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Approximately 700 diamond crystals were identified in volcanic (mainly pyroclastic) rocks of the Tolbachik volcano, Kamchatka, Russia. They were studied with the use of SIMS, scanning and transmission electron microscopy, and utilization of electron energy loss spectroscopy and electron diffraction. Diamonds have cube-octahedral shape and extremely homogeneous internal structure. Two groups of impurity elements are distinguished by their distribution within the diamond. First group, N and H, the most common structural impurities in diamond, are distributed homogeneously. All other elements observed (Cl, F, O, S, Si, Al, Ca, and K) form local concentrations, implying the existence of inclusions, causing high concentrations of these elements. Most elements have concentrations 3-4 orders of magnitude less than chondritic values. Besides N and H, Si, F, Cl, and Na are relatively enriched because they are concentrated in micro- and nanoinclusions in diamond. Mineral inclusions in the studied diamonds are 70-450 nm in size, round- or oval-shaped. They are represented by two mineral groups: Mn-Ni alloys and silicides, with a wide range of concentrations for each group. Alloys vary in stoichiometry from MnNi to Mn2Ni, with a minor admixture of Si from 0 to 5.20-5.60 at%. Silicides, usually coexisting with alloys, vary in composition from (Mn, Ni)4Si to (Mn,Ni)5Si2 and Mn5Si2, and further to MnSi, forming pure Mn-silicides. Mineral inclusions have nanometer-sized bubbles that contain a fluid or a gas phase (F and O). Carbon isotopic compositions in diamonds vary from -21 to -29‰ d13CVPDB (avg. = -25.4). Nitrogen isotopic compositions in diamond from Tolbachik volcano are from -2.32 to -2.58‰ d15NAir. Geological, geochemical, and mineralogical data confirm the natural origin of studied Tolbachik diamonds from volcanic gases during the explosive stage of the eruption. [ABSTRACT FROM AUTHOR]

In addition to a series of finds of diamond in mafic volcanic and ultramafic massive rocks in Kamchatka, Russia, a carbonado-like diamond aggregate was identified in recent lavas of the active Avacha volcano. This aggregate differs from ‘classic carbonado’ by its location within an active volcanic arc, well-formed diamond crystallites, and cementing by Si-containing aggregates rather than sintering. The carbonado-like aggregate contains inclusions of Mn–Ni–Si–Fe alloys, native β-Mn, tungsten and boron carbides, which are uncommon for both carbonado and monocrystalline diamonds. Mn–Ni–Si–Fe alloys, trigonal W 2 C and trigonal B 4 C are new mineral species that were not previously found in the natural environment. The formation of the carbonado-like diamond aggregate started with formation at ~ 850–1000 °C of tungsten and boron carbides, Mn–Ni–Si–Fe alloys and native β-Mn, which were used as seeds for the subsequent crystallization of micro-sized diamond aggregate. In the final stage, the diamond aggregate was cemented by amorphous silica, tridymite, β-SiC, and native silicon. The carbonado-like aggregate was most likely formed at near-atmospheric pressure conditions via the CVD mechanism during the course or shortly after one of the volcanic eruption pulses of the Avacha volcano. Volcanic gases played a great role in the formation of the carbonado-like aggregate. [ABSTRACT FROM AUTHOR]

On "Kamchatite" diamond aggregate from northern Kamchatka, Russia: New find of CVD-formed diamond in nature - Reply to K.D. Litasov, T.B. Bekker, and H. Kagi They contain inclusions of Mn-Ni-Fe-Si alloys and silicides, native b-Mn, tungsten carbide W SB 2 sb C, boron carbide B SB 4 sb C, and an open-framework allotrope of silicon Si SB 24 sb , most of which are never observed in synthetic diamond. The silicides have very variable compositions: from (Mn,Ni) SB 4 sb Si to (Mn,Ni) SB 5 sb Si SB 2 sb and Mn SB 5 sb Si SB 2 sb , and further to MnSi, forming pure Mn-silicides. [Extracted from the article]