DISTRIBUTION OF [SI-V]- DEFECT IN NATURAL TYPE IIA DIAMONDS

[Si-V]^- is a well-known defect in diamond. It has a great potential in electronics application and also is a very important feature for gem diamond identification in separating natural from synthetic. It is common in CVD diamonds (Martineau et al., 2004), can be doped into HPHT synthetic diamonds (Sittas, et al., 1996), and also occurs in natural type IIa diamonds (Breeding and Wang, 2008). Recent study of [Si-V]^- distribution (emissions at 736.6 and 736.9 nm) in HPHT synthetic diamonds confirmed that it was concentrated in the {111} growth sectors. Identical distribution pattern of Ni_i⁺, which has doublet emissions at 882.1 and 883.7 nm, was observed (Johnson et al., 2015), strongly indicating that [Si-V]^- and Ni_i⁺ have the same behavior during HPHT diamond growth. However, little is known how [Si-V]^- is incorporated in natural diamond lattice. In this report, we studied the distribution of [Si-V]^- defect in natural diamonds and its correlation with other emissions (defects).

Seven natural type IIa gem diamonds were analyzed in this study. A common feature of this group of diamonds is occurrence of euhedral olivine inclusions, which is very rare among natural type IIa diamonds and good evidence that these IIa diamonds were formed in the lithospheric mantle. Occurrence of [Si-V]^- in these stones were detected among enormous natural type IIa diamonds routinely analyzed in GIA laboratories. Distribution of [Si-V]^-, Ni_i⁺ and many other emissions were mapped over the table faces using various laser excitations at liquid nitrogen temperature. It was found that intensities of [Si-V]^- emission varied significantly over a small area. The distribution patterns were irregular and changed from stone to stone. Distribution of Ni_i⁺ emissions showed irregular patterns, but entirely different from that of [Si-V]^-. There is no correlation in spatial distribution of these two defects in natural type IIa diamonds, in drastic contract to that observed in HPHT synthetics. Possible causes of the contrast behaviors of [Si-V]^- distributions in natural and HPHT synthetic diamonds, and their implications in diamond formation and gem diamond identification will be discussed.