Placer-forming diamantiferous rocks and diamonds of Eastern Brazil

The aggregate geological, geochronological, petrographic, and mineralogo-geochemical data suggest the participation of endogenic, presumable mantle matter, in formation of Brazilian diamantiferous metaconglomerates. It is possible that the studied rocks from Sopa-Brumadinho Formation are not secondary, but primary source of diamonds in Eastern Brazil, which has been searched for about 200 years. The study of the Brazilian, Uralian, and Yakutian diamonds confirms existence of the uniform Brazilian-Uralian type of rounded diamonds, characteristic for special fluidi-zation type of primary diamond deposits. Photogoniometric analysis showed that curvelinear faces of the Brazilian diamonds formed in result of long-term dissolution mantle process combined with mechanical abrasion, and chemical etching, encountered mainly in endogenic conditions. New quantitative data about difference in morphology between diamonds from kimberlitic and non-kimberlitic deposits were obtained. The twinning seams of diamond of the Uralian-Brazilian type represents a fragment of lonsdaleite structure. Spectroscopic study showed the specific features of Brazilian-Uralian diamonds, such as significant variation in IR-absorption, predominance of IaB 1 type, high nitrogen aggregation degree, high concentration of platelets, enrichment by hydrogen and H3 and H4 centers often dominated over N3 centers, increased (1150-1200°С) primary temperature of crystallization, and considerable duration of post-crystallization annealing. The physical substantiation of polysynthetic twinning of diamond in high parametric quasi-equilibrium conditions is presented. The spectroscopic sturdy testifies to important features of Brazilian-uralian diamonds - very wide variations on IR-absorption, admixture and high nitrogen concentration as structural defects-platelets, enrichment by hydrogen and H3 and H4 centers, often dominating over N3 centers, increased (1150-1200°С) primary temperature of crystallization, considerable duration of post-crystallization annealing. All these features are also characteristic for rounded Uralian diamonds, but not typical for diamonds from kimberlites. Variation of spectroscopic features of Brazilian diamonds suggests their formation in unstable thermodynamic environment.