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. Individ- lattice preferred orientations with lonsdaleite 100 peak being ual nanocrystals are tightly bound to each other and nei- coaxial with diamond 111. The open triangle in (b) indicates ther pores nor impurity phases were observed at their trade–off in diffraction intensity between lonsdaleite 100 and diamond 111. grain boundaries. SAED patterns taken from the center of each image using a f 1.4 µm aperture show single– crystal–like, remarkably strong preferred orientation in flake–shaped (Fig. 1), they were analyzed from the lateral spite of the fine polycrystalline texture. A pair of dia- direction (perpendicular to the basal plane), assuming mond 111 diffraction spots are oriented perpendicular to that the platy morphology is derived from the source the lineation and lonsdaleite 100 spots are also seen ad- single–crystalline graphite, just like the case of Popigai jacent to them, as shown in the magnified insets (Figs. 3a diamond (Masaitis, 1998; Ohfuji et al., 2015). Figure 2 and 3b). These results suggest that the observed weak shows 2D diffraction patterns of the 3 yakutite samples, lineation in yakutite is likely derived from the layered in which Debye rings of diamond 111, 220, 311 are clear- structure (basal planes) of the source graphite. At high ly seen. Ya1 sample shows distinct LPOs in several di- magnification, the constituent nanodiamonds frequently rections and also shows a discontinuous ring of lonsda- showed moiré fringes, characteristic of nanoscale rota- leite 100 which is located just inside the diamond 111 tional deformation features (Ovid’ko and Sheinerman, ring. Ya3 and Ya5 show rather continuous diffraction 2012), which are also observed in synthetic NPD and rings of diamond with just a small degree of LPOs, and the impact diamonds from the Popigai crater (Ohfuji et the lonsdaleite 100 was not clearly recognized in the 2D al., 2015). It is worthy of notice that the single–crystal– patterns (only seen as a tiny peak after integration to 1D like, unique diffraction patterns obtained from Ya1 and profiles). Ya3 samples exhibit a six–fold symmetry (Figs. 3a, 3b, The presence of lonsdaleite, a metastable carbon and 3e). Such hexagonal patterns are, of course, not ex- polymorph in yakutite suggests that it formed by the mar- plained by any (single–crystal) reciprocal patterns of dia- tensitic transformation from graphite via lonsdaleite as mond with cubic structure. So, how can we interpret intermediate product. The martensitic diamond formation them? Figure 3e shows a magnified view of the diffrac- from crystalline graphite has been demonstrated in labo- tion pattern shown in the inset of Figure 3a. Note that the ratory synthesis (Britun et al., 2004; Ohfuji and Kuroki, coaxial relationship between diamond {111} and lonsda- 2009) and also reported in diamonds found in large im- leite {100}, which was produced through the martensitic pact craters (refs; Ohfuji et al., 2015) and carbon–rich transformation from graphite, is observed in all the six meteorites (Nakamuta and Toh, 2013; Garvie et al., directions intersecting to each other at 60 degrees. This 2014). Since the graphite to lonsdaleite/diamond transfor- suggests that the observed hexagonal arrangement of di- mation occurs by sliding of graphite basal planes, fol- amond {111} is derived from the reciprocal pattern of the Mineralogical and crystallographic features of yakutite diamond 49

Figure 3. TEM images and corresponding selected–area electron diffraction patterns of yakutite samples, Ya1 (a), Ya3 (b), Ya5 (c). (d) magnified view of Ya3 showing individual constituent grains with unique moiré fringes (indicated by open triangles) due to nanoscale rotational deformation. Small insets below ED patterns are magnified images of the diffraction spots at the 12 o’clock position. (e) Six–fold diffraction pattern (magnified view of the inset of (a), obtained from Ya1) composed of diamond {111} spots and lonsdaleite {100} spots lying adjacent to each other (i.e., in coaxial relation). Angle between each vector is approximately 60 degrees, which is reasonable for the reciprocal pattern projected along [001] of lonsdaleite but cannot be explained by any diamond lattices.

