Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..416o&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 416
Astronomy and Astrophysics
Astrophysics
6
Ci Chondrites, Diamonds, Interstellar Dust, Supernova, Xenon
Scientific paper
Microdiamonds in primitive chondrites are characterized by Xe-HL, which supposedly formed in a type II supernova. Several models have been proposed for the origin of the microdiamonds. These include chemical vapor deposition (CVD) [e.g., 1], interstellar shock [2], and UV-annealing of small graphite particles [3]. However, it is difficult for any of these models to explain the unique association of Xe-HL with the microdiamonds. We have suggested that a diamond formation process, proposed by Kaminsky [4], for the origin of a particular terrestrial diamond, carbonado, may apply to the microdiamonds in primitive meteorites [5,6]: Kaminsky speculated that carbonado was formed from natural coal that was enriched in uranium and hence subjected to irradiation by high-energy particles produced from the uranium and thorium. The paper in this volume by Mochizuki et al. [7] reports nanometer-sized diamondlike clusters in a uranium-rich natural coal, in accordance with Kaminsky's hypothesis. Mochizuki et al. also report the possibility of the production of nanodiamonds in graphite that was irradiated with a 50-KeV argon beam. These experimental studies strongly suggest that microdiamonds can be produced by irradiation of carbonaceous matters with energetic particles. On the basis of these experimental results, we propose a scenario for the origin of the microdiamonds in primitive chondrites. The scenario gives a reasonable explanation for the unique association of Xe-HL with the microdiamonds as well as for their formation in a supernova envelope. We assume that carbonaceous materials (amorphous carbon, graphite, and hydrocarbon grains) in the outer envelope of a supernova was irradiated by energetic particles (including Xe-HL) emitted during supernova explosion. The energetic particles then interacted with the carbonaceous matter: Most of the energy was dissipated through electronic interaction, and at the end of the journey the particles produced cascade displacement of target atoms. Suppose that an xenon atom impinged into an amorphous carbon particle of 25 nm, which is a typical grain size for interstellar amorphous carbon [8]. In order for the xenon atom to stop within the target particle, the maximum energy would be less than 0.01 MeV. For an impinging energy E(sub)O, the number (n) of cascade-displacement atoms in the target is given by the simple relation [9] n = E(sub)O/E(sub)D, which gives n ~ 250 atoms for E(sub)O = 0.01 MeV. The disturbed region, if recrystallized to have formed a cubic-shaped diamond, would be about 1 nm in size. We can also show from a simple energetic consideration that diamond would be a more stable phase than graphite at room temperature for a size smaller than a few nanometers. The latter estimation is made by comparing the excess pressure induced inside a small particle with the surface tension above which diamond phase becomes stable. The maximum size thus estimated is about a few nanometers. A similar conclusion was drawn by Nuth [10], who used a different approach. Hence, if the displaced region were to recrystallize, the region would form diamond. It is easy to see that the process necessarily leads to the association of Xe-HL with microdiamonds formed in this manner. References: [1] Anders E. (19?7) Phil. Trans. R. Soc. Lond., A323, 287-304. [2] Tielens A. G. G. M. et al. (1987) Astrophys. J., 319, L109-L113. [3] Nuth J. A. III and Allen J. E. Jr., Astrophys. Space Sci., 196, 117-123. [4] Kaminsky F. (1987) Dokl. Akad. Nauk SSSR, 294, 439-440. [5] Ozima M. and Zashu S. (1991) Nature, 351, 472-474. [6] Ozima M. and Zashu S., Meteoritics, 26 382. [7] Mochizuki K. et al., this volume. [8] Sorrell W. H. (1990) Mon. Not. R. Astron. Soc., 243, 570-587. [9] Lehmann C. (1977) Interaction of Radiation with Solids and Elementary Defect Production, 172, North-Holland, Amsterdam. [10] Nuth J. A. III (1987) Astrophys. Space Sci., 139, 103-109.
Mochizuki Kenji
Ozima Minoru
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