V.M. Kvasnytsya. IMPACT DIAMONDS FROM NEOGENE SAMOTCAN PLACER (MIDDLE-DNIPRO AREA)
IMPACT DIAMONDS FROM NEOGENE SAMOTCAN PLACER (MIDDLE-DNIPRO AREA)
V.M. Kvasnytsya http://orcid.org/0000-0002-3692-7153
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine
34, Acad. Palladin Ave., Kyiv, Ukraine, 03142
Mineralogical journal 2019, 41 (4): 3-12
Impact diamonds from the Neogene Samotkan titanium-zirconium placer located on the Middle-Dnipro megablock of the Ukrainian Shield are described. The Samotkan placer is a buried coastal-marine deposit of Miocene sands. The sands are enriched with heavy minerals, including microdiamonds of various origins. Besides endogenous (mantle) diamonds, the placer contains impact apographitic diamonds (up to 11% of the total number of diamond crystals found). The size of the impact diamonds is up to 0.3 mm. The source rocks of endogenous and impact diamonds are unknown. The results of a comprehensive study of impact diamond crystals — morphology, microtopography, microstructure, carbon isotope composition, photoluminescence, optical, infrared, and Raman spectroscopy — are presented. Impact diamond crystals have external morphological and internal microstructural signs of solid-phase transition of graphite to diamond under impact shocks, they are paramorphoses on graphite. At the macromorphological level, these include their pinacoidal and pinacoidal-prismatic habitus and lamellar and tabular shape. At the nano-microstructural level, the signs of this transition are intense separate parallel strokes on (11-21) or a system of these strokes in two or three directions with angles of 60° between them on the (0001) plane, as well as new sculptures on the (0001) planes of crystals. These sculptures are represented by various nano- and micro blocks of oval-elongated shape, as well as various etch pits, strictly oriented along [10-10]. Microstructural features of the graphite-diamond transition in the studied crystals of impact diamonds are also their polysynthetic twinning and the polycrystalline structure of the twins themselves. The carbon isotopic composition of impact diamonds ranges from –10.35 to –23.06 ‰ d13С, with an average value of –17.64 ‰ d13С. The photoluminescent and spectroscopic features of the studied diamonds indicate the absence of nitrogen defects in crystals that are characteristic of mantle diamond. Raman spectra of the studied impact diamonds indicate a defective structure of their crystals caused by their intense twinning and polycrystallinity due to shock loads. The question of the source rocks and possible routes of influx of diamond into the Samotkan placer are discussed.
Keywords: impact apographitic diamond, morphology, microtopography, microstructure, carbon isotope composition, photoluminescence, spectroscopy, Neogene Samotkan placer, the Ukrainian Shield.
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