Figure 4. Schematic illustration of graphite–diamond transformation in yakutite. See text for details. precursor lonsdaleite projected along the [001] direction. pact–induced transformation of graphite to lonsdaleite Figure 4 shows a schematic illustration of texturing and to diamond in yakutite. The observed microtexture and crystallographic transition process through the im- features and coaxial relationships between the carbon 50 H. Ohfuji, M. Nakaya, A.P. Yelisseyev, V.P. Afanasiev and K.D. Litasov polymorphs suggest that the source material of yakutite is from graphite to lonsdaleite and diamond is parallel pro- most likely single–crystalline graphite as is the case of cesses (i.e., graphite → lonsdaleite and graphite → dia- Popigai diamonds. Upon shock–compression the single– mond) rather than the sequential process (graphite → crystalline graphite is subjected to shock–induced defor- lonsdaleite → diamond), as the structural modification mation (kinking) and fracturing, which results in severe from lonsdaleite to diamond seems to require a high ac- fragmentation and misorientation (slight rotation) of the tivation energy. However, this is not likely in the present individual domains at local level, followed by the trans- case, since the coaxial relation, lonsdaleite [100]*//dia- formation to lonsdaleite/diamond, as proposed for the mond [111] was observed indeed in all the <100>* direc- texturing process of Popigai diamonds (Ohfuji et al., tions of lonsdaleite (Fig. 3e), which indicates their direct 2015). In this process, the original single–crystal frame- transformation. Experimental studies also demonstrated work of graphite collapses to form polycrystalline mosaic that the relative proportion of diamond vs lonsdaleite textures as observed (Figs. 3a–3d). However, the coaxial gradually increases with heating temperature and holding relationships between graphite, lonsdaleite and diamond time, suggesting that the lonsdaleite–diamond transfor- is maintained at least in the c–axis direction of the source mation is likely preferable at high temperatures (Irifune graphite. As lonsdaleite has a hexagonal structure, there et al., 2004; Sumiya and Irifune, 2004). are six crystallographically equivalent [100]* directions. The present study revealed that yakutite consists Upon further transformation to diamond, any of the lons- of tightly bound nanocrystals (5–50 nm) of diamond and daleite <100>* (such as [100], [1–10], [0–10], etc.) can small amount of lonsdaleite with distinct LPOs and be converted to diamond [111] (Fig. 4), because the 3D formed by the martensitic transformation from crystalline transition matrix linking between graphite and lonsdaleite graphite. These features are very well comparable to those (mentioned above) is no longer valid due to the nano– of the impact diamonds from the Popigai crater, which fragmentation of the original graphite structure. As the supports the idea inferred from the geochemical signa- result, six–fold patterns composed of diamond {111} tures that yakutite is a product of long–distance outburst and lonsdaleite {100} (Figs. 3a, 3b, and 3e) are produced from the Popigai crater. The source material of yakutite is in yakutite. Similar SAED patterns with hexagonal sym- probably the same metamorphic graphite contained in the metry were also reported in Popigai diamonds (Koeberl Archean gneiss distributed around (and also below) the et al., 1997) and are, therefore, likely an intrinsic feature Popigai crater. of impact diamonds formed by direct conversion of sin- gle–crystalline graphite. Compared with the SAED pat- ACKNOWLEDGMENTS terns, LPOs shown by XRD patterns are considerably weak and less obvious (Fig. 2, particularly in Ya3 and We thank Prof. T. Irifune of Ehime University for his Ya5). This is, of course, greatly influenced by the large useful discussion and support. This work was supported difference in sampling volume between the two methods; by JSPS KAKENHI Grant Number 26287138 and the latter is more than 100 times larger than the former. Russian Ministry of Education and Sciences (No. However, it might also indicate that the constituent nano- 14.B25.31.0032). We thank Dr. Y. 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