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A spectroscopic survey of naturally-occurring vacancy-related colour centres in diamond

1982; Institute of Physics; Volume: 15; Issue: 8 Linguagem: Inglês

10.1088/0022-3727/15/8/014

1361-6463

A T Collins,

Geological and Geochemical Analysis

A spectroscopic survey of a large number of natural brown and yellow diamonds shows that the H3 centre (a vacancy plus an A-aggregate of nitrogen) is observed in absorption quite frequently in brown diamonds and occasionally in yellow samples. The H4 centre (a vacancy plus a B-aggregate of nitrogen) occurs much less frequently, even in diamonds containing predominantly B-nitrogen, and the relationship between the H3:H4 ratio and the A:B ratio observed for diamonds irradiated and annealed at approximately 800 degrees C in the laboratory generally does not apply to naturally coloured crystals. However, annealing a laboratory-irradiated diamond at 1500 degrees C resulted in an H3:H4 ratio more nearly like that observed in natural specimens, and produced an absorption line seen previously only in natural diamonds. It is believed that this is due to partial fragmentation of the H4 centre, and that the process goes to completion at the more modest temperatures experienced over a much longer time period by natural diamonds. Those natural processes which might be responsible for the production of vacancy-related colour centres are considered, and it is concluded that plastic deformation, or gamma -ray irradiation from a localised concentration of uranium, are possible candidates.