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 by the hydroxyl-rich minerals amphibole, phlogopite, and apatite present in the R6 chondrites LAP 04840 and MIL 11207. It is considered that these chondrites experienced metamorphism of insoluble organic matter at high temperatures (~720°C) and at significant depth (tens of km) within a lithologic unit in which water with a high D/H ratio was pervasive. This deep burial is considered to be the result of reassembly following impact disruption on the R chondrite parent body (McCanta et al., 2006, 2008). In a study of these two hydroxyl-bearing R chondrites, Gross et al. (2017) reasoned that such high abundances of hydrous phases could only be established under conditions of high water vapor pressure (20700 bar), and such pressures could only be maintained on this relatively small body through the emplacement of a solid ice shield.
Noble gas analyses of the known Northwest Africa R chondrites were conducted by Vogel et al. (2014). Based on the results, they have tentatively placed these numerous R chondrites into ~16 groupings representing possible common source craters and/or fall events. They have proposed a pairing exists between NWA 1668 and NWA 2897 which show a common CRE age of ~66 m.y.
Northwest Africa 1668 is one of the freshest R chondrites found to date, next to the 1934 fall in Rumuruti and the 23.6 kg NWA 7514. 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 the R chondrite group as a result of formation in a high-sulfur fugacity; e.g., Rumuruti wi-0 contains 8.0 wt%.
In addition to NWA 1668 and Sah 99527 (both R5), several other unbrecciated R chondrites have been recovered so far, including the following: HaH 119, Ouzina, NWA 053, NWA 800, and Sah 98248 (all R4); NWA 753 (R3.9); Carlisle Lakes and NWA 978 (R3.8); NWA 755 (R3.7); and Acfer 217 (R3.85).
Further details about the R chondrite group can be found on the DaG 013 page. The photo above shows a 4.0 g partial slice of this rumurutiite.