NORTHWEST AFRICA 1222


EL5
standby for northwest africa 1222 photo
purchased 1999
no coordinates recorded

A single stone weighing 2,800 g was found in the Moroccan Desert. This meteorite was analyzed by T. Mikouchi and K. Kentaro at the University of Tokyo, Japan, and determined to be a rare EL5 chondrite, one of only three found to date. This meteorite has been heavily weathered to grade W3, but this is most obvious very near the crust. Enstatite chondrites were formed in a highly reducing environment and all contain abundant metallic FeNi.

In contrast to the EL6 meteorites, all very weakly shocked to S2, NWA 1222 is more strongly shocked to stage S3 (corresponding to a peak pressure of 10–20 GPa). The mineral sinoite (silicon oxynitride) has been found to occur in many EL chondrites, associated with crystallization of an impact melt. Presumably, these initial intense shock effects were erased during an annealing phase, but sometime later, a minor shock event left its characteristic signature in the rock.

The EL5 subgroup presently comprises only a few confirmed members, including Tanezrouft 031 (28 g), TIL 91714 (163.9 g), and NWA 1222 (2,800 g). Another E chondrite of petrologic type 5, the ambiguously grouped RKPA80259 (20.2 g), was initially included in the EL group based on its low siderophile element content. However, further studies determined that it has certain refractory element ratios more consistent with the EH group, and that its low siderophile content was probably the result of terrestrial weathering. Additional support for inclusion into the EH group comes from the presence of niningerite [(Mg,Fe)S] or keilite [(Fe,Mg)S] rather than alabandite [(Mn,Fe)S], a Si content in kamacite of 2.1 wt% (EH: 1.9–3.8 wt%; EL: 0.3–2.1 wt%), and a small chondrule size. Paradoxically, RKP A80259 displays the characteristic cathodoluminescence of enstatite found in the EL group.

The specimen of NWA 1222 pictured above is a 5.35 g polished partial slice, while the photo below shows the cut face of the main mass. Although not obvious from the angle of the lighting in these pictures, FeNi-metal is pervasive throughout the rock.

standby for northwest africa 1222 photo
Photo courtesy of R. A. Langheinrich Meteorites