METEORITES
 These rocks come from outer space!


ALL meteorites can be broken up into three basic kinds:
  • Stony (mostly silicate minerals)
  • Iron (mostly iron)
  • Stony Iron (iron and silicate minerals)
STONY METEORITES are the most abundant and consist of chondrites and achondrites. Chondrites represent 85% of all meteorite falls and contain chondrules, the earliest material in the solar system (unchanged!). Achondrites have no chondrules and include martian and lunar crustal samples.
CCSF samples include:
    Julesburg Ordinary Chondrite (L3) (#15252)
    Found: 1983 Julesburg, Sedgewick County, Colorado
    On loan from California Academy of Sciences
Carbonaceous chondrites are the most primitive of all meteorites. It appears that these meteorites condensed in the pre-solar nebulae at low pressures. As a result, they contain carbon, partially in the form of organic molecules. Amino acids, the basis of RNA and DNA found in the human body, have been found in certain carbonaceous chondrites.
CCSF samples include:
    Allende Carbonaceous Chondrite (C3V) (#15257)
    Fall: 7/10/1899 Allende, Chihuahua, Mexico
    On loan from California Academy of Sciences

    Murchison Carbonaceous Chondrite (CM2)
    Fall: 9/28/1969 Murchison, Victoria, Australia
    This piece was studied at NASA-AMES Research Center, in Moffett Field, CA, where researchers discovered the presence of amino acids. Sample on loan from K.W.
IRON METEORITES are identified in two ways. First they usually display a smooth black or oxidized surface often marked by pits called "thumbprints," which are caused when some of the meteorite melts away during atmospheric entry. The second way to identify an iron meteorite is to slice, polish and etch it with a weak solution of nitric acid. This procedure will reveal a crisscross pattern called the "widmanstatten pattern.” This structure is unique to meteorites and is not found in any terrestrial rock. It is caused by the slow cooling and growth of crystals composed of two iron-nickel alloys.

Octahedrites are iron meteorites that contain about 6-17% nickel and are the most common of the iron meteorites. Octahedrites are further classified into three main groups: coarse, medium and fine, which describe the width of the bands in the crystalline pattern. The coarser the pattern, the greater the amount of iron. The finer the pattern, the higher the nickel content.
CCSF samples include:
    Henbury Octahedrite (medium IIIA) (#9116.01)
    Found: 1931 Henbury Crater, Northern Territory, Australia On loan from California Academy of Sciences

    Gibeon Octahedrite (fine IVA) (#9517)
    Found: 1836 Gibeon, Great Namaqualand, Namibia
    On loan from California Academy of Sciences

    Canyon Diablo Octahedrite (coarse IA) (#9388)
    Found: 1891 Barringer crater, Coconino County, Arizona
    On loan from California Academy of Sciences
STONY-IRON METEORITES are a combination of silicate minerals in an iron matrix. They are quite rare and only account for about 1% of all meteorites. There are two different types of stony-iron meteorites: pallasites and mesosiderites. Pallasites contain green or golden olivine crystals embedded in a nickel-iron matrix. Like iron meteorites, they display the widmanstatten pattern in the nickel-iron matrix when polished and etched. Scientists believe that these meteorites formed when a planet was forming. The material from the molten metal core of the planet mixed with the silicate magma, and the olivine crystallized out of the silicate as it cooled. These crystals were then forced into the metal "mold" where the mass solidified.
CCSF samples include:
    Brenham Pallasite Meteorite (#15263)
    Found: 1882 Locality: Kiowa County, Kansas
    On loan from California Academy of Sciences
Tektites are not true meteorites. They are thought to represent crustal rock that was ejected during a meteorite collision. This crustal rock would have melted as was ejected through Earth’s atmosphere, perhaps even exiting the atmosphere and then falling back to earth while cooling, creating the streamlined shapes.
CCSF samples include:
    3 individual pieces – dumbbell, club, and irregular shapes – from Thailand. (#9083) *the tektites from Laos, Thailand and Cambodia are assumed to have originated from a single huge meteorite impact, the crater of which has never been identified. They are about 800,000 years old. On loan from California Academy of Sciences
METEORITE FACTOIDS
  • All meteorites (save those that are pieces of the Mars or Moon) have the same age: 4.6 billion years – the age of the initial formation of the rocky planets in our solar system.
  • A meteorite refers to natural extra-terrestrial material that lands on the ground. Materials vaporizing in the atmosphere are called meteoroids and if luminous are called meteors or shooting stars.
  • Meteorites are named for the geographical location at which they are found.
  • When a meteorite is seen landing and whole or parts are recovered it is called a "fall". If a piece of meteorite is found but not observed falling it is called a "find."
  • Tools, weapons, and jewelry determined to have been made from meteorite material go as far back as 3000BC.
  • Although there are a few early reports speculating that meteorites might be extra-terrestrial; the general public was largely skeptical until about 1800.
  • About 500 meteorites weighing more than 200 gm. fall on earth every year, only 150 fall on land. Of those, only about 20 will be eventually recovered. A meteorite weighing more than 4 tons falls about once every 10 years. Information Source: "Meteorites" by Alan Carion, Paris, France

Text above written by Jim Ambrose and Katryn Wiese.

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