Occurrence of Carbon Onions and Nanoparticles

The carbon onions are an artificially generated modification of carbon. For the time being, no terrestrial occurrence is known. Just some highly defective structures similar to carbon onions have been observed in classical soot (Section 1.2.3), yet these do not normally have a closed shell and consequently cannot be considered carbon onions in the true sense of the meaning. 

In the cosmos, however, carbon onions possibly exist. Soon after their discovery they have already been discussed as a potential reason for an up to then not interpretable absorption at 217.5 nm in the spectrum of interstellar space. The absorption spectrum of carbon onions closely resembles that of interstellar dust indeed. A red shift is observed on the occasion, yet this may be explained by the measurements being made in different media (water or vacuum, respectively Section 4.4.1.3). 

The generation of carbon onions in space has not yet been fuUy elucidated. However, it seems reasonable to assume that they originate from nanoscopic diamond particles. These may be converted into carbon onions upon heating, electron bombardment, or intensive irradiation (Section 4.3.5.4). The existence of nanodiamonds in extraterrestrial material could be confirmed by analyses on different meteorites. Especially the AUende meteorite contains significant amounts of tiny diamond particles (Section 5.1.2). 

Actually, the first signs of carbon onions existing in such objects have been found in this very meteorite. Electron microscopic pictures (Eigure 4.8) show 

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