Meteorite Identification and Information

Meteorite Classification
A meteorite is is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and impact with the ground without being destroyed. Meteors are visible event that occurs, (i.e. the flashes of light often called 'shooting or falling stars') when meteoroids (a small sand to boulder-sized particle of debris in the Solar system) pass through Earth's atmosphere. Other, smaller, pieces of extraterrestrial debris also cause meteors

Meteorites are classified into three main categories: stones, stony-irons and irons, depending on their dominant composition. Stones are further divided into chondrites and achondrites. Chondrites, achondrites, stony-irons and irons are then subdivided into groups and subgroups. In addition, meteorites are divided into falls and finds depending on how they were collected. (classification tree)

STONES
are similar to common terrestrial rocks in that their mineral composition is dominated by silicates. Stones are further divided into Chondrites and achondrites

• CHONDRITES get their name from the fact that they contain chondrules, tiny mineral spherules made mostly of silicates. Most chondrules are less than one millimeter across, although some may be as large as a few millimeters. In chondrites, chondrules are bound within a consolidated and fine-grained background matrix. Chondrites are the most primitive meteorites known in terms of when their constituents came together to form a rock, and the most unprocessed ones in terms of how little their materials have been altered since this rock formed. (see examples).
  • Ordinary Chondrites: The most common type of stone meteorite. They are further grouped by H, L and LL classifications, indicating iron content, and by the numbers 3-7, indicating the amount of change or metamorphism in the chondrules.
  • Carbonaceous Chondrites: Some of the most complex of all meteorites. They are rare, primitive and contain organic compounds. Most importantly they contain water-bearing minerals which is evidence of water moving slowly through their interiors not long after formation. They are further divided according to chemical and mineralogical differences into the sub-classes CI, CM, CV, CO, CK and CR. These sub-classes are named for the type specimen of each group, Ivuna, Mighei, Vigarano, Ornans, Karoonda and Renazzo respectively.
  • Enstatite Chondrites: A rare and unusual type of meteorite. They are unusual in their high enstatie  mineral content and most of their iron is in the form of metal or sulfide rather than taken up as oxides in silicates. This implies they were formed in an area of the solar nebula that was very poor in oxygen. They are further classified by H and L for their iron content and by petrologic grades 3 to 6. EH chondrites contain approximately 30% iron, while EL chondrites are about 25% iron.
  • Rumuruti or R-Chondrites: Named after the fall at Rumuruti, Kenya
• ACHONDRITES make up about 10% of all stony meteorites. Achondrites lack chondrules and originate from different areas of the solar system. These meteorites vary quite widely and can have a pale chalky interior, sometimes with other colored inclusions or shapes. They can have fusion crust, metal and other meteorite traits but will not have the chondrules.
  • HED Group: Contains the related types Howardites, Eucrites and Diogenites. Very careful analysis  provides compelling evidence that this group of meteorites may come from asteroid 4 Vesta.
  • SNC Group: Contain the classifications Shergottite, Nakhlite, Chassignite and the single specimen (ALH84001) Orthopyroxenite. The three main groups have similar chemical and isotopic characteristics, but are unusual in that they are relatively young for meteorites.
  • Aubrites: Very similar to enstatite chondrites and are often called enstatite achondrites. They show a similar high content of the mineral enstatite and are almost iron free. Aubrites also formed in a low oxygen area and are believed to have formed by the melting and differentiation of E chondrites.
  • Ureilites: A very rare class of meteorite with a relatively high 2% carbon content, most in the form of graphite found in veins within an olivine matrix, but some of the carbon takes the form of microscopic diamonds. Because of the high carbon content and other trace elements, it is thought that ureilites could have formed from or in conjunction with carbonaceous chondrites.
  • Primitive Achondrites: Those meteorites whose bulk compositions are approximately chondritic, but have been texturally modified by partial melting or metamorphic recrystallization. Group contains acapulcoite. brachinite, lodranite, and Winonaite classes.

IRONS
are mostly metallic in composition; they consist of alloys of iron (Fe) and nickel (Ni), in varying proportions.

