Structures, origin and evolution of various carbon phases in the ureilite Northwest Africa 4742 compared with laboratory-shocked graphite

Abstract

International audienceMineralogical structures of carbon phases within the ureilite North West Africa 4742, a recent find, are investigated at various scales by high-resolution transmission electron microscopy (HRTEM), Raman microspectrometry and X-ray diffraction. Ureilites are the most carbon-rich of all meteorites, containing up to 6 wt.% carbon. Diamond, graphite and so-called "amorphous carbon" are typically described, but their crystallographic relationships and respective thermal histories remain poorly constrained. We especially focus on the origin of "amorphous carbon" and graphite, as well as their relationship with diamond. Two aliquots of carbon-bearing material were extracted: the insoluble organic matter (IOM) and the diamond fraction. We also compare the observed structures with those of laboratory-shocked graphite. Polycrystalline diamond aggregates with mean coherent domains of about 40 nm are reported for the first time in a ureilite and TEM demonstrates that all carbon phases are crystallographically related at the nanometre scale. Shock features show that diamond is produced from graphite through a martensitic transition. This observation demonstrates that graphite was present when the shock occurred and is consequently a precursor of diamond. The structure of what is commonly described as the "amorphous carbon" has been identified. It is not completely amorphous but only disordered and consists of nanometre-sized polyaromatic units surrounding the diamond. Comparison with laboratory-shocked graphite, partially transformed into diamond, indicates that the disordered carbon could be the product of diamond post-shock annealing. As diamond is the carrier of noble gases, whereas graphite is noble gas free, graphite cannot be the sole diamond precursor. This implies a multiple-stage history. A first generation of diamond could have been synthesized from a noble gas rich precursor or environment by either a shock or a condensation process. Thermally-induced graphitization of chondritic-like organic matter could have produced the graphite, which was then transformed by shock processes into polycrystalline nanodiamond aggregates. The formation of the disordered carbon occurred by diamond post-shock back-transformation during post-shock heating. The noble gases in the first generation diamond could then be incorporated directly into the disordered carbon during the transformation

Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material

“…Its formation was attributed to shock-induced transformation of graphite within the meteorite upon impact with Earth, and its occurrence was used as an indicator of shock [1][2][3] . It has since been reported from several meteorites as well as from terrestrial sediments and has been attributed to asteroidal impacts, both extraterrestrial and on Earth [4][5][6][7] .…”

“…1a,b) or match those of graphite, but well-resolved X-ray reflections for lonsdaleite have not been reported. Selectedarea electron diffraction (SAED), Raman, electron energyloss spectroscopy (EELS) and high-resolution transmission electron microscopy have also been used for identification of lonsdaleite [4][5][6][7]14,15,18,23,24 ; however, interpretation of data is ambiguous (Supplementary Note 1). In spite of the many diffraction and spectroscopic studies, unambiguous data that prove the existence of lonsdaleite as a distinct material have not been reported.…”

“…These patterns are consistent with diamond projected along o0114 or o1214, respectively, although the streaking and hexagonally arranged 111 reflections are incompatible with defect-free single-crystal cubic diamond. Such data have been interpreted as evidence for 'lonsdaleite' projected along o0104 and o0014 [4][5][6][7]14,15,20 . The circularly integrated intensity profiles of these SAED patterns ( Supplementary Fig.…”

Diamond-like-carbon nanoparticle production and agglomeration following UV multi-photon excitation of static naphthalene/helium gas mixtures

“…The work presented here may be of significant interest for astro-chemists concerning the UV photo-excitation of polycyclic aromatic hydrocarbons (which are believed to be abundant in the interstellar medium 58,59 ), and the possible pre-solar formation of NDs observed in meteorites develops through a chemical pathway. 60 Although the density and temperature in the experiments outlined here differ substantially from those in the interstellar medium, the results suggest that NDs can indeed be formed in the gas phase through a chemical pathway.…”

Organic and volatile elements in the solar system

“…Carbon can also occur as diamond, either presolar or solar. Nevertheless, it requires high pressure to be formed on a meteorite parent body and only shocked objects exhibit diamonds formed from organic matter that was initially accreted on the parent body (see the example of a shock chondrite in [97]). In primitive chondrites, presolar nanodiamonds, likely formed by CVD condensation, have been detected [96].…”