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 is generally considered that diamonds found in ureilites, as well as those found in iron meteorites, were formed by impact shock pressures, or possibly through chemical vapor deposition 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.
In the least-shocked, diamond-free ureilite, ALH 78019, the absence of primordial noble gases in graphite, along with a heavy N-isotopic signature in graphite, was found to be inconsistent with the theory that graphite was a precursor to nanodiamond formed by 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 which 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.
Large sub-millimeter-sized diamonds of solar origin are found in 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 different types of diamonds are found in the meteorite Acfer 214—nanometer-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.
