Yoshihiro Nakamuta

Bulk mineralogy of individual micrometeorites determined by X-ray diffraction analysis and transmission electron microscopy

Abstract Bulk mineralogy of individual fine-grained micrometeorites from 50 to 200 μm in diameter was determined on the basis of the powder X-ray diffraction patterns and the observation of internal textures by a transmission electron microscope (TEM). X-ray diffraction analysis of 56 micrometeorites indicated that 42, 11, and 3 samples are olivine-rich, pyroxene-rich, and phyllosilicate-rich micrometeorites, respectively. Among the phyllosilicate-rich micrometeorites, one contains saponite and other two contain serpentine. No samples contain both saponite and serpentine. We found that saponite-rich micrometeorite was weakly heated, which results in shrinkage of 001 basal spacing of saponite down to 9.7 A, and that cronstedtite, which is commonly contained in CM chondrites, occurs in serpentine-rich micrometeorites. Micrometeorites that consist entirely of anhydrous minerals and amorphous phases are predominant in the samples studied. The major phases of such micrometeorites are olivine, low-Ca pyroxene, magnetite, and Fe-sulfide and the average abundances are 65, 17, 11, and 7 wt%, respectively, when the total abundance of the four minerals are normalized to 100 wt%. The relative mineral abundance varies greatly between samples: low-Ca pyroxene/olivine ratios range from 0 to 3.5, with a mean of 0.3. TEM observations of inner portions of some micrometeorites revealed that they are aggregates of very small equigranular grains (∼100 nm) of olivine + magnetite, or low-Ca pyroxene + olivine + magnesiowustite. The textures are very similar to those of hydrous carbonaceous chondrite that was experimentally heated to temperature below melting point, thus suggesting that the micrometeorites had been hydrous particles but were decomposed by the brief heating upon atmospheric entry. It is newly found that magnesiowustite was formed in micrometeorites instead of magnetite as a product of phyllosilicate decomposition under low oxygen fugacity. The decomposed hydrous micrometeorites gave two types of characteristic X-ray diffraction patterns: (1) broad olivine and magnetite reflections or (2) variable intensities of magnesiowustite reflections together with magnetite, low-Ca pyroxene, and olivine reflections. Twenty-nine olivine- or pyroxene-rich micrometeorites showed such diffraction patterns, thus suggesting that more than half of micrometeorites investigated must be decomposed hydrous particles. The results confirmed that hydrous dust particles are much more abundant in the interplanetary space than in the micrometeorites recovered on the Earth.

Transformation of graphite to lonsdaleite and diamond in the Goalpara ureilite directly observed by TEM

Abstract This study reports on the structural relationship between graphite, lonsdaleite, and diamond extracted from the Goalpara ureilite and propose a model for the formation of lonsdaleite and diamond in these stony meteorites. The study is based on data from reflected-light microscopy and laser Raman spectroscopy of a polished thin section (PTS) of the Goalpara ureilite and X-ray powder diffraction (XRPD) analyses of the grains taken out of it. Selected-area electron diffraction (SAED) analyses and high-resolution TEM (HRTEM) observations were carried out in the three unique directions of pristine graphite with two thin slices prepared from a carbon grain directly taken out of a PTS. SAED patterns reveal the relative crystal-axes orientations between graphite (Gr), lonsdaleite (Lo), and diamond (Di) as (001)Gr//(100)Lo//(111)Di, [210]Gr//[001]Lo//[21̅1̅]Di, and (12̅0)Gr//(1̅20)Lo//(02̅2)Di. The shapes of diffraction spots in the SAED patterns reveal that the transformation of graphite to lonsdaleite and diamond is initiated by sliding of hexagonal carbon planes of graphite along the [210] of the graphite structure. These results suggest that lonsdaleite and diamond in ureilites formed directly from graphite through boat-type buckling and chair-type puckering of hexagonal carbon planes of graphite, respectively. The results of this study confirm the shock origin of diamond in ureilites.

Shimazakiite-4M and shimazakiite-4O, Ca2B2O5, two polytypes of a new mineral from Fuka, Okayama Prefecture, Japan

Abstract Shimazakiite occurs as greyish white aggregates up to 3 mm in diameter. Two polytypes, shimazakiite- 4M and shimazakiite-4O, have been identified, the former in nanometre-sized twin lamellae and the latter in micrometre-sized lamellae. Shimazakiite was discovered in an irregular vein in crystalline limestone near gehlenite-spurrite skarns at Fuka mine, Okayama Prefecture, Japan. Associated minerals include takedaite, sibirskite, olshanskyite, parasibirskite, nifontovite, calcite and an uncharacterized hydrous calcium borate. The mineral is biaxial (-), with the following refractive indices (at 589 nm): α = 1.586(2), β = 1.650(2), γ = 1.667(2) and 2Vcalc = 53º [shimazakiite-4M]; and α = 1.584(2), β = 1.648(2), γ = 1.670(2) and 2Vcalc = 54.88º [shimazakiite-4O]. Quantitative electronmicroprobe analyses (means of 28 and 25 determinations) gave the empirical formulae Ca2B1.92O4.76(OH)0.24 and Ca2B1.92O4.76(OH)0.24 for shimazakiite-4M and shimazakiite-4O, respectively. The crystal structure refinements: P21/c, a = 3.5485(12), b = 6.352(2), c = 19.254(6) Å , β = 92.393(13)º, V = 433.6(3) Å3 [for shimazakiite-4M]; and P212121, a = 3.55645(8), b = 6.35194(15), c = 19.2534(5) Å , V = 434.941(18) Å3[for shimazakiite-4O], converged into R1 indices of 0.1273 and 0.0142, respectively. The crystal structure of shimazakiite consists of a layer containing B2O5 units (two near-coplanar triangular corner-sharing BO3 groups) and 6- and 7-coordinate Ca atoms. Different sequences in the c direction of four layers are observed in the polytypes. The five strongest lines in the powder-diffraction pattern [listed as d in Å (I)(hkl)] are: 3.02(84)(022); 2.92(100)(104̅) 2.81(56)(104); 2.76(32)(113); 1.880(32)(118̅,126̅,126,118) [for shimazakiite-4M]; and 3.84(33)(014); 3.02(42)(022); 2.86(100)(104); 2.79(29)(113); 1.903(44)(126,118) [for shimazakiite-4O].

