Chondrite Meteorite Classification

Chondrite classification takes into account the amount of free and chemically bound iron and the amount of alteration of the chondrules due to heating and pressure within the parent body. The transformation of the chondrules occurred shortly after the parent bodies accreted. This is thought to have occurred between 4.5 and 4.56 billion years ago and indicates weak thermal activity.

Chondrites are divided into three broad groups:

Ordinary chondrites; By far the most numerous of the meteorites that hit the earth (about 85%), these chondrites are classified by iron content and further by the degree of metamorphic alteration of chondrules seen in the individual specimen. There are three types of ordinary chondrites.

High iron (H) 25 - 31 % total iron with 15 - 19% metallic iron. A Fayalite ratio range of Fa15-21 is typical. The "H" chondrites are considered to have formed outward from the sun beyond the earth's orbit with the "L" parent body forming farther out and the "LL" parent body farther yet from the sun.

Normal range iron (L) 20-25 % total iron with 4 - 10% metallic iron and a Fayalite ratio range of Fa22-26.

Low iron (LL)19-22 % total iron with 1- 3% metallic iron and a Fayalite ratio of > Fa28-32.

Alteration grades for ordinary chondrites run from "3" with un-altered chondrules to "7" with the chondrules being completely obliterated by alteration. A petrologic grade of "3" never exceeded 600° C, "4" was heated to between 600-700° C, "5" around 700-750° C, and "6" between 750 and 1000°C. Chondrules are not commonly seen in petrologic grade 5 and above stones.

An additional classification factor considered is the degree of shock alteration by collisions seen in the specimen. These grade from S1 to S6. S3 will generally have visible shock veins.

The final consideration is alteration by terrestrial processes after the meteorite arrives at the surface of the earth. Weathering has '6' step grades. W0 is fresh with little or no weathering. If the specimen is completely oxidized a weathering grade of W4 is given. If an old fall has weathered with the silicate minerals being replaced by minerals of terrestrial origin a weathering grade of W6 is indicated.
Adding it all up: An H3 S3 W1 meteorite will be from the "H" parent body, have clearly defined chondrules, visible shock veins and metallic iron, and show some rust stains where the metallic iron has started to oxidize.

Enstatite chondrites; were heated, but not to the 1200° C melting point. Composed of the minerals pyroxene and plagioclase feldspar, they are graded into high iron content (EH) and low iron content (EL) groups. These chondrites can have the highest content of free metal and sulphides found in meteorites. Petrologic grading follows that used in the ordinary chondrites described above.

Carbonaceous chondrites; show alteration due to aqueous action completely unlike the alteration seen in ordinary chondrites. They are graded from "3" that indicates little alteration to "1" that indicates the chondrules have been obliterated due to metamorphic alteration. These primitive meteorites are rare with classifications named for the specimen that originally had the type. Thus, "CI" meteorites are named for the Ivuna stone, which fell in Tanzania in 1938. It is a "CI 1" stone with 3-5% carbon, 20% water in silicates, and magnetite. There are also some amino acids present. The grade "1" indicates the chondrules have been obliterated by the action of the water.
The type "CM" group is named for the Mighei stone that fell in the Ukraine in 1889. It is a "CM 2" class stone with 0.6 to 2.9% carbon and 13% water, visible chondrules and some amino acids.
The type "CO" group is named for the Ornans stone that fell in Doubs, France in 1868. It is a class "CO 3.4" specimen with 0.2 to 1% carbon and 1% water. Chondrules, while present, are small.
The type "CV" group is named for the Vigarano stone that fell in Italy in 1910. It is a class "CV 3.3" stone with less than 0.2% carbon and less than 0.03% water. Chondrules are present. The Allende meteorite that fell in Mexico is one of this type. It also contains mysterious "CAI" inclusions, which are calcium, aluminum minerals not found on earth. These white, irregular shaped inclusions do not derive from the same process as the chondrules. Their origin is subject of current speculation.

Three recent additions have been added as new, rare, stones have been found. These are "CK" from Karoonda, Australia that appear to be similar to "CO" and "CV" meteorites, "CR" for a stone from Renazzo Italy formerly classified as a "CM" but shown to have visible metal and "CH" which is similar to "CR" but with a very high metallic iron content.

There are two additional and extremely rare types. Type "R" or Rumurutiites are breccias with very little free metal. "K" or Kakangari type is an extremely rare stone that is unusually rich in combined iron.

NOTE: The carbonaceous meteorites are generally observed falls. This is because they rapidly weather beyond recognition. Prompt recovery is essential to identification of these stony meteorites.