A single small stone weighing 57.1 g was probably found near Boudnib, Morocco, and was subsequently purchased in Erfoud. Analysis is ongoing at the Muséum National d'Histoire Naturelle in Paris. The analyses thus far have determined that NWA 1763 is a carbonaceous chondrite of the CV group with a petrologic type of 3.5 (Fa[mol%]: 0.27–34.91; Fs[mol%]: 0.69–2.34). It is both unshocked (S0) and unweathered (W0), with an unoxdized matrix and an abundance of intact sulfide. The pristine condition of this CV3.5 chondrite should prove helpful in studies of this carbonaceous group.
Northwest Africa 1763, along with most carbonaceous chondrites, contains many refractory inclusions of calcium and aluminum (CAIs) that were probably formed by condensation at temperatures high enough to vaporize the 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, newly formed Jovian gap. It was further proposed that surface flares from Jupiter then formed chondrules, and both components were finally incorporated into a carbonaceous assemblage. More recently, theories have attributed the formation of chondrules to shock waves caused by gravitational instabilities.
Yet, another theory of CAI origin places the formation at a later period, when the accretion phase was over and the Sun was in its T Tauri phase. The solar wind swept the volatile-rich gas from the outer layers of the nebular disk leaving only refractory-rich dust behind. Shock wave heating then evaporated the Fe and Mg silicates, leaving the dust enriched in Al. This dust finally coalesced, and was melted to form the CAIs that are present in most carbonaceous meteorites. More recently, studies have shown that CAIs were accreted rapidly into larger bodies, heated by the decay of 26Al, and thermally metamorphosed. Thereafter, these bodies were disrupted, and the CAIs were returned to the nebula to be remixed and recycled into later carbonaceous chondrite bodies. Layered rims surrounding some CAIs, called Wark–Lovering rims, or accretionary rims, were possibly formed by a flash heating event measured in fractions of a second, resulting in a loss of volatiles, enrichment of the refractory component, and the subsequent diffusion of O and Mg. During the flash heating event, temperatures at the rim are inferred to have approached 3000°C, steeply decreasing to temperatures of ~1700°C just 1 mm below the rim. Subsequent chemical and isotopic exchange, corresponding to the grain size and porosity of specific minerals, most likely occurred in situ on the parent body.
The CV3 group was subdivided into three subgroups (McSween, 1977; Weisberg et al., 1997):
Reduced subgroup including Arch, Efremovka, Leoville, Vigarano, and QUE93429.
Oxidized-Allende subgroup including Allende, Axtell, Tibooburra, and ALH84028.
Oxidized-Bali subgroup including Bali, Grosnaja, Kaba, and Mokoia.
The oxidized and reduced subgroups are separated on the basis of metal abundance and Ni content of sulfide (Howard et al., 2010). The previously used discriminator, magnetite abundance, has been shown to overlap between oxidized and reduced subgroups. The oxidized-Bali subgroup has a higher degree of aqueous alteration than oxidized-Allende (for more mineralogical relationships, see Appendix I, Carbonaceous Chondrites). The subgroup to which NWA 1763 belongs has not yet been published, but it likely belongs to the Oxidized-Allende subgroup. The specimen pictured above is a 3.35 g slice of NWA 1763 (some saw marks are evident in the photo). A photo of the complete, fresh stone as found is shown below.
