Description: We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum group element-rich (PGE) samples from the Merensky Reef of the Bushveld Complex in South Africa. Chromitites and pegmatitic melanorite from the Northam and Rustenburg platinum mines contain Ru-Rh-Pt arsenide nanocrystals (〈50 nm in size) as inclusions in pyrrhotite and pyrite, as well as Ru-Rh-Os, Ru-Rh-Pt-Ir, and Ru-Rh-Pt sulfide nanocrystals in pyrrhotite and pentlandite. Chalcopyrite did not incorporate nanometer-sized PGE-bearing minerals (PGM). These nano-scale PGM inclusions form a new population that was hitherto unknown from the Merensky Reef. The PGM inclusions are found as idiomorphic, plate-like nanocrystals, which are differently oriented with respect to the host base metal sulfide (BMS). In one TEM foil we observed Ru-Rh-Pt-sulfide nanocrystals exsolved from a pentlandite matrix with a common crystallographic orientation that is visible in TEM dark-field images. However, this phenomenon is only patchily developed and ahead of the exsolution front the pentlandite still contains Ru-Rh in solid solution. Occasionally the PGM inclusions form clusters, develop a flow texture, or are aligned like pearls on a string. These observations are inconsistent with sub-solidus exsolution and suggest that the nano-scale PGM crystallized before solidification of the sulfide melt and the formation of their BMS hosts. There is evidence from recently performed laboratory experiments on silicate melts with additives of Ru and Pd that discrete Fe-rich Fe-Ru-Pd alloys (nanometer-sized) can indeed crystallize early. These experiments raise the possibility that the PGE-bearing nanocrystal inclusions found in the BMS of the Merensky Reef might represent preserved relicts of an early phase of magmatic PGM that precipitated from the silicate melt and afterwards were collected by the sulfide melt. Combined FIB/HRTEM techniques open up a window through which the nano-scale mineral population can be viewed. PGE previously thought to be present in BMS solid solution might actually be an artefact of inadequate spatial resolution.

Description: A microcrystalline polymineral aggregate was identified as an inclusion in a diamond from the Rio Soriso area, Mato Grosso State, Brazil. It is composed of iron carbides, Fe-rich periclase (with exsolved magnesioferrite), and two orthorhombic, postspinel phases — Mg-Cr-Fe and Ca-Cr oxides, which are new mineral phases. This association was formed during several stages and may be considered as a rock microfragment from the lower mantle. In addition to the carbide-oxide fragment, other phases were identified as inclusions in the diamond, such as parascandolaite KMgF 3 (that is stable at pressures of up to 50 GPa) in association with orthorhombic MgO, and porous dolomite occurring together with nanoinclusions of calcite, apatite, spinel, Fe-sulfide, and an assemblage of periclase plus wüstite. These minerals belong to the carbonatitic association and, along with syngenetic nanoinclusions of fluid nitrogen, represent the media of diamond formation. The assemblage of periclase and wüstite within the carbonatitic matrix points to the origin of diamond from a carbonatitic environment within the lower mantle under pressure conditions of ≥ 85–86 GPa (~1,900 km depth).

Description: Samples of carbonado from Brazil and the Central African Republic were studied with the use of electron backscatter diffraction (EBSD) for quantitative textural analysis (QTA) and transmission electron microscopy (TEM). The grain size distribution in carbonado is either random or may be approximated by a log-normal distribution model with a mode at 6–8 μm. No bimodal distribution, as suggested previously for some carbonado samples, was observed. The crystallographic orientations of diamond grains in carbonado are quasi-random. The following minerals were identified among syngenetic mineral inclusions, enclosed in diamond grains of carbonado: garnet, apatite (including fluorapatite), phlogopite (or high-silica mica), SiO 2 , Ca-Mg-Sr- and Ca-Ba-carbonates, halides (sylvite and bismocolite BiOCl), native Ni and metal alloys (Fe-Ni, Cr-Fe-Mn, and Pb-As-Mo), oxides (FeO, Fe-Sn-O, TiO 2 , SnO 2 , and PbO 2 ), and Fe-sulfides, as well as fluid inclusions. Most of these occur over a very wide range of stability conditions. Only bismocolite, which is characteristic of the weathered crust, can be considered an entirely crustal mineral phase, which implies a possible crustal origin of the entire mineral association. Among syngenetic liquid inclusions in diamond grains comprising carbonado, silicate-carbonate ones overwhelmingly predominate. In addition to the usual silicate components, such as Si, Ti, Al, and Fe, they have Ca, K, and Cl; the latter three comprise 11.9 at.% of the only analyzed fluid inclusion.

Description: Eleven new minerals were identified in deep mantle primary carbonatitic association as inclusions in diamond from the Juina area, Mato Grosso, Brazil. Specifically, two carbonates [magnesite and eitelite Na 2 Mg(CO 3 ) 2 ], two phosphates [mixed-anion phosphate Na 4 Mg 3 (PO 4 ) 2 (P 2 O 7 ), Fe-diphosphate Fe 2 Fe 5 (P 2 O 7 ) 4 ], two fluorides [oskarssonite AlF 3 and Ba-rich fluoride (Ba,Sr)AlF 5 ], three sulfides [pentlandite (Fe,Ni) 9 S 8 , violarite FeNi 2 S 4 , and millerite NiS], hematite, and metallic Ni-iron were detected; the two phosphates and Ba-rich fluoride are observed in the natural environment for the first time. The mineral compositions of the analyzed inclusions are variable, even at a nanometer scale, which indicates variability in the source media during the formation of diamond. Volatiles, represented in the form of porosity, played a significant role in this process. Most mineral phases contain volatile elements, as well. Carbonatitic inclusions most likely originated from high-density fluid (HDF) microinclusions encapsulated in diamond during its growth. During the ascent of diamond, HDF inclusions underwent disintegration in composition and crystallized as polymineralic inclusions. Formation of diamond in the studied case took place in a carbonatitic, carbonate-halide-phosphate-fluoride medium, which was enriched in volatiles and acted as an open system during diamond formation.