Carbonaceous Chondrites CH 

CH Chondrites

Synonyms: High-iron-type, or ALH 85085-like carbonaceous chondrites


General: The chondrites of the CH group are somewhat of an exception since their group name isn't derived from a particular type specimen but from one of the typical properties of these meteorites. The "H" stands for "high metal" since the CH chondrites contain up to 15% nickel-iron. The first CH chondrite has been recovered from the Antarctic Allan Hills and has been named ALH 85085, so it can be regarded as the original type specimen of this group.


Description: Dark, metal-rich carbonaceous chondrites of different appearance. Besides their high abundance of nickel-iron they show sparsely distributed fragmented chondrules, only a few remaining intact. Most of these chondrules as well as the less abundant CAIs are very small which is rather typical for the chondrites of the CH group. All members recovered so far belong to petrologic types 2 or 3.


Mineralogy: As with the closely related CR chondrites, the members of the CH group contain certain amounts of phyllosilicates and other traces of aqueous alteration that took place during their history of formation. CAIs are also present within the fine grained silicate matrix. Some CH chondrites are known to contain rare mineral phases, and in one member, NWA 470, the mineral Ca-monoaluminate has been found for the first time in nature.


Origin and Formation: Some researchers have suggested that the CH chondrites formed in close proximity to the Sun. This is reflected in the abundance of certain trace elements as well as in mineralogy. It is believed that the precursor material of the CH chondrites condensed in a very early stage from the hot primordial nebula - inside what is today the orbit of Mercury -  and have been later transported to outer and cooler regions of the nebula where they have been more or less preserved to this day. An interesting coincidence is that the planet Mercury might have formed from similar, metal-rich material. This would explain its high density and its extraordinary large metal core that makes Mercury unique among all other terrestrial planets in our solar system.


Members: There are only about 12 meteorites known which belong to the CH group if one excludes all probable pairings. Most of them have been recovered from the blue-ice fields of Antarctica, or from the hot deserts of Africa and Asia, such as Acfer 241, NWA 470, or SaU 290 - rare CH members that get available to privat collectors from time to time. No witnessed CH chondrite fall has been recovered, thus far.


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