A single fusion-crusted stone weighing 1,101 g was found in Kelb Ellouz, Morocco by the French team of Caillou Noir, under the organization of Michel Franco. A portion was sent for analysis to the Museum National d'Histoire Naturelle, France (M. Denise), the Universite Blaise Pascal, France (B. Devouard), and the Université des Sciences et de la Technologie Houari Boumediene, Algeria (M. Messaoudi).
Northwest Africa 4290 was determined to be a highly unequilibrated ordinary chondrite, and was published as the first known LL3.10 chondrite (NWA 1756 was previously classified as type LL3.0/3.2, but on-going studies have revealed that it is most consistent with type LL3.10). Northwest Africa 4290 has been shocked to stage S3 and has been weathered to grade W3. The main mass of NWA 4290 was subsequently purchased by collector S. Turecki, from which the specimen above was derived.
Northwest Africa 4290 is a primitive chondrite, among the very few to have escaped significant metamorphic processes on its parent asteroid. It preserves many primordial features such as the following: 1) a wide range of olivine fayalite compositions (Fa0.4Fa47), and ferrosilite compositions (Fs0.7Fs35); 2) a high chondrule/matrix ratio with 65% Type-II (FeO-rich, oxidized) chondrules; 3) well-defined chondrules (0.1 mm to 3 mm, with some up to 10 mm) having dark fine-grained rims; 4) chondrule mesostasis consists of isotropic glass; 5) whereas Cr is rapidly lost from chondrules as metamorphism increases, NWA 4290 contains an average of 0.38 (± 0.21) wt% Cr (in olivines from type-II chondrules).
Recently, a new petrologic scheme was proposed by J. Grossman (2004) and J. Grossman and A. Brearley (2005). It is more discriminating at the lowest petrologic types, those associated with the highly unequilibrated chondrites (3.03.2). This new classification scheme, based on a sensitive technique utilizing the variation in the distribution of Cr in ferroan olivine, is virtually unaffected by the processes of terrestrial weathering and aqueous alteration. The petrologic scale of the new decimal system has been extended as follows:
In subsequent studies of chromite zoning profiles along with the chromite content of individual ferroan olivine grains, Grossman (2008) was able to further resolve the petrologic type for chondrites at the lowest metamorphic stages. These two petrographic features provide a reference for a sequencial history of increasing thermal metamorphism that is consistent among olivine grains within each meteorite. For metamorphic types 3.003.03, chromite zoning profiles are smooth and correlate with igneous FeO zoning profiles. In addition, at this lowest metamorphic stage chromite contents account for 0.30.5 wt% in the chondrite groups studied. While chromite contents in type 3.053.10 chondrites still reflect the lowest degrees of metamorphism, chromite now exhibits igneous zoning profiles which are no longer smooth. Upon reaching a degree of metamorphism equivalent to type 3.15, chromite zoning has diminished considerably, and chromite abundance is now only 0.10.2 wt%. With metamorphic types of at least 3.2, no zoning is observed and chromite abundance is mostly less than 0.1 wt%.
Following the scheme of J. Grossman and A. Brearley (2005), only the LL chondrite Semarkona and the ungrouped (probably CO-related; Simon and Grossman, 2015) carbonaceous chondrite Acfer 094 (Kimura et al., 2006) were assigned to the least equilibrated subtype 3.00; however, Semarkona has more recently been determined to represent a petrologic subtype 3.01 (Kimura et al., 2008). This specific metamorphic type for Semarkona is also consistent with findings based on the FeNi-metal component, the features of which provide one of the most sensitive indicators for the onset of thermal metamorphism. The technique reveals that primary martensite decomposes to fine-grained plessite during very low degrees of thermal metamorphism in Semarkona, but which did not occurred in Acfer 094. Furthermore, they found that metal in and around Semarkona chondrules does not show a solar ratio of Co/Ni like that in Acfer 094, reflecting the greater degree of metamorphism that affected Semarkona. Moreover, low temperature aqueous alteration has occurred in Semarkona as attested by the presence of secondary alteration products such as smectite.
Another very sensitive method for determining low degrees of metamorphism employs Raman spectra to reveal the maturity of the matrix organic matter (Bonal et al., 2006). Results from this technique for NWA 4290 produce values between that of Semarkona (LL3.00) and Bishunpur (L/LL3.15), again consistent with a value of 3.10.
The CO-group meteorite ALHA77307 is consistent with a subtype 3.03, while three ordinary chondritesQUE 97008, MET 00526, and EET 90161have been assigned the next lowest petrologic subtype of 3.05. Several meteorites share the less rigorously defined 3.1 subtype. In addition, Kimura et al. (2008) included the carbonaceous chondrites of groups CR, CH, CB, and CM as probable 3.00 subtype specimens, notwithstanding their current designation of type 2 due to aqueous alteration features. In light of this petrologic typing paradox, they proposed that a separate scale be adopted to describe aqueous alteration which is distinct from the scale currently used for thermal metamorphism.
Further information on the various means of classification of the highly unequilibrated chondrites can be found on the NWA 1756) page. The photo above shows a 0.692 g very thin partial slice of NWA 4290.