Pyroxene Pallasite
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 34.5% orthopyroxene and 40.2% olivine—the highest pyroxene content recorded for a pallasite. Minor troilite and chromite are also present.
Three other meteorites with a pallasite-like composition were studied previously: Vermillion, Y-8451, and Zinder. 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. Vermillion has recently been included in the IAB iron-meteorite complex by Wasson and Kallemeyn (2002). 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 are modally equivalent to ~1.6 vol% pyroxene (Boesenberg et al., 2000). The 46 g Zinder pallasite has a high modal abundance of pyroxene, more similar to that of NWA 1911, estimated to be 28 vol% (Wittke and Bunch, 2003).
All four pyroxene pallasites (when including Vermillion) are compositionally distinct from each other. The low-Ca pyroxene in Zinder and NWA 1911 is entirely composed of orthopyroxene, while that of Y-8451 comprises both orthopyroxene and clinopyroxene (Niekerk, 2005). Zinder contains a higher abundance of chromite compared to both Vermillion and Y-8451. The O-isotopic compositions of all four meteorites are distinct from each other—NWA 1911 plots in the field of the main-group pallasites, while Zinder plots on the TFL (Bunch et al., 2005).
From all of the data gathered so far, it can be reasonably argued that the pallasites in our collections represent at least four separate parent bodies: (1) main-group, (2) Eagle Station grouplet, (3) Zinder, and (4) Y-8451. In addition, the anomalous pallasites Milton, Springwater, and Glorieta Mountain could increase this number of parent bodies to seven (O-isotopic ratios for Milton and the Eagle Station grouplet plot within the CV field). Further information on the pyroxene pallasites can be found on the Vermillion page. The specimen shown above is a 6.47 g slice of NWA 1911.
