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Mars Meteorite / Orthopyroxenite
General: This subgroup consists of just one single member, Allan Hills 84001, a meteorite that was recovered from the ice fields of Antarctica in late 1984. It was initially misclassified as a diogenite, but further research subsequently showed it to be related to the meteorites of the SNC group, and to be a genuine, but strange type of martian rock.
Description: Allan Hills 84001 is a grey-greenish rock, exhibiting a coarse-grained texture, and a dull black fusion crust. Since it is the only martian orthopyroxenite known, it can't be said if it's a typical member of its group, or if other members actually might look different.
Mineralogy: ALH 84001 is a cumulate rock consisting of 97% Mg-rich orthopyroxene, and minor plagioclase (maskelynite), chromite, and carbonate. The presence of oxidized iron in the chromite of ALH 84001 led to its reclassification as a martian meteorite; a fact that has been subsequently confirmed by its oxygen isotope composition.
Formation history: With a crystallization age of 4.4 billion years, ALH 84001 is by far the oldest Mars meteorite. It most likely represents a sample of the early martian crust, providing evidence for the earliest geologic history of Mars. However, public attention has been focused on a minor aspect of this unique rock during the past years - the presence of small orange-colored carbonate spherules that formed 3.9 billion years ago. These "orangettes" are barely visible to the naked eye, ranging up to 200 microns in size, and they seem to have formed in the presence of liquid water within fractures inside ALH 84001. In 1996, McKay and his co-workers published a paper announcing the discovery of traces of fossil martian life within the orangettes, consisting of organic molecules, biominerals, and microfossils that resemble terrestrial nanobacteria. Ever since, his discovery has been vigorously debated, splitting the scientific community into advocates and prosecutors of the existence of (former) primitive life on Mars.
Origin: Planetary. Comparisons between various characteristics of the members of the SNC group, and data obtained about Mars by space probes and landers, such as Viking, Pathfinder, and the new Mars rovers Spirit and Opportinity, have provided strong proof for the martian origin of the SNCs, and today it is widely accepted that these achondrites actually represent genuine Mars rocks that have been blasted off of the surface of the Red Planet by major impacts. Although ALH 84001 is older than all other Mars meteorites known, its mineral, and oxygen-isotopic compositions clearly prove its martian origin beyond any doubt.
Members: Up to this day, Allan Hills 84001 remains the only martian orthopyroxenite in our collections. Since it's an Antarctic find, all of it is locked away in institutions, and none is available for the private sector. We can only hope that among the multitude of finds from the hot deserts of Africa, and Arabia, a second martian orthopyroxenite will be recovered, and indentified, allowing a comparison with the data obtained from ALH 84001, and samples to be distributed to institutions, museums, and private collections.