NWA 1911

PyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. pallasiteOne of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, sometimes peridot (green gem quality clear olivine crystals), embedded in a matrix of Fe-Ni metal. The type specimen, weighing 680 kg, was found in the mountains near, ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung).

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% olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion and 34.5% orthopyroxene—the highest pyroxene content recorded for a pallasite. Minor troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. are also present, as well as trace merrillite.

In a study of NWA 1911 conducted by Boesenberg and Humayun (2019), they determined that the metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn 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).
Diagram credit: Boesenberg and Humayun, 50th LPSC, #1438 (2019) Interestingly, the pyroxene pallasite Zinder was previously found to contain metal with a composition that is chemically identical to that of group IIIF irons (Boesenberg et al., 2017; Humayun et al., 2018). However, the IIIF irons formed in the carbonaceous region of the Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. beyond Jupiter, whereas the negative ε54Cr and δ26Mg* values of Zinder indicate that it formed in the non-carbonaceous region of the inner Solar SystemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with (Wimpenny et al., 2019). See the Appendix Part III for further details about the two regions.

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% orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur, 0.4% clinopyroxene, and 0.4% augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite basalt.. 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.
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 acapulcoitesPrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated and lodranites, and both NWA 1911 and NWA 10019 plot on the eucriteMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after/mesosiderite fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. 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 seven separate parent bodies: 1) main-group; 2) Eagle Station group; 3) Milton; 4) Choteau + Vermillion + Y-8451; 5) Zinder + NWA 1911; 6) NWA 10019; 7) 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 chondritesMeteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich, and Choteau might be derived from the acapulcoite–lodraniteRare type of primitive achondrite named after the Lodran meteorite that fell in Pakistan in 1868. Initially, lodranites were grouped with the stony-iron meteorites because they contain silicates (olivine, orthopyroxene, and minor plagioclase) and Fe-Ni metal in nearly equal proportions. However, since discovery of the closely related acapulcoite group, lodranites parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today.. 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.