NORTHWEST AFRICA 1617


Acapulcoite
Acapulcoite–Lodranite Clan
standby for northwest africa 1617 photo
Purchased June 2002
no coordinates recorded

A small 23 g weathered stone was purchased in Agadir, Morocco by Moroccan dealer A. Habibi on behalf of American collector N. Oakes. A portion of the meteorite was submitted for classification to the University of Washington in Seattle (A. Irving and S. Kuehner), and it was initially classified as a winonaite (MetBull 88); however, this classification was eventually changed. Northwest Africa 1617 is composed primarily of magnesian bronzite and forsteritic olivine, along with minor FeNi-metal, troilite, chromite, and sodic plagioclase.

The classification as a winonaite was based on those components in NWA 1617 which represent a high degree of oxidation, such as is manifest in the lack of daubréelite and the occurrence of very Cr-rich chromite. More oxidizing conditions are consistent with the later-crystallizing, silicate-dominated inclusions of the high-Ni IAB irons, whereas more reducing conditions are expected to prevail in the graphite–troilite inclusions that predominate among the later crystallizing, low-Ni IAB irons. Also significant is the fact that the Ni content of metal in these irons was found to be positively correlated with the contents of fayalite and daubréelite (Benedix et al., 1995), consistent with what is found in NWA 1617. The plagioclase composition of NWA 1617 (Ab81) is almost identical to that found in the NWA 1457 winonaite. Likewise, the O-isotopic composition of NWA 1617 plots close to the winonaite group. Therefore, it was proposed that NWA 1617 was an unusual winonaite that extends the compositional limits of the winonaite/IAB iron group.

Notably, in the initial analysis it was found that the silicates in NWA 1617 are more FeO-rich (Fa11.6, Fs11.2) than those in typical winonaites (the most FeO-rich winonaite previously studied is Fa5.3, although silicate inclusions in the IAB-related iron Udei Station are compositionally close [Fa8, Fs9]). In fact, the values are more consistent with those of acapulcoites. In 2005, a method to distinguish acapulcoites from winonaites was devised by D. Rumble, III et al. (2005). They utilized a diagram comparing the Fa mol% of olivine vs. the Δ17O‰, and on this diagram NWA 1617 (Fa11.6; Δ17O = –0.86 ±0.02‰) plots well within the acapulcoite field close to Monument Draw. Northwest Africa 1617 has now been reclassified as an acapulcoite (MetBull 90). The average grain size of NWA 1617 is 350 µm, which distinguishes it from the more slowly cooled lodranite component of this parent body which are distinguished by grain sizes larger than 500 µm. However, with many new members it is now evident that a continuum exists for the grainsizes of these two groups, and it has been proposed by Bunch et al. (2011) that an arbitrary group division is no longer justified; the term ‘acapulcoite–lodranite clan’ should therefore be applied to all members of the combined group.

For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The specimen of NWA 1617 shown above is a 0.53 g partial slice. The photo below shows a cut face of the main mass of this small stone.

standby for northwest africa 1617 photo
Photo courtesy of N. Oakes—Meteorites–R–Us