A single 40.03 g ureilite meteorite was found in the Libyan Sahara Desert. Like most ureilites, DaG 868 is composed of grains of olivine (82 vol%) and pigeonite (11 vol%) along with carbonaceous material forming rims and veins. The small amount of metal present has been extremely weathered. The olivine in DaG 868 has a high CaO content and a high fayalite value of 20.6, which places it in Berkley's subgroup I and Goodrich's subgroup 1.
In contrast to most other ureilites, DaG 868 contains unshocked olivine without undulose extinction, but still contains sub-millimeter-sized diamonds in the graphite that occur within pigeonite crystals. These diamonds have a solar signature inferred by their C and N isotope compositions. It has been generally considered that diamonds found in ureilites, as well as those found in iron meteorites, were formed by impact-shock pressures and/or through chemical vapor deposition (CVD) processes. While DaG 868 has forced a reconsideration of diamond origins, a new mechanism, catalytic transformation of graphite to diamond, is under consideration to account for the production of diamonds in this ureilite. Under conditions of relatively low pressure and high temperature, certain molten metals can serve as solvent catalysts leading to diamond formation. An alternative scenario ascribing diamond formation to CVD processes has been propounded by Langendam and Tomkins (2012). They envision a smelting mechanism involving methane to explain observed smelting within fractures, as well as the finding of discontinuous smelting at grain boundaries. Still, the predominance of crystalline graphite accompanying diamond is more consistent with shock being the major driver of diamond synthesis in ureilites (Ross et al., 2011).
Conversely, the least-shocked, diamond-free ureilite, ALH 78019, lacks primordial noble gases in the graphite component, and contains a heavy N-isotopic signature in the graphite, observations which are inconsistent with graphite as a precursor to the nanodiamond which was subsequently formed through in situ shock conversion processes (Rai, et al., 2002). Utilizing the ureilite NWA 4742, Guillou et al (2009) studied this paradox in which graphite precursor material is depleted in noble gases, while the nanodiamonds into which it was transformed are noble gas-rich. Their investigation led to a proposal that a mixture of two diamond populations is present; i.e., an early population of unknown origin that contains noble gases, and a later population that was formed by shocked graphite depleted in noble gases. They further suggest that the presence of a noble gas-containing graphitic phase surrounding some nanodiamonds could be the result of back-transformation of the early population of diamonds under conditions of slow cooling following a late shock event.
To differentiate between the two competing scenarios for diamond formation on the ureilite parent body, i.e., impact shock vs. chemical vapor deposition (CVD), Nagashima et al. (2012) utilized micro-Raman spectroscopy to study of carbonaceous material in a number of ureilite samples. The resulting spectral data obtained for the major parameters for diamond (peak position, band intensities, and full width at half maximum [FWHM]) were a better match to diamond produced under CVD rather than shock pressure. Moreover, they demonstrated that there was no correlation of the diamond:graphite ratio to the shock level, and found the noble gas and N-isotopic compositions of graphite, amorphous carbon, and diamond to be in accordance with the CVD model, but not with the shock model. Their results suggest a scenario of chemical deposition of graphite, amorphous carbon, and diamond directly onto high-temperature condensates in the primitive solar nebula, with the formation of each phase being associated with specific variations in CH4:H2 ratios commensurate with temperature and pressure changes. The migration of carbonaceous material to silicate grain contacts, as well as the occurrence of compressed graphite in conjunction with diamond, was the result of later shock events on the ureilite parent body.
Large sub-millimeter-sized diamonds of solar origin are also found in some unshocked meteorites, such as the enstatite chondrite Abee. By contrast, diamonds present in primitive chondrites are nanometer-sized, and contain anomalous C and N, reflecting a circumstellar origin. Remarkably, two types of diamonds are found in the meteorite Acfer 214nanometer-sized diamonds similar to those found in primitive chondrites, and larger micrometer-sized diamonds with unique isotopic characteristics, combustion temperatures, and C/N ratios.
A synopsis of current models for ureilite formation is presented on the Kenna page. The specimen of DaG 868 pictured above is a 0.39 g partial slice with crust.