An Introduction to Meteorites and Meteoritics

Meteorites are classified into three basic categories: irons, stones, and stony irons. One important fact to keep in mind is that the vast majority of all meteorites are magnetic. Those who work in the field searching for meteorites keep a magnet handy. This is always the first test; if the suspect is not magnetic, it is nearly always a meteorwrong!

Researchers and collectors learn quickly how to identify meteorwrongs since the purchase of what may be represented as a very rare stone meteorite at $200 a gram could turn out to be an expensive piece of junk. Over the years, the classic meteorwrong is a specimen known as a Cumberlandite. For years, Cumberlandites have been found in Rhode Island, originating from an outcrop in Cumberland, Iron Mine Hill. Cumberlandites are igneous rocks and are still being offered for sale as meteorites.

There are four major visual clues in recognizing a meteorite:

1. It attracts a magnet due to the fact that the meteorite contains metallic iron.
2. It has an usually dark, thin, melted surface layer called a fusion crust, due to the melting of minerals on the meteorite's surface during the plunge through the earth's atmosphere.
3. It exhibits an aerodynamic shape acquired during the high speed flight.
4. Its surface is covered with thumbprint–shaped indentations called regmaglyphs, made during the entry through the atmosphere.

Iron Meteorites

Iron meteorites, or “irons” as they are commonly known, are actually a combination of iron, nickel, and cobalt. Iron is the predominant metal, with 5% up to 50% nickel (as an alloy with iron in most iron meteorites), and a trace of cobalt. Iron meteorites are divided into three basic groups. These groups depend on the iron-nickel ratio and the rate at which their parent body originally cooled: Hexahedrites, Octahedrites, and Ataxites.

Hexahedrites

The word “hexahedrite” comes from the type of crystalline form, a hexahedron or six-sided crystal. Hexahedrites contain large, hexahedron crystals of kamacite. Kamacite is an iron-nickel crystal containing iron and up to 7.5% nickel. Other elements also often found in minute quantities include carbon, chromium, cobalt, phosphorous, silicon, and sulfur.

Octahedrites

Octahedrite, like hexahedrite, comes from the fact that they contain octahedron, or eight-sided crystals in addition to the iron and nickel matrix. Octahedrites also can contain chromite, cohenite, diamond, and schreibersite, along with nodules of graphite and troilite, which are often surrounded by kamacite. Kamacite is an iron-nickel metal alloy up to 7.5% nickel. Octahedrites are divided into several groups, depending on the iron-to-nickel ratio. These groups are Finest, Fine, Medium, Coarse, and Coarsest.

People will often ask how one knows for certain that a particular magnetic rock is a meteorite. Octahedrites produce a crystalline pattern of lines after being cut, polished and etched with a weak solution of nitric acid. These patterns are called the Widmanstätten Pattern, after its discoverer, Count Alois von Widmanstätten, a porcelain manufacturer from Vienna, Austria. Widmanstätten described the pattern in 1808. The Widmanstätten Pattern is unique to iron meteorites and is due to the broad kamacite bands sandwiched between narrow taenite bands. These bands are parallel to the octahedron's pairs of faces.

Hexahedrites also can produce a series of lines or bands, called Neumann Lines, named for its discoverer, Johann Neumann, who detailed them in 1848. These are a very fine series of lines, sometimes crossing each other. Neumann Lines, much like the Widmanstätten Pattern, are seen when the sample is cut, polished and acid-etched. However, the reason for Neumann Lines is that the sample experienced higher pressures and temperatures than Octahedrites.

Ataxites

Ataxites are also called Silicated Irons. This class of irons contains significant amounts of nickel, forming a unique nickel–iron alloy. Also ataxites do not exhibit Neumann Lines or Widmanstätten Pattern when cut, polished and etched with nitric acid.

Stone Meteorites

Stone meteorites, also called “stony meteorites” or stones, make up over 94% of observed falls. They are mostly believed to be material from the crust and mantle of asteroids. A few stony meteorites are thought to be from comets. Stone meteorites contain approximately 75% to 90% silicate materials, such as olivine and pyroxene. Olivine and pyroxene are types of silicate minerals, which contain silicon, oxygen, and one or more metals. The majority of stones also contain an iron–nickel alloy. There are two major groups of Stones: Chondrites and Achondrites.

Chondrites

Chondrites are so named due to the fact that they contain small spherical crystals of minerals such as olivine and pyroxene imbedded in the stony material, called chondrules. Edward Howard and Jacques–Louis Comte de Bournon first described chondrules in 1802. The term “chondrules” comes from the Greek “chondros”, meaning a grain of seed. Chondrules are (approximately) millimeter-sized spheroids.

