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Revision 1 1 Transformation of Graphite to Lonsdaleite And Revision 1 1 Transformation of graphite to lonsdaleite and diamond in the Goalpara ureilite 2 directly observed by TEM 3 4 Yoshihiro Nakamuta1 and Shoichi Toh2 5 1University Museum, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan 6 2HVEM Laboratory, Kyushu University, Motooka, Fukuoka 819-0395, Japan 7 8 ABSTRACT 9 Reflected-light microscopy and laser Raman spectroscopy of a polished thin section (PTS) 10 of the Goalpara ureilite and x-ray powder diffraction (XRPD) analyses of the grains taken 11 out of it elucidate the mode of occurrence of carbon grains which contain lonsdaleite and 12 diamond in the Goalpara ureilite. The results enable us to get samples with which precise 13 analyses can be made in order to know the formation mechanism of lonsdaleite and 14 diamond in ureilites. Selected area electron diffraction (SAED) analyses and high 15 resolution TEM (HRTEM) observations were carried out in the three unique directions of 16 pristine graphite with two thin slices prepared from a carbon grain directly taken out of a 17 PTS. SAED patterns reveal the relative crystal-axes orientations between graphite (Gr), 18 lonsdaleite (Lo) and diamond (Di) as (001)Gr // (100)Lo // (111)Di, [210]Gr // [001]Lo // [2-1- 19 1]Di and (1-20)Gr // (-120)Lo // (0-22)Di. The shapes of diffraction spots in the SAED patterns 20 reveal that the transformation of graphite to lonsdaleite is initiated by sliding of hexagonal 21 carbon planes of graphite along the [210] of graphite structure. These results suggest that 22 lonsdaleite and diamond in ureilites formed directly from graphite through boat-type 23 buckling and chair-type puckering of hexagonal carbon planes of graphite, respectively. 24 The results of this study confirm the shock origin of diamond in ureilites. 25 Keywords: Ureilite, graphite, lonsdaleite, diamond, transformation mechanism, TEM 1 Revision 1 26 INTRODUCTION 27 Diamond was found in the Novo Urei meteorite, one of typical ureilites, first in 28 meteorites in 1888 (Kunz, 1888). Lonsdaleite, hexagonal polymorph of diamond, was 29 found in the Canyon Diablo iron and the Goalpara ureilite first in nature (Bundy and Kasper, 30 1967; Frondel and Marvin, 1967; Hanneman et al., 1967). So far, mineralogical properties 31 of carbon minerals in ureilites have been only roughly determined by using acid-treatment 32 residues because they exist in small quantities in ureilites (Hanneman et al., 1967; Le 33 Guillou et al., 2010; Ringwood, 1960; Urey et al., 1957; Valter et al., 2003; Vdovykin, 34 1970). Then, the origin of diamonds in ureilites has been controversial (Arrhenius and 35 Alfvén, 1971; Fukunaga et al., 1987; Lipschutz, 1964; Urey, 1956). 36 Berkley and Jones (1982) reported that euhedral graphite blades occur without diamond 37 in the Allan Hills (ALH) 78019 ureilite. Euhedral tabular crystals of graphite were also 38 found in the Nova 001 and Nullarbor 010 ureilites and hard lumps in graphite crystals were 39 supposed to be diamond formed by shock (Treiman and Berkley, 1994). Nakamuta and 40 Aoki (2000) investigated carbon minerals in very weakly and weakly shocked ureilites and 41 observed blade-like shaped euhedral graphite crystals, being tan-grey color and bearing 42 metallic luster, in the very weakly shocked ALH 78019 and Yamato (Y)-82100 ureilites 43 with an optical microscope under a reflected light. They also observed blade-like shaped 44 grains in weakly shocked ureilites, Kenna, Y-79158 and ALH 77257, although these grains 45 are black in color and dull in luster. X-ray powder diffraction (XRPD) patterns of the grains 46 directly picked up from the polished thin section (PTS) of the Kenna ureilite reveal that the 47 grains are a mixture of graphite, compressed graphite and diamond. Lonsdaleite was not 2 Revision 1 48 found in these weakly shocked ureilites. The results suggest that diamond in ureilites was 49 formed from well-crystallized graphite having euhedral shapes by shock. In order to 50 confirm the origin of diamonds in ureilites and to elucidate the transformation mechanisms 51 of graphite to lonsdaleite and diamond, direct observation of structures of graphite, 52 lonsdaleite and diamond at the same area of a carbon grain is important but was not carried 53 out until now. The Goalpara ureilite is one of heavily shocked ureilites and is known to 54 contain lonsdaleite together with diamond (Hanneman et al., 1967). In this study, SAED 55 analyses and HRTEM observations were carried out in three directions with two slices 56 prepared from a carbon grain directly taken out of the PTS of the Goalpara ureilite and the 57 transformation mechanisms from graphite to lonsdaleite and diamond were proposed. 