Internal Texture and Syngenetic Inclusions in Carbonado

Carbonado is a black or dark brown cryptocrystalline aggregate of diamond. Its age ranges from 2.6 to 3.8 Ga (billions of years), according to lead isotopic composition. Carbonados have been found only in alluvial placer deposits, such as those in Brazil and Central Africa, and never in kimberlite. Many hypotheses, including non-kimberlitic, crustal origin of carbonado were proposed by previous researchers. Analysis of internal texture and syngenetic inclusions is important to understanding the genesis.

Carbonado samples were collected from alluvial placer deposits of the Macaubas River basin in the Brazilian state of Minas Gerais and the Ubangi River basin in the Central African Republic. They were dark or nearly black, with a smooth and shiny surface. The samples contained pores ranging from a few hundred nanometers to a few micrometers. Secondary electron images revealed a typical polycrystalline structure, which showed fine-grained texture with irregularly shaped diamond grains averaging 5 to 15 µm. Grain boundaries were in irregular, zigzag patterns. Numerous pores, up to 20 µm, were found inside grain boundaries. They were usually filled with postgenetic minerals such as kaolinite, flurencite, and quartz. Dislocation lines were also observed within some diamond grains. The pole figures suggested that crystallographic orientations of these grains were quasi-random.

Syngenetic inclusions ranging from 10 to 50 nm were identified using the combined FIB/TEM technique: garnet, apatite (including fluorapatite), phlogopite, silica, Ca-Mg-Sr- and Ca-Ba-carbonates, halides (sylvite, KCl, and bismocolite, BiOCl), native nickel and metal alloys (Fe-Ni, Cr-Fe-Mn, and Pb-As-Mo), oxides (FeO, Fe-Sn-O, TiO₂, SnO₂, and PbO₂), Fe-sulfides, and fluid inclusions. An intergrowth of fluorapatite, phlogopite, and silica was also observed. Almandine-pyrope garnet inclusions with silica-enriched amorphous rims were confirmed. Fluid inclusions contained Si, Al, Fe, O, Ti, Ca, S, Cl, K, and carbonate. These fluid inclusions suggested the carbonados were formed in a chloride-rich environment. Crystal inclusions were not pressure-indicator minerals. Instead, they were stable in a wide range, from crust to lower mantle, except bismocolite, which could only be formed in the crust. Although this whole inclusion assemblage was possibly of crustal origin, conclusive research is still needed. The authors propose that these samples were crystallized and grown in one stage, which was followed by sintering. This hypothesis could be confirmed by comparison with one-stage growth process of a polycrystalline diamondite.