Chondrites can be as porous as sandstone and can be easy to break or crush. There are several subgroups of chondrites:

Amphoterites or LL Chondrites

Contain very little iron (“LL” stands for low iron and low metal); these tend to be composed of fragmented rock.

Carbonaceous or C Chondrites

Contain organic compounds. Carbonaceous Chondrites are very rare, contain little metal, exhibit well defined chondrules, and tend to have a black to gray matrix. There are four subclasses of Carbonaceous Chondrites (C1 through C4) depending on their composition and state of alteration.

Enstatite or E Chondrites

Contain the silicate enstatite, an iron-free pyroxene. Two subclasses (H and L) of Enstatite Chondrites are dependent on iron content. Enstatite Chondrites are very rare.

Olivine-Bronzite or H Chondrites

The most abundant class of meteorites, contains a high degree of iron (thus the “H” Chondrite) both in metal flakes and mineral form.

Olivine-Hypersthene or L Chondrites

Contains less iron (“L” Chondrite) than Olivine-Bronzite Chondrites.

Some additional classes of Chondrites have been suggested, including B Chondrites (more that 50% iron-nickel alloy) and R Chondrites (highly oxidized, olivine-rich, and little iron-nickel alloy).

Achondrites

Achondrite meteorites are so named due to the lack of chondrules or metal flakes, but are rich in silicates. Achondrites are very rare, relatively speaking. Achondrites are believed to have undergone advanced geological processing on their parent bodies, such as lava or magma flows and impact breccias. There are also a number of Achondrite subgroups.

Eucrites

The most abundant of the Achondrites, these are calcium-rich basaltic meteorites, meaning they are like volcanic rocks. Eucrites are nevertheless chemically different from terrestrial basalt and appear to be from the asteroid Vesta.

Diogenites

(Calcium-poor basaltic meteorites) are related to the Eucrites. They also appear to be from the asteroid Vesta.

Aubrites

Calcium- and iron-poor meteorites consisting mostly of enstatite (Aubrites might be related to the Enstatite Chondrites).

Ureilites

Calcium-poor meteorites, consisting mainly of olivine, pyroxene, and carbon in the form of either graphite, diamond, or lonsdaleite (a rare pure carbon mineral like diamond, but with a different crystal structure), as well as a significant amount of iron and nickel. Their parent source is highly debated today.

Lunar meteorites

A small number of lunar meteorites have been recovered. Lunar meteorites are impact breccias, rocks formed by the re-welding of loose fragments that were shattered during impact events. Lunar meteorites may be identified by a fusion crust with slightly green hues and by a gray interior with angular inclusions of often brighter materials. Lunar meteorites are believed to have been blasted off the Moon as ejecta from high-velocity impact events.

SNC or Martian meteorites

A small number of meteorites are apparently from Mars. Martian meteorites are commonly referred to as the SNC meteorites after Shergotty, Nakhla, and Chassigny (the first three subgroups known). These Martian meteorite subgroups are distinguished on the basis of mineralogy, but all share characteristics which together point to a Martian origin.

There are a number of Achondrites that do not fit into any of the preceding groups or subgroups. Some are the only meteorite known of their kind.

Stony–Iron Meteorites

Stony–iron meteorites, or stony–irons, are mixtures of silicates and iron–nickel in roughly equal proportions. Only about 1% of all falls have been stony-irons. There are two main stony–iron groups: Pallasites and Mesosiderites.

Pallasites

Pallasites, the most common stony-iron, consist of olivine crystals embedded in an iron-nickel alloy matrix. The olivine crystals can be as large as 10 millimeters across. Many consider Pallasite sections as simply the most beautiful form of meteorite representation when they are cut and polished. Pallasites are believed to have been formed at their parent body's core-mantle boundary. The iron-nickel, Octahedrite in nature, would have come from the core and the olivine from the mantle's base.

Mesosiderites

Exhibiting characteristics of multiple impacts, Mesosiderites, or “Mesos” as they are often called, are often referred to as the “wastebasket” form of meteorites. The iron–nickel alloy of Mesosiderites does not form a network as in the Pallasites, but instead form grains and nodules of the alloy. Like Pallasites, the iron-nickel alloy exhibits the same properties as octahedrites.

Tektites

Tektites are fused glassy material classified as dry — that is, they contain little water, are silica–rich, and somewhat similar in composition to volcanic glasses. Their exact origins are still debated, however the best explanation is that tektites are formed when an Impactor strikes the Earth with such force that surround terrestrial rock is vaporized and ejected into space. There the ejecta cools and re–enters Earth's atmosphere where it develops shape characteristics.

Impactites

Impactites are impact-caused fused silica–based material. These are often the only evidence in addition to a crater of very old celestial impacts and were first discovered in 1932 around the Australian Henbury craters. This slag–like glassy material is rock melt due to the high heat and pressure of the meteoritic impact.