Ordinary Chondrites - LL Group

Synonyms: Soko-Banja-like chondrites, amphoterites

General: Like the chondrites of the H and the L group, the members of the LL group are named for their 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 historic Serbian chondrite fall from 1877.

Description: Freshly fallen LL chondrites show a sturdy black fusion crust, and a grey interior. But other than the other OCs, the LLs show just a few sparkling metal flakes dispersed in their matrix. Petrologic types range from 1 – 7, with a distinct peak at type 5. About 2,300 chondrites of this group have been classified as LL5, about 1,100 as LL6, and just about 200 as LL4 and as LL3, respectively. Obviously, the unequilibrated LLs are rare in comparison to other ordinary chondrite groups.

Mineralogy: The olivine in LLs 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 LLs often as "amphoterites" since they were once thought to be the missing link between chondrites and achondrites, but this name is anachronistic, rather misleading, and thus no longer in use.

Origin & Formation: Scientists are eagerly searching for a probable parent body for the LLs. For some time, the small main belt asteroid 3628 Boznemcová has been considered a good candidate, but recent studies of its reflectance spectra showed it to be more similar to the angrites, a group of basaltic achondrites. However, the preliminary results of the Hayabusa (MUSES-C) mission  - an asteroid sample return project - have shown that near-Earth asteroid Itokawa has a composition similar to LL chondrites, indicating that the LL members might be derived from a swarm of related S-type NEAs.

Members: The LL group represents the least common major class of ordinary chondrites since it comprises just about 4,100 members, including abundant probable pairings. Historic falls include famous members such as Ensisheim, Bishunpur, Krymka, Paragould, Parnallee, and Siena. More recent, but also highly covetted witnessed falls include Benguerir, Bensour or Kilabo – just to name a few. LL members with rather low or exceptionally high petrologic grades are also very much covetted among collectors, such as the ultra-rare LL7 Sahara 97037 – just about 20 of these highly metamorphosed LLs have been recovered up to this day.