• HEXAHEDRITES: Composed almost exclusively of the nickel-iron alloy kamacite, and are lower in nickel content than the Octahedrites.
• OCTAHEDRITES: The most common class of iron meteorites. They are composed primarily of the nickel-iron alloys: taenite - high nickel content, and kamacite - low nickel content. Due to a long cooling time in the parent asteroid interior, these alloys have crystallized into intermixed millimeter-sized bands (from about 0.2 mm to 5 cm). When polished and acid etched the classic Widmanstätten patterns of intersecting lines of lamellar kamacite, are visible. The Octahedrites can be further divided up on the basis of the properties of their Widmanstätten patterns.
• ATAXITES: Rare and composed mainly of the alloy taenite with plessite, troilite, and microscopic lamellae of kamacite. They have no visible Widmanstätten pattern. They are the most nickel-rich meteorites known (usually contain over 18% nickel). The largest meteorite ever, the Hoba meteorite, belongs to this class.

A newer Chemical classification based on the proportions of trace elements separates the iron meteorites into classes corresponding to distinct asteroid parent bodies: IAB, IC, IIAB , IIS, IID, IIE, IIF, IIG, IIIAB, IIICD, IIIE, IIF, IVAS, IVB, and Ungrouped Irons.

STONY-IRONS
are combinations of both and contain silicate and metallic phases in approximately equal amounts.

• MESOSIDERITES: Are a breccia of an approximately equal mixture of silicates and metal that is indicative of multiple and repeated impacts. The metals found in mesosiderites are very uniform, in contrast to the range of metal compositions found in iron meteorites.
• PALLASITES: very scarce and believed to have formed on differentiated bodies in the transition area between the metal-rich core and the olivine-rich mantle where the olivine could cool slowly enough to form relatively large crystals. 

FALLS are meteorites whose arrival on Earth was witnessed and recorded. Their time of fall is thus relatively precisely known. These meteorites were usually recovered shortly after their arrival. Falls give a reasonable estimate of the general population of meteorites reaching the Earth. The vast majority of falls are stones (92.8%), most of which turn out to be chondrites (85.7% of all falls). Irons are rare (5.7% of all falls); stony-irons rarer still (1.5%). In other words, by far most meteorites falling on Earth are chondrites.

FINDS are meteorites whose fall was not directly observed but were subsequently discovered on the ground, often long after they landed. The time and circumstances of their arrival on Earth are thus not well documented. The vast majority of meteorites in museum and private collections around the world are finds, not falls. Stony meteorites tend to look like ordinary terrestrial rocks and are easily overlooked making stone finds rare in spite of their commonness among falls. Finds in meteorite collections are predominantly dominated by irons, which have a distinctive appearance and therefore easier to spot; also resist weathering longer than stones and are easily found by metal detectors. Stony-irons are also less common among finds because of their their extreme rarity among falls in the first place in addition to their lesser resistance to weathering compared to irons.

Meteorite Naming & Cataloguing

Meteorites, whether falls or finds, are usually given the name of the locality nearest the site where they were recovered. In cases where many meteorites are found within a relatively small area (such as Antarctica's blue ice fields), the meteorites are designated by locality name, sometimes abbreviated (the same name for all meteorites from that area) followed by a serial number. For Antarctic meteorites the year of find is also mentioned. ALH81005, for instance, is meteorite number 5 among those recovered in the Allan Hills area of Antarctica during the 1981-1982 field season. Meteorite Classification Australia List

When sent to a testing facility for analysis and determined to be a meteorite, the finder is required to submit a finder's report to the Meteoritical Society. Twenty grams of a meteorite or 20% of a small meteorite is donated to fulfill the requirement for official classification and inclusion in the Catalogue of Meteorites. This amount will be placed in a permanent collection for scientists to have access to after the analysis is complete.

Meteorite Identification
Think you have found a rock that is unusual and it might be a meteorite? It is important to note that this information is a guide only! A meteorite can only be termed so, when it has been formally classified by a recognized institution. Here is some simple guidelines to help you get started on finding out for sure.

