Chondrites are so-called because they contain chondrules. Chondrules were created when molten droplets of silicates solidified. Then they accreted (form by gradual accumulation or coalescence) in space together with free metals to form the first asteroids 4550 million years ago. Chondrites account for approximately 93% of stony meteorites. They are older than the Earth and the most abundant and complex type of stony meteorite.

Chondrites have different degrees of metamorphism and are thus divided into different petrologic types. They are categorized numerically from 1 to 7 in which 3 is the virtual center. Some scientists only consider numbers from 1 to 6.

Aqueous metamorphism is represented by the numbers 1 and 2, with 1 having obliterated chondrules and 2 containing altered chondrules.

Thermal metamorphism is represented by the numbers 3 to 7, with 3 being unaltered pristine chondrules and 7 being destroyed chondrules. Numbers 4, 5 and 6 are graduations between the two.

Stony meteorite

1. Ordinary

Ordinary chondrites are the most common type of chondrites. This type represents about 94% of all chondrites, 87% of stony meteorites in general, and 83% of all meteorites. They are divided according to the amount of iron within their structures and by the degree of thermal metamorphism (read chondrite). Depending on their metallic iron content, they are divided into 3 groups: H, L and LL. H means high iron content, L is low iron content and LL is very low iron content. Consequently, an ordinary chondrite with very well defined chondrules and a high metallic iron content would be classified as an H3. On the other hand, if it has no apparent chondrules and a very low iron content it would be an LL6 or LL7. If the chondrite has semi-defined chondrules and a low iron content it would be classified as an L4 or L5.

L6 ordinary chondrite slice

2. Carbonaceous

This is a complex group of chondrites which has many subgroups. The meteorites in this group were originally named carbonaceous because of their supposed higher carbon content. Later this was found to not be true in the case of all subgroups. The name ‘carbonaceous’ was maintained, but their scientific differentiation from ordinary chondrites is now based their higher abundance of refractory elements like calcium and aluminum. Carbonaceous chondrites come from very diverse locations in our Solar System and they can exhibit both thermal and aqueous metamorphism (read chondrite). Each type is abbreviated with a C for ‘carbonaceous’ then another letter indicating its subgroup. This second letter derives from the fall location of the first meteorite described for each subgroup (known as type-specimens). Therefore, meteorites that are like the carbonaceous meteorite that fell in Vigarano (Italy) are named CV. The ones like the Mighei (Ukraine) carbonaceous meteorite are CM. Other subgroups are Karoonda (CK), Ivuna (CI), Renazzo (CR), Ornans (CO), and Bencubbin (CB). There is only one exception to this nomenclature rule and that is a rare carbonaceous chondrite which is high in iron and is abbreviated CH. Carbonaceous chondrites are abundant in fascinating ingredients. Some have a high water content, amino-acids and sugars. They also have amazing materials and inclusions. The Solar System is 4800 million years old – some presolar grains can be 7000 million years old. Carbonaceous meteorites are presumably the scientifically more important type of meteorite.

CV3 meteorite slice

3. Enstatite

Also known as E chondrites, enstatite chondrites account for about 2% of all chondrites. They take their name from the mineral enstatite, of which they have a high content. They differ from ordinary chondrites primarily by having almost no iron oxide content. Interestingly, enstatite chondrites are the driest objects in the Solar System, consisting of only 0.01% water. Asteroid 16 Psyche has been proposed as the parent body of E chondrites. Their petrologic types range from 3 to 7 and they have two main subgroups: high and low iron which are abbreviated as EH and EL respectively.

4. Rumuruti

Also known as R chondrites, they are a very rare type of chondrite accounting for only 0.4% of all chondrites. They are abbreviated with an R, after the Rumuruti meteorite which fell on 28th January 1934 in the Rift Valley, Kenya. It consisted of a single piece weighing only 67 grams. R chondrites differ from ordinary chondrites mainly because most of their metal content is in the form of sulfides, they are more oxidized, and contain very little metallic iron and nickel. They have a matrix with more dust (about 50%), a higher trace element concentration of zinc and selenium, and a higher oxygen-17 ratio. Implanted solar wind is found in more than half of the rumuruti chondrites that have been analyzed for noble gases. This suggests that R chondrites come from the regolith of an asteroid (a surface with loose solid materials).

5. Kakangari

Also known as K chondrites, they are an exceptionally rare type of chondrite accounting for less than 0.01% of all chondrites. They are abbreviated with a K, after the Kakangari meteorite which fell on 4th June 1890 in India. It left just two pieces weighing only 350 grams. Kakangari chondrites differ from ordinary chondrites primarily by their reduced silicate mineralogy.

Easy chondrite meteorite classification