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LL-Group (Low-Iron, Low-Metal)
Synonyms: amphoterites, Soko-Banja-like chondrites
General: Like the ordinary chondrites of the H and the L group, the chondrites of the LL group are named for their respective metal content. 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. Their type specimen is Soko-Banja, a witnessed Serbian chondrite fall from 1877.
Description: Similar to the H and L chondrites, freshly fallen LL chondrites show a thick black fusion crust, and a grey interior. But different from the other groups, the LLs show just a few sparkling metal flakes dispersed in their matrix. Petrologic types range from 1 - 7, but the distribution of types shows no distinct peak. The most common LL chondrites are LL6 and LL5 types with about 550 members each. More unequilibrated types such as LL4 and LL3 are much more rare with just about 100 members each.
Mineralogy: The olivine in LL chondrites is considerably 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.
Origin and Formation: Scientists are still searching for a probable parent body for the LL group. One small main belt asteroid, 3628 Boznemcová, 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 is 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 the Earth's orbit.
Members: This group represents the least common class of ordinary chondrites since it includes just about 1,400 members, including many probable pairings. Historic witnessed falls include famous meteorites such as Albareto, Ensisheim, Parnallee, Siena, and Soko-Banja. More recent, but also highly covetted witnessed falls include Benguerir, Bensour, and Kilabo - just to name a few. LL chondrites with low or exceptionally high petrologic grades are also much covetted among collectors, such as our LL3.5 find, Sahara 98175, or our ultra-rare LL7, Sahara 97037, - only a handful of these highly metamorphosed LL7 members have been recovered up to this day.