Pyroxene pallasite, ungrouped
standby for nwa 1911 photo
Purchased March 2003
no coordinates recorded

A fresh (W1), complete stone, weighing 53.07 g, was retrieved by M. Farmer from a batch of meteorites shipped to him from Rissani, Morocco; this is the first pallasite recognized to be found in Northwest Africa. Northwest Africa 1911 was analyzed and classified at Northern Arizona University (Wittke and Bunch, 2003), and was found to have a modal composition of 24.3% FeNi-metal and 75% silicates, with the silicates consisting of 40.2% olivine and 34.5% orthopyroxene—the highest pyroxene content recorded for a pallasite. Minor troilite and chromite are also present, as well as trace merrillite.

In a study of NWA 1911 conducted by Boesenberg and Humayun (2019), they determined that the metal composition was extremely similar to that of the Zinder pyroxene pallasite. Utilizing a coupled Fe/(Fe+Mg) vs. Al/(Cr+Al) diagram in an analysis of chromite for various pallasites, they demonstrated that chromite in both NWA 1911 and Zinder contains a relatively high Al content and plots in a common unique compositional space (see diagram below).

standby for chromite fe vs. al diagram
Diagram credit: Boesenberg and Humayun, 50th LPSC, #1438 (2019)

It is noteworthy that the pyroxene pallasite Zinder, and by association NWA 1911, were previously found to contain metal with a composition that is chemically identical to that of group IIIF irons, particularly Cerro del Inca, Moonbi, and St. Genevieve County (Boesenberg et al., 2017; Humayun et al., 2018; Zhang et al., 2021 #1640). However, considerable data indicates that the IIIF irons formed in the carbonaceous region beyond Jupiter, whereas the negative ε54Cr and δ26Mg* (‰) values (Wimpenny et al., 2019, Fig. 8b) as well as the O-isotopic composition of Zinder indicate it was formed in the non-carbonaceous region within the inner Solar System. See the Protoplanetary Disk page for further details about the NC and CC regions. Interestingly, Rubin (2018, Fig. 1) has shown that group IIIF irons are poorly resolved from the NC-iron groups with respect to Ni and Ir contents. Continued investigations will help resolve whether or not a genetic relationship exists between the IIIF irons and the Zinder and NWA 1911 pallasites.

To date, seven pyroxene-bearing meteorites having a pallasite-like composition have been characterized: the 'Vermillion pallasite grouplet' (Choteau, Vermillion, and Y-8451), Zinder, NWA 1911, NWA 10019, and LoV 263. Vermillion is composed of 86 vol% FeNi-metal and 14 vol% silicates, with the silicates consisting of 93% olivine and 5% pyroxene (4.9% opx and 0.1% cpx)—equivalent to a modal composition of ~0.7 vol% pyroxene. Wasson and Kallemeyn (2002) recognized that Vermillion might be related to the IAB complex iron meteorites. The 54.8 g Y-8451 pallasite contains 57 vol% silicates consisting of 97% olivine, 2% orthopyroxene, 0.4% clinopyroxene, and 0.4% augite. The silicates in Y-8451 are modally equivalent to ~1.6 vol% pyroxene (Boesenberg et al., 2000). The 46 g Zinder pallasite has a high modal abundance of pyroxene, similar to that in NWA 1911, estimated to be 28 vol% (Wittke and Bunch, 2003). The modal abundance of silicates in NWA 10019 is ~60%, comprised of olivine (~43–51 vol%) and orthopyroxene (~9–17 vol%) with pyroxene accounting for ~1–5 vol% of this pallasite (Boesenberg et al., 2016). The silicates in the 4.88 kg LoV 263 pallasite are comprised of approximately equal proportions of olivine and orthopyroxene.

standby for o-isotopic diagram
Diagram credit: Gregory et al., 47th LPSC, #2393 (2016)

In a study conducted by Gregory et al. (2016), it was ascertained that Choteau is compositionally and isotopically similar to both Vermillion and Y-8451, and it was concluded that these three pyroxene pallasites form a grouplet; they suggested that these meteorites should be termed 'Vermillion pallasites' (see the Vermillion page for additional details). The low-Ca pyroxene in Zinder, NWA 1911, NWA 10019, and LoV 263 is composed entirely of orthopyroxene (orthopyroxene in NWA 10019 contains ~100µm-sized clinopyroxene inclusions; Boesenberg et al., 2016), while that in the Vermillion pallasites comprises both orthopyroxene and clinopyroxene (Niekerk, 2005; Irving and Kuehner, 2013). Zinder contains a higher abundance of chromite compared to the Vermillion pallasites. The O-isotopic compositions of the Vermillion pallasites are distinct from the other four pyroxene-bearing pallasites, and many are associated with a number of established O-isotopic trends: the Vermillion pallasites plot near the field of acapulcoites and lodranites, and both NWA 1911 and NWA 10019 plot on the eucrite/mesosiderite fractionation line, which remains incompletely resolved from the bimodal fractionation trend of the main-group pallasites (Ziegler and Young, 2011; K. Ziegler, 2015). Although Zinder has been demonstrated to be associated with NWA 1911 (Boesenberg and Humayun, 2019), it plots on the terrestrial fractionation line due to a difference in δ17O values; however, terrestrial weathering may be the reason for this difference.

Based on all of the data gathered so far, it could be concluded that the pallasites in our collections represent at least eight separate parent bodies: 1) main-group high-Δ17O; 2) main-group low-Δ17O; 3) Eagle Station group; 4) Milton; 5) Choteau + Vermillion + Y-8451; 6) Zinder + NWA 1911; 7) NWA 10019; 8) LoV 263. In addition, several pallasites with anomalous silicates (e.g., Springwater) and anomalous metal (e.g., Glorieta Mountain) could possibly increase the number of unique parent bodies. Notably, the O-isotopic ratios for both Milton and the Eagle Station group pallasites plot on an extension of the trend line for the CV chondrites, and Choteau might be derived from the acapulcoite–lodranite parent body. Further information on the pyroxene pallasites can be found on the Vermillion page. The specimen of NWA 1911 shown above is a 6.47 g slice.