Types of Meteorites

Meteorite Classification Table

(Click on letter designations for photo catalogs and information pages for specific meteorites.)

Category

Composition Type

Distinguishing Features/ Chondrule Character

Letter Designation

Chondrites

Stony Meteorites are characterized by chondrules--small spheres (average diameter of 1 mm) of formerly melted minerals that have come together with other mineral matter to form a solid rock. Chondrites are believed to be among the oldest rocks in the solar system. Click here to link to an article on the Nature and Origin of Chondrules. Click here to see a close-up picture of chondrules. 82 percent of meteorite falls are chondrites.

Number designations for chondrites: Most classified chondrites are given a number designation to catagorized their "petrologic type." The number refers to the alteration of the chondrules. A designation of "3" indicates unaltered chondrules. Numbers greater than 3 indicate increasing thermal metamorphism. A designation of 7 indicates complete obliteration of chondrules. Numbers less than three indicate increasing aqueous alteration. A designation of 1 indicates that chondrules have been obliterated by aqueous alteration. A lack of numeric designation means that no alteration designation has been assigned. Chondrites of Petrologic Type 3 are referred to as "unequibrated" because of their lack of metamorphism. Here is a table from Weisberg indicating the criteria for petrologic type"

Further reading, King, ed., Chondrules and Their Origins. 1983

Note on type 7 Chondrites. There is no firm agreement among meteorite researchers on the definition of type 7 chondrites. Many meteorites previously described as type 7 are either type 6 or are impact melts. Impact melting is not considered a proper basis for type 7 classification. Future research is expected to clear up this issue.

Here is a diagram from Weisberg showing petrologic types for each group:

Letter Designations: The E stands for Enstatite. H indicates high metallic iron and L indicates low metallic iron. Very Rare--1.5% of falls. Minerals:  Enstatite, metal, sulfides, plagioclase, and occasional olivine (EH).

The most common meteorite from observed falls--73.5%.

The initial letter designation for ordinary Chondrites, "O", is usually deleted. Brecciated meteorites with clasts of more than one type of alteration are designated with the range of alteration present in the meteorite. The letter designation refers to the iron content for the meteorite as a whole. E.g., Zag bears the designation H3-6.

Some meteorites are classified as transitional between the main groups, e.g., H/L, L/LL

When a researcher is not sure of a class, it may be designated with a notation like L(LL) with the less certain class being in parentheses.

High Iron (12 to 21% metallic iron) (also called Bronzite Chondrites) 31.4% of falls. Minerals:  Olivine, pyroxene, metal, plagioclase, sulfide.

Low Iron (5 to 10% metallic iron) (also called Hypersthene Chondrites) 34.8% of falls. Minerals:  Olivine, pyroxene, plagioclase, metal, sulfide.

Low Metal Content (about 2% metallic iron) (also called Amphoterites) Principle minerals are bronzite, olivine, and minor oligoclase. 7.2% of falls.

These rare meteorites contain elemental carbon, a basic building block for life. 3.6% of falls.

Explanation of Letter Designations:

The "C" stands for Carbonaceous Chondrite. The second letter refers to type localities (except the H designation). The number refers to alteration.

Achondrites

Stony Meteorites without chondrules. Scientists believe that some of these meteorites originated on the surface of the Moon or Mars. 7.8 percent of meteorite falls are achondrites.

Note on Groupings:

Howardites, Eucrites, and Diogenites have been grouped as HED meteorites. These types may originate on the asteroid Vesta. Chassignites, Shergottites, and Nakhlites are grouped as SNC meteorites. These types are believed to originate on Mars.

See H. McSween, Meteorites and Their Parent Planets for and excellent explanation.

Irons (structural classification)

These meteorites are made of a crystalline iron-nickel alloy. Scientists believe that they resemble the outer core of the Earth. 4.8 percent of meteorite falls are irons.

6% to 17% Ni. Contains both kamacite and taenite and has Widmanstatten pattern.

Irons (Chemical Classification)

A second scheme for classifying iron meteorites is by their chemistry. The determining factors are groupings of meteorites with similar ratios of trace elements to nickel. Generally, the higher the Roman numeral of the classification, the lower the concentration of trace elements. The casual observer cannot see this as one can with the Widmanstatten bandwidth that is the determining factor for structural classification. Chemical classification is important because it suggests that certain iron meteorites share a common origin or were formed under similar conditions. The Handbook of Iron Meteorites by Vahn Buchwald has a more complete description. Here is a diagram taken from Weisberg, et al.

Stony Irons

These meteorites are mixtures of iron-nickel alloy and non-metallic mineral matter. Scientists believe that they are like the material that would be found where the Earth's core meets the mantle. 1.2 percent of meteorite falls are stony irons.

Mesosiderites have been divided into a classification grid that resembles that for chondrites. There are three broad petrologic groups designated A, B, and C. Within these groups there is a numeric classification of metamorphic grade. Grade 1 is fine grained and fragmental, 2 and 3 show progressive recrystallization, and 4 is a melt breccia.