Penetration twins of potassium chloride

Abstract A large number of penetration twins were formed during growth of KCl crystals from aqueous solution. All of them are interpenetrating octahedra or cuboctahedra with well defined rotation angles about the [100], [110] and rarely the [111] axis common to two individuals. Twin formation is favored by Pb 2+ and relatively high supersaturation. The maximum yield is obtained within the range 0.4–0.6 g Pb 2+ per liter H 2 O and σ=0.1–0.4. Both geometry and reticular aspects are discussed in terms of rotation twinning, on the ground of coincidence-site lattices. Good agreement is obtained between the rotation angles observed in the twins and those expected from the application of coincidence-site lattice theory.

Dendritic diamonds synthesized by simple hot-filament-assisted chemical vapor deposition

Abstract Dendritic diamond crystals have been formed by using a simple method of hot-filament-assisted chemical vapor deposition. A dendritic single crystal consists of three square prisms elongated to the direction of [100], [010] and [001], and one to four triangular pyramids elongated to 〈111〉. Twins and aggregates attached to dendritic individuals are also found. Dendritic diamonds are formed under the conditions of higher filament temperature and substrate temperature than those for polyhedral diamond growth.

Orthorhombic polymorph of rengeite from Ohmi region, central Japan

Abstract Rengeite, Sr4ZrTi4(Si2O7)2O8, is a Sr-analog mineral of perrierite and occurs as an accessory mineral in natrolite, pectolite, and itoigawaite veins associated with jadeitic rock from Ohmi-Itoigawa region, Japan. The symmetry of this mineral is reported to be monoclinic. Here, we found an orthorhombic polymorph of rengeite that occurs as micro domains up to 5 pm in width in monoclinic rengeite crystals using electron diffraction and high-resolution TEM analyses. X-ray diffraction analysis using a Gandolfi camera also revealed the presence of the orthorhombic phase in monoclinic rengeite crystals. The unit-cell dimensions of the orthorhombic polymorph are a = 14.0, b = 5.7, c = 21.9 Å, V= 1748 Å3, and Z = 4. Although the space group of orthorhombic polymorph of rengeite has not been determined, the possible space group is Pbca or a similar one from theoretical consideration by Ito (1950). The chemical composition was determined by ATEM-EDS analysis to be SiO2 = 22.5, TiO2 = 30.2, SrO = 39.4, Nb2O5 = 0.2, Fe2O3 = 0.3, and ZrO2 = 7.5, totaling 100.1 wt%, and the ideal chemical formula is Sr4ZrTi4(Si2O7)2O8. Detailed HRTEM imaging revealed that the unit cell of the orthorhombic polymorph of rengeite can be interpreted as a superstructure of monoclinic rengeite, accompanied by repeated twinning on (001) plane. The HRTEM images taken were consistent with the simulated images

Arrowhead-like diamond crystals formed by hot-filament chemical vapor deposition

Abstract Arrowhead-like diamond crystals have been formed by using a simple method of hot-filament-assisted chemical vapor deposition. These are contact twins with {111} as the twin plane, of which each individual is composed of {100}, {110} and {111} faces. These twins flatten along {110} face and elongate parallel to {111} contact plane. The flattened {110} face consists of many {110} terraces sided by 〈110〉 and 〈112〉 steps. So the twinned crystal looks like an arrowhead. These twins are formed just underneath the uppermost substrate temperature for diamond growth.

Formation of tetrapod-like crystals of diamond formed by hot-filament chemical vapor deposition: effects of preformation of tungsten carbide on the substrate

Abstract A large number of diamond crystals with tetrapod-like form were obtained when a tungsten thin plate was used as the substrate of hot-filament-assisted chemical vapor deposition. In this case, the substrate was covered by hexagonal tungsten carbide (WC) thin films and subsequently diamond crystals grew on them by facing its own {111} to the substrate. The sites of carbon atoms on {111} of diamond are in good accord with those on (001) of WC. Hence WC microcrystals provided nucleation sites for subsequent diamond growth. Namely on (001) of WC, {111} of diamond grew epitaxially, constructing the coincidence site lattice (CSL).

Regional Patterns of Diagenetic Alteration in the Tertiary of the Kyushu-Korea Region: Implications for Plate Tectonics at the Eastern Asian Continental Margin

ABSTRACT Studies of organic and inorganic diagenesis of Tertiary successions in the Kyushu-Korea region was combined with systematic correlation of the onshore geology. The degree of diagenesis can be attributed to two types of alteration: regional burial diagenesis in each locality was accelerated by additional heat supply from a zone of high paleotemperature in the Sea of Japan. This zone is related to the opening of the Sea of Japan in the Eocene.