58 59 MATERIALS AND EXPERIMENTAL METHODS 60 The Goalpara ureilite is a monomict ureilite (Papike, 1998; Vdovykin, 1970). Urey et al. 61 (1957) found diamond in an acid-tretment residue of the meteorite based on an XRPD 62 analysis. Lonsdaleite was also found in an acid-tretment residue of the meteorite by an 63 XRPD analysis (Hanneman et al., 1967). 64 A PTS was prepared from a fragment of the Goalpara ureilite and observed by an optical 65 microscope. The meteorite mainly composed of coarse-grained olivine and pigeonite with 66 minor amounts of blade-like shaped carbon grains, being black in color and dull in luster 67 under a reflected light and up to 0.5 mm in size. Olivine and pyroxene crystals partly show 68 a mosaic texture, suggesting that the meteorite has been heavily shocked. Melting of 69 silicate minerals was not observed. 3 Revision 1 70 Micro Raman spectra of carbon grains in the PTS were recorded with a Jobin-Yvon 71 T64000 triple-grating spectrometer equipped with confocal optics and a nitrogen-cooled 72 CCD detector. A microscope was used to focus the 514.5 nm Ar exitation laser beam to a 1 73 µm spot. Accumulations lasting 120 seconds were made. The laser power on the sample 74 was 2 mW. 75 After micro Raman analyses, a typical carbon grain, a hundred µm in size, was removed 76 from the PTS and mounted on a glass fiber of ~10 µm in diameter for an XRPD analysis 77 using a 114 mm diameter Gandolfi camera (Gandolfi, 1967). A rotating anode x-ray 78 generator with a Cr-anode, 0.2 x 2 mm fine-filament, and a V-filter was used as an x-ray 79 source. The x-ray powder diffraction pattern was two-dimensionally recorded on an 80 imaging plate (IP), 35 mm wide and 350 mm long, and the intensity data on IP were read 81 by the 50 x 50 µm pixel size with a Fuji-film BAS-2500 IP-scanner. Intensities of 82 diffracted x-rays for each Bragg angle were obtained at intervals of 0.025º (2θ) by 83 averaging those along a Laue cone over 300 pixels of the central part of IP that had the 84 width of about 560 pixels. 85 SAED and HRTEM analyses were performed by using two thin slices of about 150 nm 86 thick which were prepared from the grain removed from a glass fiber used for the x-ray 87 analysis. FIB sample preparation was performed with a Ga ion beam by Hitachi FB-2000K 88 equipped with a micro-sampling unit at the Art, Science and Technology Center for 89 Cooperative Research, Kyushu University. One of the sides of the rectangular-shaped slice, 90 10 x 10 µm in size, was attached to the chipped edge of a copper disk of 3 mm in diameter 91 and set into TEM. SAED patterns and HRTEM imags were obtained by JEM-3200FSK 4 Revision 1 92 with a 300 kV accelerating voltage and a 400 mm camera length and recorded by a CCD 93 camera at the Research Laboratory for High Voltage Electron Microscopy of Kyushu 94 University. Diffraction patterns of Au were also obtained and used in order to correct d- 95 values of reflections of carbon minerals. 96 97 RESULTS 98 The mode of occurrence of carbon grains 99 Carbon grains occur along the grain boundaries of olivine with blade-like shapes (Fig. 100 1a). The rims of olivine crystals in contact with carbon grains include many tiny metal 101 blebs. The rims are thought to have been produced by redox reaction between olivine grains 102 and carbon when the parent body of ureilites has been break up (Goodrich, 1992; Goodrich 103 et al., 2004). The carbon grain, C1 in Figure 1a, was taken out of the PTS. The grain is 104 platy in shape with a thickness of 20 µm and a width of 100 x 100 µm (Fig. 1b). The shape 105 of the grain implies that the carbon grain has originally crystallized as a single crystal of 106 graphite (Berkley and Jones, 1982; Nakamuta and Aoki, 2000; Treiman and Berkley, 107 1994). 108 109 Raman spectroscopy 110 Raman spectra were measured at twenty points selected at random on the surface of the 111 C1 grain in the PTS (Fig. 1a). All spectra obtained are very similar to each other and show 112 a very broad Raman band spreading over the positions between 1200 and 1650 cm-1 and 113 high-fluorescence back ground (Fig. 2). The broad band has two peaks at around 1330 and 5 Revision 1 -1 -1 114 1590 cm , which may correspond to one-phonon band of diamond at 1332 cm and E2g 115 band of graphite at 1580 cm-1 (Hanfland et al., 1989), respectively. 116 117 XRPD analysis 118 An XRPD pattern was obtained by using the whole grain taken out of the PTS before 119 digging a hole to take a slice for TEM analysis.
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