Pyroxene Pallasite
Vermillion grouplet
standby for vermillion photo
Found May, 1991, recognized 1995
39° 44.18′ N., 96° 21.68′ W. A 34.36 kg mass was found by M. and G. Farrell while planting in a grain field in Marshall County, Kansas, in the vicinity of the Black Vermillion River. This was purchased by a dealer with the belief that it was a Brenham mass. Upon cutting, it was discovered to be unique from Brenham. Vermillion consists of ~86 vol% FeNi-metal and ~14 vol% silicates with grains much smaller than normal (Boesenberg et al., 1995). The silicates consist of olivine (~93 vol%), orthopyroxene (~5 vol%), chromite (~1.5 vol%), and merrillite (~0.5 vol%). Vermillion was compared to the pallasite Yamato 8451 (54.9 g) which has a very similar silicate composition consisting of olivine (~94 vol%), orthopyroxene (~4.8 vol%), clinopyroxene (~1.1 vol%), and merrillite (~0.1 vol%) reported by Yanai and Kojima (1995). Pyroxene accounts for ~0.7 and ~1.6 vol% of the silicate fraction of Vermillion and Y-8451 respectively, the remainder being mostly olivine. By comparison, the main-group pallasites contain all olivine with only trace amounts of pyroxene. The coexistence of both olivine and pyroxene in these two pallasites might indicate a lower crystallization temperature. In light of the compositional and isotopic similarities between Vermillion and Y-8451, Boesenberg et al. (1995) proposed they be recognized as a grouplet.

Subsequent studies have determined that Vermillion shares a similar pyroxene composition, mineralogy, O-isotope composition, and REE pattern not only with Yamato 8451, but also with the more recently discovered pyroxene pallasite Choteau (8,474 g), and it was proposed by Gregory et al. (2016) that these three pyroxene pallasites be recognized as members of a new pyroxene-pallasite grouplet termed ‘Vermillion pallasites’. standby for o-isotopic diagram
Diagram credit: Gregory et al., 47th LPSC, #2393 (2016) However, significant differences that exist between these three Vermillion pallasites are not yet resolved, including differences in texture (Y-8451 contains 4× the vol% of silicates as Vermillion) and siderophile trace element composition, as well as the presence of the carbide cohenite in Vermillion. In addition, although Vermillion, Y-8451, and Choteau all contain both low- and high-Ca pyroxenes of similar compositions, some differences are evident. Vermillion and Y-8451 contain both pyroxene types in the form of large grains, inclusions in olivine, and grains bordering olivine. However, in Choteau high-Ca pyroxene has only been identified as inclusions in olivine, while low-Ca pyroxene is present as both individual grains and along boundaries of high-Ca pyroxene grains (Gregory et al., 2016). Furthermore, plagioclase has not been found in either Vermillion or Y-8451, but is present in Choteau as inclusions in both low- and high-Ca pyroxene and as small veins in low-Ca pyroxene. Notably, plagioclase is also present in the ungrouped pyroxene pallasite NWA 10019, but it is compositionally distinct to that in Choteau.

The olivine fayalite composition of these pyroxene-bearing pallasites plots at the magnesian end of the main-group pallasite range. As a comparison, the Eagle Station pallasite group has the most ferroan composition, as well as a high Ge/Ga ratio in the metal and a unique O-isotope composition. The high Ir content in the Eagle Station pallasites suggests crystallization from the inner core region of its parent body, below the core–mantle interface in which the main-group and pyroxene-bearing meteorites probably formed on their respective parent bodies.

Siderophile trace element and oxygen isotopic compositions clearly resolve the pyroxene-bearing meteorites from the main-group and Eagle Station pallasites, and therefore they represent additional parent bodies on which pallasite-like textures were formed. In addition to the Vermillion trio, several other pyroxene-bearing meteorites have been found and studied. A 46 g pyroxene-rich pallasite named Zinder was found in Niger in 1999. It contains 28 vol% pyroxene and 27 vol% olivine in a network of FeNi-metal. In 2003, a 53 g pyroxene-rich pallasite designated NWA 1911 was purchased in northwest Africa. It has a modal composition of about 24% FeNi-metal and 75% silicates, with the silicates consisting of 34.5% orthopyroxene and 40.2% olivine. Separate fragments of the ungrouped pyroxene-bearing pallasite NWA 10019, weighing together 606 g, were found in 2015.

On a Ni vs. Au coupled diagram, Vermillion plots just outside of the low-Au end of the IAB main group irons, and also plots along an extention of the low-Au, medium-Ni (sLM) subgroup into lower Au compositions (Wasson and Kallemeyn, 2002). Furthermore, the O-isotopic composition of Vermillion is within the range of IAB irons, and therefore, Vermillion could be considered genetically related to members of the low-Au division of the IAB iron-meteorite complex.

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. A beautiful etched slice of Choteau is shown in ‘Meteorite Picture of the Day’ for June 20, 2013, courtesy of Ruben Garcia. The specimen of Vermillion pictured above is a 67.9 g partial slice. The photo below shows a close-up of the etched reverse side. standby for vermillion photo

Leave a Reply