Andreichak V. Нawk's and tiger's eye from ferruginous rocks of Kryvyi Rih basin: topomineralogy, genesis, gemology

Object of study - hawk's and tiger's eyes manifestations of the Kryvyi Rih basin. The aim of the study consist in establishment laws localization manifestations, morphology individuals and aggregations, variability of the mineral and chemical composition and gemologycal parameters hawk's and tiger's eyes. Crystallochemical, morphological, anatomical changes of the gem were confirmed by X-ray diffraction, X-ray fluorescence, magnetometric analyses. It was found that hawk's and tiger's eyes manifestations of the Kryvyi Rih basin are the most abundant within the Gleyuvatske deposit of magnetite quartzites. They colour and gemology value of grey, light blue and blue hawk's eye and gold brown tiger's eye are determined by mineral composition, structure, texture and genesis of the stone. Cummingtonite, magnesioriebeckite or riebeckite asbestos of Alpine veins in dynamothermal metamorphites and sodium metasomatites of iron-banded formation were initial material for the gem formation. Amphibolite asbestos silification at regressive stages of metamorphism and metasomatosis were the following stage of the gem formation. The tiger's eye is a product of hypergene changes of hawk's eye accompanied by formation of complex pseudomorph of dispersed goethite and hypergene quartz with inclusions of montmorillonite at fibrous individuals of amphiboles. Five gem gemological varieties were specified taking into account genesis, mineral composition, morphology and anatomy of mineral individuals. Their classification is used when evaluating stones as operating supply.