A single mass of 3,636 g was found by a German team in the Ténéré region of the Sahara Desert in north-central Niger, specifically at a location known as Grein. Provisionally named Te-1 (also associated with the name Grein 004), it was classified by J. Otto and A. Ruh (Universitat Freiburg) as a metal-rich, coarse-grained, primitive achondrite. The meteorite has a recrystallized texture with 120° triple-junctions. Olivine grains are mostly 0.10.4 mm in size, but larger grains occur. Large poikilitic pyroxene grains are present, as well as small agglomerates of crystals, sometimes called "Sammelkristalle", which usually form during melting and recrystallization processes. Unlike chondrules, these structures are composed primarily of plagioclase poikilitically enclosing minor olivines and pyroxenes, and are often accompanied by FeNi-metal. Te-1 is a freshly fallen meteorite with a weathering grade of W0, and it has a shock stage of S12.
*Previously, Floss (2000) and Patzer et al. (2003) proposed that the acapulcoite/lodranite meteorites should be divided based on metamorphic stage:
primitive acapulcoites: near-chondritic (Se >1213 ppm [degree of sulfide extraction])
typical acapulcoites: FeNiFeS melting and some loss of sulfide (Se ~512 ppm)
transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
enriched acapulcoites (addition of feldspar-rich melt component)
A similar distinction could be made among the winonaites in our collections, as well as among members of the newly proposed group ténéréites (Agee et al., 2020). One of the most "primitive" members identified in this new group is NWA 7317, which contains relict chondrules comparable to a petrologic type 6 chondrite. However, most ténéréites have experienced more extensive thermal metamorphism involving incipient melting and now exhibit highly recrystallized textures, characteristics analogous to the "typical" acapulcoites. Metamorphic progression in other ténéréites involved higher degrees of partial melting and even separation of a basaltic fraction (e.g., NWA 011 pairing group). Samples representing such an advanced metamorphic stage are known as lodranites in the acapulcoite/lodranite metamorphic sequence, while the term "evolved" could be used to represent a similar metamorphic stage in the ténéréite group.
This primitive achondrite has a chemical and mineral composition unlike that of any other meteorite. It has an O-isotopic composition distinct from any other achondrite group, plotting within the CR-field, and interestingly, very near to that of the ungrouped basaltic meteorite NWA 011. Oxygen isotopes are similar to those of the acapulcoitelodranite parent body but are not an exact match. The mineral composition and noble gas content of Te-1 are very similar to that of the brachinites and the brachinite-like meteorite Divnoe; moreover, the olivine and pyroxene compositions are nearly identical to those of Brachina. In addition, the composition of chromite and metal in Te-1 is indicative of a very close relationship with Divnoe. At the time of its classification, these varied characteristics were considered to be most consistent with the grouping of Te-1 as a brachinite-like primitive achondrite.
It was asserted by Agee et al. (2020) that the similarity in O, Cr, and Ti values among the CR2 carbonaceous chondrites and the many ungrouped equilibrated meteorites like Te-1 is coincidental, and that significant geochemical and other differences exist which make a common parent body untenable. They contend that the thermally metamorphosed CC meteorites represent a unique group for which they propose the name 'ténéréites' (see list and oxygen isotope diagram below).
Diagram credit: Agee et al., 51st LPSC, #2292 (2020)
'Northwest Africa 12869: Primitive Achondrite From the CR2 Parent Body or Memeber of a New Meteorite Group?'
Te-1 was reported to have a CRE age of ~45 m.y., although the CRE age ascertained for Tafassasset is ~76 m.y. See the Tafassasset page for updated information about this meteorite. The specimen shown above is a 1.72 g partial slice with fresh fusion crust on one end, acquired in 2001 from Norbert Classen. The top photo below shows the main mass of Te-1 with an end slice removed. At the bottom are two different lighting angles for a thick slab of Te-1, shown courtesy of Stephan Kambach.
click on photos for a magnified view
Photos courtesy of Stephan Kambach