit by a meteorite. The Benld Meteorite impacted at about 9 AM on 29 September 1938 in the town of Benld, Macoupin County, south-central Illinois, USA. It struck a small wooden garage, crashed through the garage ceiling & then the car roof, and even through the car's back seat and floorboards. It bounced off the car's muffler & stopped in a seat cushion. The original muffler & back seat & garage roof are on display at Chicago's Field Museum of Natural History (FMNH).
Above: Benld Meteorite (cast of FMNH Me 2259). The original mass weighed 1.77 kg (= 3.9 pounds). Benld is an H-chondrite, principally composed of olivine, orthopyroxene, plus some Fe-Ni metal.
(More info. on the Benld Meteorite)
Benld Meteorite display at Chicago's Field Museum.
Benld Meteorite display - original car seat & dented muffler struck in 1938 by the Benld Meteorite.
Bath Furnace Meteorite - some meteorites have a conical shape, formed by melting during their descent through the atmosphere while maintaining one orientation during flight. Bath Furnace is an L6 chondrite that impacted the evening of 15 November 1902 at the town of Bath Furnace, Bath County, northeastern Kentucky, USA. (public display, Field Museum of Natural History, Chicago, Illinois, USA)
(More info. on the Bath Furnace Meteorite)
Ochansk Meteorite - a fusion-crusted H4 chondrite found in 1887 in Russia (FMNH Me 1442, Field Museum of Natural History, Chicago, Illinois, USA).
(More info. on the Ochansk Meteorite)