Nakhlites
Synonyms: augite-olivine achondrites, diopside-olivine achondrites
General: The members of this group are named for Nakhla, a witnessed fall that occurred in El Nakhla, Egypt, in 1911. The story of one of the Nakhla stones having allegedly killed a dog is wide spread, but recent research has shown that this story probably isn't true but just another urban legend. The more interesting part of the Nakhla story is that scientific research has shown it to be a genuine rock from our red neighbor, the planet Mars.
Description: All members of this group are of a very similar appearance, and homogenous composition. Nakhlites are diopside-green rocks, displaying a sugary, fine-grained texture, and a glossy black fusion crust.
Mineralogy: Nakhlites are Ca-rich clinopyroxenites, consisting of green cumulate augite crystals with minor olivine in a very fine-grained mesostasis. This mesostasis is composed of plagioclase, alkali feldspar, pyroxenes, iron-titanium oxides, sulfides and phosphates. Traces of pre-terrestrial aqueous alteration products are present in the form of hydrated minerals, e.g. amphibole, clay minerals similar to iddingsite or smectite, and carbonate and sulfate salts.
Formation history: All Nakhlites crystallized about 1.3 to 1.4 billion years ago, and two competing scenarios have been proposed to explain their very recent formation. According to an older theory, the nakhlites are cumulates that formed in magma plutons deep inside the martian crust. A more recent theory places their origin in a lava flow near the martian surface, similar to Theo's flow, an extrusive Archean lava flow in Canada that formed pyroxenites similar to the nakhlites. However, subsequent aqueous alteration of the resulting rocks took place much later as indicated by the formation ages of the clay minerals in nakhlites; these show ages of just about 700 million years. Perhaps in the not-too-distant past Mars wasn't the dry and hostile place that we know today.
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. Recent studies, based on data provided by the Mars Odyssey Orbiter, and THEMIS (Thermal Emission Imaging System), suggest that both, the nakhlites, and the chassignites, were ejected from a large impact crater in the northeast region of the Syrtis Major volcanoes.
Members: For decades, only 3 nakhlites were known - Nakhla, Lafayette, and Governador Valadares. For some time, the latter two finds were even suspected of being dislocated or mislabelled individuals of Nakhla due to their virtually identical mineralogical and textural features. However, detailed studies showed them to belong to different falls, though probaly source crater paired, i.e., blasted off of Mars by one and the same impact event. During the last few years, four new nakhlites were recovered - two from the ice fields of Antarctica, and two from the hot deserts of Northwest Africa, the unusual mesostasis-rich NWA 817, and the orthopyroxene-bearing NWA 998 - raising the number of all known nakhlites to 7 separate falls.