Identifying Stone Meteorites

• Black or Rusty Brown on Outside: Meteorites pass through the atmosphere of the Earth initially at thousands of miles per hour. They interact with the thin air high above Earth and become melted on the outside surface creating a 'Fusion Crust'. It is often black freshly fallen meteorites but will turn more and more brown as time passes looking like a dark brown varnish as the iron grains and minerals rust and weather. Unless very weathered, almost all meteorites will have traces of fusion crust, which is distinctly different from the interior matrix. So rusty brown or black on the outside should be the first thing to look for in your suspect rock.
• Rounded Corners: Meteorites are rarely ever a ball or sphere type shape, but actually quite irregular and will usually have rounded corners. Where the stones have large flat surfaces, it could be possible that it broke on entry.
• Heavy for Its Size: These meteorites look allot like a rock since they are made of primarily of materials similar to many rocks that originate on Earth, but true meteorites are often much heavier for their size than an equivalent size Earth rock. So 'Heavy for Size' is the first thing to check for in your suspect rock.
• Solid Inside: If the rock is broken it will be solid inside. It may have small round structures like tiny balls showing on the broken surface (chondrules) and many stone meteorites (the chondrites) will have them. It will not have holes inside it or be porous, like lava rock. They will not be layered or banded with different strips of mineral types. and most stone meteorites will not have shiny crystals in them.
• Has Metal Grains: Stone meteorites often have grains of nickel-iron in them. Metallic iron in rocks from the Earth is very rare because of Earths moist atmosphere, almost all native iron has been turned into some other chemical form of iron long ago. If you grind off a small spot on your suspect rock and find bright shiny metal spots it is another good indication that you might have a stone meteorite. NOTE: The metal spots in meteorites will be actual metal; they will look the way the chrome on a car looks and not appear as just a metallic luster or shininess.
• Responds to a Magnet: Because meteorites often have iron metal in them they will interact with a standard iron magnet brought near them. If a magnet will stick to your rock or will pull the rock when it is hung from a string it may mean there is iron metal in the suspect rock.
• Streak: Many rocks on Earth contain iron forms that also respond to a magnet, including magnetite and hematite. You can help distinguish them from a meteorite by a simple streak test. Vigorously scratch the suspect rock on the unglazed side of a floor or wall tile. If it leaves a black gray streak (like a soft leaded pencil) the sample is likely magnetite, and if it leaves a vivid red to brown streak it is likely hematite. A stone meteorite, unless it is very heavily weathered will not normally leave a streak on the tile. Also these rocks don't have metal grains when ground and the powder produced when grinding them will be black. Most meteorites will make a brown powder when ground (fresh meteorites may not).

If your rock has these characteristics there is a chance that it is a meteorite and you will want to find a meteorite expert to look at it. If your rock had some of the characteristics but not all it may still be a type of stone meteorite. Some stone meteorites have little or no metal so there is none to see when you grind them and they will respond weakly or none to a magnet. You will have to rely on external appearance, so if you think it is fusion crusted and appears to have flight marking from traveling through the atmosphere while melting on the outside, send it to be examined.

Identifying Iron Meteorites
Much more rare to find are meteorites made almost entirely of nickel-iron.

• Black or Brown on Outside:  These meteorites will be black or brown on the outside, often as thick crust that flakes away. Unless very weathered, there will be traces of blue-black fusion crust.
• Very Heavy: Because the are composed entirely of metal, they will be very heavy for their size.
• Strongly Magnetic: Because they are iron metal, a magnet will interact strongly with them.
• Solid Inside: There will be no holes inside.
• Show Metal when Ground: If you file or grind on them they will show metal under the colored outside like any piece of iron that is rusted.
• Surface "Thumbprints": Iron meteorite and other types may have surface by depressions that look like thumbprints, called regmaglyphs scientifically. They look like the marks made by pushing your thumb into potter's clay. Do not confuse these with holes which meteorites don't have; they are depressions on the surface.

Iron meteorites are easily confused with rusted pieces of manmade iron and steel and things like old mill balls, cannon balls, airplane, car, and motorcycle parts become good imitators of meteorites after years of rusting. All iron meteorites have nickel along with the iron so testing for the presence of nickel is often required to determine for sure if a chunk of iron is meteoritic.

Identifying Stony-Iron Meteorites
As the name indicates they have characteristics of both the previous types (see above). They are made of a mixture of nickel-iron and stone.

• Heavy for Size: They will be heavy since they contain a lot of iron.
• Contain Metal: It will be possible to file or grind and find abundant metal. But it will often be possible to find spots on the surface where a regular file is useless since the material is mineral.
• Rusted: Due to their iron content they are often very rusted and any olivine color and crystals in the mineral portion may have discolored or fallen out from weathering and rusting.
• Yellow Green or Yellow Brown: The mineral portion can be dark silicate minerals or it can be yellowish green or yellowish brown olivine crystals.

Links to additional information about meteorites and tektites available at www.meteorite.com

Sources: meteoriticalsociety.org www.meteorites.com.au, www.meteorlab.com, www.meteorite.com,