Nitrogen nanoinclusions in milky diamonds from Juina area, Mato Grosso State, Brazil

A unique set of diamonds with a 'milky' optical appearance from the Rio Soriso placer deposit in the Juina area, Mato Grosso, Brazil was studied by combined transmission electron microscopy (TEM) and fourier transform infrared (FTIR) spectroscopy. The main characteristics of the studied samples are large numbers of randomly distributed {111}-faceted octahedral defect nanostructures. The dislocation densities of the focused ion beam (FIB) foils are generally low. Dislocation loops are observed only around larger inclusions. The inclusion size shows a bimodal distribution and spreads around values of 20 and 200 nm. Electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) spectroscopy mapping of both subsets yield high nitrogen contents for all sealed inclusions. In cases where the nanoinclusions touch the surface of the FIB section no nitrogen signal could be detected, indicating the loss of a fluid or gas phase as the carrier of nitrogen. FTIR mapping of the same regions showed a strong correlation between structurally bound nitrogen, hydrogen and the abundance of nanoinclusions. We propose that the most likely phase included in these nanoinclusions is NH3. These nanoinclusions could be the result of a high-temperature episode or of long residence times at shallower depths and lower temperatures. Thus they might represent the last stage of the nitrogen aggregation, or they may be syngenetic trapped NH-bearing source fluids.