Diamonds from V. Grib kimberlite pipe: Morphology and spectroscopic features

Abstract

Research subject. The article presents the results of studying 500 diamond crystals from the core of exploration boreholes of crater and diatreme facies of the upper part of V. Grib kimberlite pipe. Crystals from tuffs and tuffites (crater), tufobreccia, autolith kimberlite breccia and porphyry kimberlite (diatreme) were analyzed separately. aim. To carry out a morphological and spectroscopic study of diamonds from V. Grib pipe. Materials and Methods. An VERTEX-70 spectrometer with a Hyperion 1000 microscope was used to determine the total nitrogen concentration and the share of nitrogen in the form of B defects (NBS), the absorption coefficients of the “platelet” bands B', 3107 cm-1. Images of photoluminescence (PL) were recorded at 360 nm excitation by an ALROSA VIEW PRO device, in order to detect inhomogeneities and color of PL. The PL spectra were recorded by a Horiba FL-3 spectrometer with an excitation of 350 and 450 nm at 300 K and by an InVia Renishaw spectrometer with an excitation of 405, 488 and 787 nm at 77 K. Results. The distribution of diamonds in terms of nitrogen concentration and NBS is specific; about 50% of the crystals are located along one isotherm, the proportion of low-nitrogen crystals is 5%. The PL spectra of 25% of crystals showed the lines of 883/885 nm of an elementary Ni-containing defect. Most crystals with this defect have less than 400 ppm of nitrogen, but they occupy the entire range according to NBS. This system is found in the spectra of 76% of twins and intergrowth; the latter are of 36% of diamonds with the lines 883/885 nm. The studied diamonds differ from those found in M.V. Lomonosov deposit in terms of a much lower content of crystals of cubic habitus (2 and 15%), a lower degree of dissolution (octahedra, respectively, 33 and 15%), a low proportion of colored stones, as well as a wide range of nitrogen concentration and NBS. Conclusion. The revealed features indicate the specific growth conditions of diamonds from V. Grib deposit, which differed from those of diamonds from other kimberlitic bodies of the East European, Siberian platforms and the Urals. The high proportion of low-nitrogen crystals and Ni-containing diamonds is a possible indicator for the presence of large CLIPPIR-type crystals.