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    New nitride minerals in carbonado diamond

    Jones, A.P. and Beard, A.D. and Milledge, J. and Cressey, G. and Kirk, C. and DeCarli, P. (2003) New nitride minerals in carbonado diamond. In: Eighth International Kimberlite Conference, 2003, Victoria, Canada. (Unpublished)

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    Abstract

    Very few nitride minerals are known, but some occur widely in certain classes of meteorites; theyare simple compounds. We have found delicate crystals of several new nitride minerals inside a single broken sample of carbonado diamond, not previously reported from Earth or meteorites, but comparable to synthetic compound mixtures between Ti-Cu-N. In addition to chemical stoichiometries from electron microprobe data, we have obtained in-situ non-destructive X-raydiffraction data, of the actual cluster of crystals imaged by the SEM, using a high brightness microbeam. Three new nitride phases have been identified; Ti3CuN (tetragonal), CuTiN2(orthorhombic) and a Cu-N compound. However, we are less confident of the exact stoichiometry of the Cu-N phase and although there are several candidates ranging from Cu3N to CuN2 (azide), the X-ray diffraction pattern is most consistent with Cu3N (tetragonal). The three new Ti-Cu nitride minerals coexist in a sub-mm scale cavity with minor copper nitrate, silver chloride and faceted polycrystalline host diamond. The delicate nitride crystals (<20 microns) contain between ~6 and ~20 wt % N and occur in three growth forms: (a) slender hollow needles (b) platycrystals with round holes and (c) euhedral spinel-shaped equant grains. In addition to major Ti and Cu, which vary sympathetically, the crystals also contain variable amounts of silver (up to ~4 wt %). The unusual metal association (Ti-Cu-Ag) of the nitride, and delicate growth forms within unconnected cavities suggest an unusual origin. Occasional probe detection of oxygen probablyreflects surface alteration to oxynitride, similar to synthetic behaviour. Our working hypothesis for origin of the nitrides, is growth from a high temperature vapour; nitrides such as osbornite (TiN) can form at temperatures of ~2500. Such a high temperature is found in the mid-lower mantle at a depth in excess of 2000 km. This is consistent with recent synthesis of polycrystalline diamond in a few minutes at 2,300-2,500oC at mantle pressures (Irifune et al, 2003). If the new nitride minerals are stable within the Earth’s mantle, they may represent a significant reservoir for nitrogen, and could explain the large isotopic imbalance between nitrogen in mantle diamond and atmospheric nitrogen, as predicted by Javoy (1998).

    Metadata

    Item Type: Conference or Workshop Item (Paper)
    School: Birkbeck Schools and Departments > School of Science > Earth and Planetary Sciences
    Depositing User: Sarah Hall
    Date Deposited: 10 Jun 2019 14:50
    Last Modified: 11 Jun 2019 08:36
    URI: http://eprints.bbk.ac.uk/id/eprint/27774

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