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Mineralogical and Crystallographic Features of Polycrystalline Yakutite Diamond

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Mineralogical and Crystallographic Features of Polycrystalline Yakutite Diamond Journal of Mineralogical and Petrological Sciences, Volume 112, page 46–51, 2017 LETTER Mineralogical and crystallographic features of polycrystalline yakutite diamond Hiroaki OHFUJI*, Motosuke NAKAYA*, Alexander P. YELISSEYEV**, Valentin P. AFANASIEV** and Konstantin D. LITASOV**,*** *Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790–8577, Japan **V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, RAS, Novosibirsk, 630090, Russia ***Novosibirsk State University, Novosibirsk, 630090, Russia This study revealed for the first time the microtexture and crystallographic features of natural polycrystalline diamond, yakutite found in placer deposits in the Siberian Platform, Russia. Yakutite consists of well–sintered nanocrystalline (5–50 nm) diamond and small amount of lonsdaleite showing distinct preferred orientations. Micro–focus X–ray and electron diffractions showed a coaxial relationship between lonsdaleite 100 and dia- mond 111, suggesting the martensitic formation of yakutite from crystalline graphite. These textural and crys- tallographic features are well comparable to those of the impact diamonds from the Popigai crater located in the central Siberia and strongly support the idea that yakutite is a product of long–distance outburst from the Popigai crater, which has been inferred merely from the geochemical signatures. Keywords: Yakutite, Impact diamond, Nanocrystalline diamond, Microtexture, TEM INTRODUCTION values ranging from −10 to −20‰, while the latter shows −23 to −31‰ (Kaminsky, 1991). Latter studies rather con- Yakutite is a type of polycrystalline diamond occurring in cerned the possibility of yakutite as impact diamonds that alluvial deposits in Northern Yakutia of Russia. It is char- are products of long–distance outburst from the Popigai acterized by a massive, dark brown to black appearance astrobleme, created about 35 Ma ago in the central Siberia similar to carbonado, which is another type of polycrystal- (Vishnevsky et al., 1997; Afanas’ev et al., 2011). Vish- line diamond found exclusively in alluvial deposits in nevsky et al. (1997) compared the optical properties in- Central African Republic and Brazil. Yakutite and carbo- cluding photoluminescence signals in addition to density nado both show geochemical features clearly distinct from and carbon isotopic signature between yakutite and im- monocrystalline diamonds found in kimberlites, for exam- pact diamonds from the Popigai crater. They both showed ple, rare earth element patterns indicative of crustal origin characteristic photoluminescence bands at 600, 693, 718 (Shibata et al., 1993) and relatively low carbon isotope and 777 nm and the same order of magnitude (<1015)of ratio compared with typical mantle–derived diamonds paramagnetic nitrogen impurity (C–centers per cm3)as (Kaminsky, 1991; Cartigny et al., 2014). There are some well as δ13C values, suggesting that they have the same notable differences between them: yakutite consists of origin. However, no further information such as microtex- 0.1–1 µm and contains small amount of lonsdaleite (so ture and crystallographic feature has been provided to called ‘hexagonal diamond’), while carbonado consists come to the conclusion. mainly of irregular–shaped diamond crystals of a few to Despite a number of studies on natural impact dia- several tens µm and lacks lonsdaleite, as summarized by monds including those from the Popigai crater, the details Kaminsky (1991). They are also different in terms of car- of their microtexture and crystallographic feature re- bon isotope ratio, δ13C: the former has relatively heavier mained uncertain for a long time. We recently reported that the impact diamonds from the Popigai crater have a doi:10.2465/jmps.160719g unique microtexture composed of tightly bound nanodia- H. Ohfuji, [email protected]–u.ac.jp Corresponding author monds of 5–50 nm (Ohfuji et al., 2015) similar to that of Mineralogical and crystallographic features of yakutite diamond 47 nano–polycrystalline diamond (NPD) obtained by direct conversion of graphite at static high pressure and high temperature (Irifune et al., 2003). It seems that stress–in- duced local fragmentation of the source graphite and sub- sequent rapid transformation to diamond in the limited time scale (of the shock event) result in multiple diamond nucleation and suppression of the overall grain growth, producing the unique nanocrystalline texture of the natu- ral NPD (Ohfuji et al., 2015). Such a nano–polycrystal- line texture may be used as an indicator for discovering further impact–produced diamonds. In the present study, we investigated the microtex- ture and crystallographic features of yakutites for the first time and compared them with those of impact diamonds from the Popigai crater to discuss their origin. Figure 1. Microscopic images (right) and Raman spectra (left) of yakutite samples. The samples have tabular morphologies