Ordinary Chondrites

Chondrites of this clan are designated as "ordinary" just because they are the most common class of stony meteorites, representing more than 85% of all witnessed chondrite falls. As genuine pieces of primordial matter, ordinary chondrites are everything else but ordinary since they are more rare than gold or diamonds and older than any mineral known on Earth. More importantly, they might not be that common at all when it comes to the actual distribution of chondritic matter in our solar system. Some researchers have suggested that the high percentage of ordinary chondrite falls might only be due to the coincidence of the crossing orbits of certain Near Earth Asteroids - so-called NEOs - and the orbit of the Earth. If the majority of those NEOs would be of ordinary chondritic composition this would of course result in a higher percentage of ordinary chondrite falls. Thus, ordinary chondrites might be not that ordinary at all. In terms of mineralogy, ordinary chondrites are primarily composed of olivine, orthopyroxene, and a certain percentage of more or less oxidized nickel-iron. Based on the differing content of metal and differing mineralogical compositions the ordinary chondrites have been subdivided into three distinct groups that are designated as H, L, and LL chondrites.

H Group

This large group counts more than 7.000 members (including lots of probable pairings), and it has been named for the high content of free nickel-iron that is characteristic for the meteorites of this group - the "H" standing for "high iron". H chondrites contain a weight percentage of 25 to 31% total iron whereas only 15 to 19% nickel-iron is found in its free, reduced form. Hence, all H chondrites are attracted to a magnet quite easily.

The H chondrites belong to petrological types 3 - 7, with a characteristic peak at type 5. More than 3,400 members of this group have been classified as H5, about 1,800 are H6, and about 1,400 are H4. There are only about 200 H3s as well as some rare, heavily brecciated members that contain lithic clasts of several petrological types. The primary minerals are olivine and the orthopyroxene bronzite. For this reason the H chondrites have also been called "olivine bronzite chondrites" or "bronzite chondrites", but those names are no longer in use.

The comparisons of the reflectance spectra of the H chondrites to the spectra of several main belt asteroids have yielded a probable parent body - the asteroid 6 Hebe. However, 6 Hebe might not be the direct source of the H chondrites but only some sort of ancestor. Probably 6 Hebe collided with another asteroid at one time of his history and larger parts of it were dislodged into an elliptical near-earth orbit. The resulting swarm of NEOs, the children of 6 Hebe, are thought to be the true parent bodies from which the H chondrites are derived. >> top...

L Group

With about 6,500 members (including probable pairings), the chondrites of the L group represent the second largest group of ordinary chondrites. The "L" stands for "low iron" - especially in its free form. L chondrites contain a weight percentage of 20 to 25% total iron, but only 4 to 10% nickel-iron is found as free metal. Therefore, L chondrites are also attracted to a magnet, but much less than their cousins of the H group.

The L chondrites belong to the petrological types 3 - 7, with a characteristic peak at type 6. More than 4,000 have been classified as L6, about 1,300 as L5, just 400 as L4, and only about 300 as L3. Brecciated members that show clasts of several petrologic types do occur, but they are more rare than in the H group. Besides magnetite and nickel-iron, the L chondrites are composed of olivine and the orthopyroxene hypersthene. Consequently, they have been called "olivine hypersthene chondrites" or "hypersthene chondrites" in older literature and papers. However, this name is not fitting modern meteoritics and the Meteoritical Society discourages the use of these names.

When it comes to the origin of the L chondrites it has been suspected that they might be former parts of the near-Earth asteroid 433 Eros which has been intensely studied by the spacecraft NEAR-Shoemaker recently. The reflectance spectra of 433 Eros and the L chondrites seem to match closely - however, most L chondrites show signs of severe shock metamorphism suggesting a violent history of its parent body. Maybe the real parent of the L chondrites was some kind of relative or a former part of 433 Eros that has been entirely disrupted when it collided with another asteroid. >> top...

LL Group

This group represents the least common class of ordinary chondrites since it includes just about 1,100 members (again, including probable pairings). The "LL" stands for "low iron" and "low metal" reflecting that LL chondrites usually contain a weight percentage of 19 to 22% total iron, but only 1 to 3% free metal. Hence, they are only weakly attracted to a magnet.

Like the other ordinary chondrites, the LL chondrites show petrologic types from 1 - 7, but the distribution of types shows no distinct peak. The most common LL chondrites are LL6 and LL5 with about 400 members each. More unequilibrated types such as LL4 and LL3 are much more rare with just about 70 members each. The olivine in LL chondrites is more iron-rich than in the other ordinary chondrites, and this implies that the LL chondrites must have formed under more oxidizing conditions than the H or L chondrites. Older literature lists the LL chondrites often as "amphoterites" since they were thought to be a connecting link between chondrites and achondrites, but this name is misleading and no longer in use.

Scientists are still searching for a probable parent body for the LL group. One small main belt asteroid, 3628 Boznemcová, has been spotted which exhibits a similar reflectance spectrum to the spectra of the LL chondrites, but with a diameter of just 7 km it seems to be too small to be regarded as the original parent body of the LL members. Maybe it's just a fragment of a common ancestor which links the LL chondrites to 3628 Boznemcová, and further research will still have to find the real source of the LL chondrites within the dense population of NEOs crossing Earth's orbit. >> top...