UDC 549.211


V.MKvasnytsya, DrSc (Mineralogy and Crystallography), Prof., Head of Department. 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
E-mail: vmkvas@hotmail.com

Language: Ukrainian
Mineralogical journal 2020, 42 (1): 12-23

Abstract: Using the methods of scanning electron microscopy and goniometry, the crystal morphology of microdiamonds from the Neogene Samotkan placer on the Middle Dnipro megablock of the Ukrainian Shield was studied. Crystal sizes do not exceed 0.3 mm in diameter. More than a hundred crystals are studied, selected from more than a thousandth collection. Crystal morphological features of microdiamonds are established. Among polyhedra of Samotkan microdiamonds, crystals with flat and smooth faces of various simple forms characteristic in the hexoctahedral symmetry class of diamond crystals (cube, rhombic dodecahedron, trigon- and tetragontrioctahedra, tetrahexahedra and hexoctahedra) are relatively common. Such forms are almost always incomplete and developed mainly on crystals of an octahedral habit. The diversity and dominance of cubic forms is a characteristic feature of Samotkan microdiamonds. As a rule, cubic crystals are complicated by the faces of the octahedron and rhombic dodecahedron (rounded dodecahedron). Two contrasting types of cubic crystals are distinguished by the mechanism of their formation — tangential in case of the degeneration of the (111) planes and normal (columnar) growth. Various twins and aggregates from cubes or octahedrons are another features of Samotkan microdiamonds. The twins of octahedrons and cubes after spinel law are widespread, as well as fivefold twins of octahedrons after spinel law and parallel crystal intergrowths occur. Crystallization of skeletal and vertex forms is also relatively often manifested on polyhedra. Octahedral crystals with signs of natural dissolution were found. Microtopography of the faces of various simple forms of polyhedra is common. In general, the crystallomorphological features of Samotkan microdiamonds indicate their crystallization in a carbon-saturated medium and also the rapid growth of crystals. The questions of the origin of Samotkan microdiamonds, their possible crystallization parental medium and their bedrock are considered. The mantle eclogite medium of diamond crystallization and the Early Proterozoic age of their primary sources are predicted.
Keywords: microdiamonds, crystal morphology, microtopography, microstructure, origin, Neogene Samotkan placer, the Ukrainian Shield.


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