EXPERIMENTAL with dark brown to black colors. The diamond Raman peak at ~ 1330 cm−1 is considerably weak and broad due to nanocrys- fi Yakutite samples (Ya1, Ya3, and Ya5) used in this study talline nature of yakutite and signi cant background increase by strong fluorescence (even weaker and broader than that of syn- are a part of collections from placer deposits of Olenek, thetic NPD, as shown for comparison). Kelimyar, Nikabyt and Khorbusuonka rivers, which are located hundreds of kilometers far from the Popigai crater (Vishnevsky et al., 1997), in the Siberian Platform, Rus- RESULTS AND DISCUSSION sia. The samples were first examined by a high–resolution digital microscope (Keyence, VHX–2000) at magnifica- Figure 1 shows typical Raman spectra collected from 3 tions up to ×1000 to observe the morphology and surface yakutite samples and that of NPD synthesized graphite feature of the samples. At first glance, they appear to be polycrystallite at high pressure and temperature as a com- opaque and dark brown to black in color (Fig. 1), but parison. Yakutite samples show only a small and very partly translucent showing dark yellowish to brawn colors broad diamond Raman peak at ~ 1330 cm−1: the observed at high magnifications. One of the samples (Ya5) showed peak is even broader and less distinct than that of synthetic distinct striations (layered texture) at some regions. NPD. Yakutite and NPD both show a significant back- Micro–focused X–ray diffraction (XRD) measure- ground increase toward the higher frequency side. This ments were conducted by using Rigaku Rapid II equipped is attributed to strong fluorescence from the samples and with MoKα radiation (λ = 0.7107 Å, collimated to f 0.1 indicative of the nanocrystalline nature of the constituent mm) operated at 50 kV and 24 mA. Each sample was diamond grains (Sumiya et al., 2009; Ohfuji et al., 2010). mounted on a thin (170 µm thick) glass plate by dou- Similar features were also observed in the impact dia- ble–stick tapes, which was then placed onto a sample monds from the Popigai crater (Ohfuji et al., 2015). Al- holder. Diffraction patterns were collected in transmitting though we analyzed more than 10 points both in relatively geometry by an imaging plate detector. 1D profiles were transparent yellowish regions and in darker greyish re- obtained by integrating the intensity of the 2D diffraction gions, no obvious changes in spectra including the detec- patterns over definite angular sectors. Micro–Raman anal- tion of graphite peak (at ~ 1580 cm−1) were observed. We ysis was conducted by using JASCO NRS–5100 equipped then further examined the sample by a UV micro–Raman with a diode laser (λ = 532 nm) as the excitation source. using a 325 nm He–Cd laser to minimize the fluorescent Focused ion beam (FIB) systems (JEOL, JEM–9310FIB; signal from the samples and detected an intense peak at FEI, Scios) were used to prepare thin cross–section foils 1332 cm−1 characteristic to diamond. The result of the UV of approximately 10 × 7 × 0.1 µm thick for microtexture Raman as well as photo–luminescence spectroscopies are observation using TEM. Prior to the FIB milling, samples presented in a separate report (Yelisseyev et al., 2016). were coated with a thin osmium layer (5 nm thick) using We also analyzed the yakutite samples by a micro– Meiwafosis Neoc–STB. TEM observation was performed focus XRD to detect the possible presence of polymor- by using JEOL JEM–2100F operated at 200 kV and phic phases (such as graphite and lonsdaleite) and lattice equipped with two CCD cameras (Gatan, Orius 200D preferred orientation (LPO) of the constituent crystals. and UltraScan1000). Since the yakutite samples studied are all platy to 48 H. Ohfuji, M. Nakaya, A.P. Yelisseyev, V.P. Afanasiev and K.D. Litasov lowed by buckling/puckering of the layers, unique crys- tallographic relationships are maintained between graph- ite (G), lonsdaleite (L) and diamond (D): (001)G //(100)L // (111)D, [210]G //[001]L //[2–1–1]D and (1–20)G //(–120)L // (0–11)D (Britun et al., 2004; Nakamuta and Toh, 2013; Garvie et al., 2014). The coaxial relation of (100)L // (111)D was indeed observed in the diffraction pattern of Ya1 (Fig. 1b), although 002 peak of the source graphite was not detected (i.e. fully converted to lonsdaleite/dia- mond mixture) in the present case. It is interesting to note the trade–off in diffraction intensity between L100 and D111 particularly at the center of Figure 1b, which also suggests the sequential transformation from graphite to lonsdaleite to diamond. Figure 3 shows bright–field TEM images and corre- sponding selected–area electron diffraction (SAED) pat- terns of the three yakutite samples. All the three samples consist of nanocrystalline diamond grains of 5–50 nm and Figure 2. 2D XRD patterns of yakutite samples: (a) Ya1, (b)–(c) show a weak lineation in the horizontal direction of each Ya3, and (d) Ya5. The observed Debye rings are indexed with image, which are well comparable to the textural features 111, 220, and 311 of diamond. Ya1 shows the most distinct of the impact diamonds from the Popigai crater.
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