Other, less common chondrite groups provide important insight into our early solar system history, as well as details on chondrite formation. In our previous discussion on carbonaceous chondrites, we explained how specific groups formed under variable oxidizing conditions based upon their distance from the Sun. For example, the CI and CM chondrite groups formed under the more oxidizing conditions present in the outer regions of the solar nebula. In contrast, groups such as the CO and CH chondrites formed under more reducing conditions, indicative of a closer proximity to the Sun. The same holds true for the other chondrite groups and they can be placed into a continuous sequence along with the ordinary chondrites. On one extreme, we find the highly reduced enstatite, or E chondrites that must have formed more closely to the Sun than the H, L, or LL chondrites. At the other extreme, we find the highly oxidized rumurutiites, or R chondrites that attest to a formation further from the Sun. We will elaborate on both groups below. Finally, we'd like to introduce two other groups or grouplets of chondrites that don't easily fit into existing schemes. The kakangariites, or K chondrites consist of only three members, while the forsterite, or F chondrites are a more hypothetical grouplet. Its "members" are merely inclusions that have been found as xenoliths inside certain brecciated
The chondrites of this group are named for their primary mineral, enstatite, and they differ in many respects from ordinary and carbonaceous chondrites. They must have formed in an oxygen-depleted environment because nearly all of the iron in E chondrites is present in its reduced, metallic form. Even the pyroxene is depleted in iron, and consequently, it is only found as the pure magnesium-rich end-member - enstatite. After taking into consideration all of the possible pairings among the meteorites recovered in the strewn fields of Africa and in Antarctica, approximately 90 different E chondrites have been identified.
Similar to the ordinary chondrites, the enstatite chondrites have been further subdivided based on their content of total iron; members of the EL group contain less iron than members of the EH group. Moreover, there are mineralogical aspects that separate the two groups. The EL chondrites show petrologic types 3 to 7, and there is a distinct peak at the equilibrated type 6. The members of the EH subgroup exhibit petrologic types from 3 to 6 with a less distinct peak at the unequilibrated type 3.
Despite the differences, most researchers believe that both subgroups originated on the same asteroid, most probably representing different layers of the parent body. Some scientists think that we should look for this asteroid inside the orbit of Venus or even Mercury since the E chondrites formed under highly reducing conditions in an oxygen-depleted environment. Other researchers suggest that a formation in the inner asteroid belt would have provided the same conditions in the early solar system. A more recent comparison of the reflectance spectra of different asteroids to the spectrum of the EH chondrite Abee suggests that the main belt asteroid 16 Psyche might be the common parent for the enstatite chondrites.
This group was formerly known as the Carlisle Lakes group, for a meteorite that was found in Australia in 1977. It is now named for the type specimen Rumuruti that fell in Kenya, Africa, in 1934. Rumuruti is the only witnessed fall of this group and just one small individual has been preserved in the collection of the Humboldt Museum Berlin, Germany, since 1938. It was thought to be an anomalous chondrite until it was reclassified in 1993 and the R group was formed. There are just 25 R chondrites known if we exclude all probable pairings.
Most R chondrites belong to petrologic type 3 or are heavily brecciated members that show different lithologies from petrologic type 3 to 6. There are only a few R chondrites of higher petrologic types that are not brecciated, e.g. our find Ouzina, an R4 from Morocco. The R chondrites are quite different from ordinary chondrites and they are the opposite of the E chondrites when it comes to mineralogy and their state of oxidation. The members of this group are highly oxidized, containing high amounts of iron-rich olivine. There is practically no free metal inside the R chondrites since most of the iron is either oxidized or found in the form of iron sulfides. The iron-rich olivines, along with the oxidized nature of the iron, give most R chondrites a typical red appearance. However, some fresh and unequilibrated R3 chondrites, such as our new find, NWA 753, show a light-greyer coloured matrix.
The meteorites of this group contain fewer chondrules than ordinary chondrites or enstatite chondrites, but they often contain xenolithic inclusions that indicate a regolith origin, representing samples of the surface of an asteroid. Another indicator for a regolith origin is the fact that most members of the R group contain high amounts of noble gases implanted into the rock by the solar wind. The parent body of the R chondrites has yet to be found, but it surely must have been subjected to many impact events during its history resulting in the high degree of brecciation that most R group members exhibit.
The chondrites of this grouplet are named for their type specimen Kakangari, a meteorite that fell in Tamil Nadu, India, in 1890. There are just three K chondrites known: Kakangari, Lea County 002, and LEW 87232. With a total known weight below 400 grams, they represent one of the most rare meteorite groups found on Earth and we dearly hope that additional K chondrites will show up in the wealth of new meteorites that are currently found in the deserts of Northwest Africa and Oman.
All K chondrites so far known belong to the unequilibrated petrologic type 3. They are rich in the iron sulfide, troilite, and show numerous primitive, armored chondrules. They are unique in their chemical composition and show an oxygen-isotopic signature that distinguishes them from all other chondrite groups and clans. All of this indicates that the K chondrites must have had their origin in a small, primitive parent body that has yet to be identified.
This strange grouplet is known solely from certain lithologies that have been found in two achondrites, both members of the aubrite group: ALH 78113 from Antarctica, and Cumberland Falls, a meteorite that fell in Kentucky, USA, in 1919. Both are polymict breccias that contain dark clasts of chondritic material that does not fit into any established chondrite group or clan. Those clasts have been provisionally named for the fact that the olivine found in these lithologies consists of the pure magnesium-rich end-member of olivine called forsterite. Hence, the grouplet was named forsterite group or F chondrites.
All F lithologies are highly unequilibrated and can be assigned to petrologic type 3. Their mineralogy and oxidation state places the F chondrites between the H group of the ordinary chondrites and the E chondrites. When it comes to their origin, it is believed that they are derived from a small and primitive asteroid of F chondritic composition that collided with the aubrite parent body short after their formation in the early solar system.