Unusual green Type Ib–Iab Dniester–type diamond from Ukrainian placers

Abstract

Among placer diamond occurrences in Ukraine, a group of microdiamonds have been distinguished that have specific morphological, color and spectral characteristics, not observed in other natural diamonds. These diamonds, termed “Dniester–type diamonds”, have tetrahexahedroidal and rhombododecahedroidal morphologies, green coloration, and high concentrations of single–atom, unaggregated nitrogen in the form of C–centers (66–74% of all N atoms), along with low ratios of nitrogen aggregation (0–13% agrregation ratio) and high total nitrogen content (892–1493 atomic ppm). With these characteristics, Dniester–type diamonds are approximate the Type Ib-Iab classification. The predominance of single–atom, unaggregated nitrogen indicates a short residence time under high–temperature conditions. These Dniester–type diamonds have a narrow range of carbon isotopic compositions, from δ13С = -10.52‰ VPDB tо -12.82‰ VPDB (average δ13С = -11.85‰ VPDB). They are distributed in Quaternary and Neogene sediments of the southwestern part of the Ukrainian Shield. This distribution forms a local halo within the Dniester and Southern Bug rivers interfluve and Black Sea beach sediments, approximately 650 km in length. This implies their endemic character and the likely nearby presence of primary source(s) of unknown, possibly non–kimberlitic type.