Numerous pieces of a carbonaceous chondrite, having a combined weight of 780 g, were found in Northwest Africa and subsequently sold in the Rissani meteorite market. Northwest Africa 2086 was analyzed and classified as an unshocked (S1 = <5 GPa) CV3 that exhibits only minor terrestrial weathering (W1).
In-depth analyses of NWA 2086 were conducted at the University of Szeged, Hungary (Fintor et al., 2013), and it was determined that this chondrite is a member of the CV3-reduced subgroup (now a distinct group termed CVred). Many petrographic features of NWA 2086 are consistent with those observed in reduced CV3 chondrites such as Vigarano, Efremovka, and Leoville; e.g., and close similarities were observed in both the chondrule types (primarily plagioclase-rich with porphyritic rims) and CAI types among these meteorites. However, the significant hydrothermal alteration observed in this meteorite is similar to that in the oxidized Allende-like chondrites. By comparison, the relatively high matrix:chondrule ratio of 52:48 for NWA 2086 is most similar to that of the CVoxB subgroup, although this could be due in part to a high degree of pre-accretionary comminution of the chondrules which constitute the NWA 2086 matrix (Kereszturi et al., 2014). A subgroup designation of CVoxB is used here for NWA 2086 in accord with Dunn et al. (2016). The 887 g CV3 NWA 10670 is probably paired with NWA 2086 (R. Bartoschewitz, MetBull 105).
Chondrules are present in a variety of types including poikilitic, granular, porphyritic, barred, and glassy, with about 20% of these having layered rims formed through accretionary processes and sometimes displaying subsequent aqueous alteration modification (Kereszturi et al., 2014). This alteration has similarly affected the matrix, and a progression of alteration is observed in a decreasing Si and Mg content from the chondrule interior outward into the matrix. Genomict clasts exhibiting higher thermal metamorphism have been identified in this meteorite (Zolensky et al., 2013). In contrast to other CVoxB chondrites like Bali and Mokoia, impact shock effects and other mechanical changes are not observed in the chondrules and rims of NWA 2086, which may attest to a source location at a greater depth (Kereszturi et al., 2014).
A new meteoritic mineral, dmisteinbergite (hexagonal CaAl2Si2O8), was identified by Raman spectroscopy within some CAIs in NWA 2086 coexisting with anorthite, and this was subsequently identified in CAIs in the oxidized CV3 chondrite Allende. Dmisteinbergite has been associated with high-temperature terrestrial environments in which rapid quenching occurred, but it also occurs as a secondary mineral formed through hydrothermal alteration processes (Fintor et al., 2014). Petrographical characteristics of meteoritic dmisteinbergite indicate that it is most likely a secondary phase formed under hydrothermal conditions.
Carbonaceous chondrites representing the two CV groups (CVox and CVred) contain the highest volume of refractory inclusions of all meteorite groups, many rich in calcium and aluminum (CAIs). It was determined through HfW systematics that they probably formed ~4.5676 b.y. ago by nebular condensation at temperatures high enough to vaporize Fe and Mg silicates. One theory places their formation early in nebular history when the heat source was the gravitational energy of the accreting stellar disk. Outward diffusion mechanisms allowed some CAIs to escape solar accretion and become stabilized in the outer zero-drag envelope of the newly formed Jovian gap. It has been argued that multiple episodes of recycling ensued, during which time thermal, chemical, mechanical, and redox processing occurred, eventually resulting in both CAIs and chondrules being incorporated into various carbonaceous assemblages (Ruzicka et al., 2008).
Further details on the formation of CAIs can be found on the Refractory Phases page, while details from numerous in-depth studies on the CV-group meteorites can be found on the Allende page. The CAI-rich specimen of NWA 2086 shown above is a 2.99 g partial slice.