R5, rumurutiite
Purchased October 2002
coordinates not recorded
A very fresh 710 g stone with black fusion crust was found in Morocco or Algeria. It was subsequently purchased in Denver from a Moroccan dealer by D. Gregory. Analysis and classification was completed at the Department of Earth and Space Sciences, University of Washington (A. Irving and S. Kuehner), and this meteorite, named NWA 1668, was determined to be an R5 genomict breccia.
As is typical for R chondrites, chondrules are more sparsely distributed than in other chondrites. They have an average diameter of ~0.3 mm, which is larger than those in CO3 chondrites and smaller than those in ordinary chondrites (Imae and Zolensky, 2003), a size consistent with what is expected to occur at large heliocentric distances. With respect to mineralogy, NWA 1668 is primarily composed of olivine (Fa38.9), clinopyroxene, orthopyroxene, troilite, and Ti-chromite, along with minor sodic plagioclase, and rare, high-Ni metal (awaruite, composed of 72% Ni). The sparsity of metal, or lack thereof in some R chondrites, indicates that R chondrites experienced highly oxidizing conditions, probably both in the nebula and on the parent body. Parent body metamorphism in an oxidizing, water-rich environment is attested to by the hydroxyl-bearing minerals amphibole (magnesiohornblende) and phlogopite present in the R6 chondrite LAP 04840. This chondrite is considered to have experienced metamorphism of insoluble organic matter at high temperatures at a significant depth of tens of km, in a lithology in which water with a high D/H ratio was pervasive. This burial may have been the result of reassembly following impact disruption on the R chondrite parent body (McCanta et al., 2006, 2008).
Northwest Africa 1668 is one of the freshest R chondrites found to date, included with the W0/1 NWA 2897 and NWA 4545 (along with provisional NWA 5035 and a newly found 958 g beauty) in the weathering scale of Wlotzka, (1993); only the 1934 fall in Rumuruti is slightly better preserved. Recently, a more useful weathering index (wi) was developed by Rubin and Huber (2005) for those oxidized meteorite groups lacking significant FeNi-metal phases, such as the CK and R chondrite groups. This index is based on the modal abundance of brown-stained silicates as visually determined on a thin section in transmitted light at ~100× magnification. It is thought that the brown staining in R chondrites (and CK chondrites) is caused by the terrestrial decomposition and mobilization of sulfides (mainly pyrrhotite and pentlandite), which are typically prevalent in these meteorites; e.g., Rumuruti wi-0 contains 8.0 wt%. Further details about the R chondrite group can be found on the Dar al Gani 013 page. The photo above shows a 4.0 g partial slice of this rumurutiite.
