Micron-sized Crystals Research Articles

High spatial resolution SIMS investigation of oxides formed on stainless steel under PWR conditions

High spatial resolution SIMS has been used to measure the variation in cobalt concentrations in oxide layers of stainless steel. Mass spectra from micron-sized crystals formed on the oxidised surface were recorded and depth profiles were taken through the inner oxide layer extending into the base metal. These measurements, used in conjuction with results from other more quantifiable techniques, allowed estimates to be made of the variation in cobalt concentration within the surface layers of the sample.

Three-dimensional structure analysis of the crystalline moiety of A-starch

Despite the fact the first first X-ray diffraction diagrams of starch were reported in the 1930s (I) , unambiguous description of the native crystalline arrangement is not yet complete. Kainuma and French (2) were the first to suggest a double-stranded helix for the B polymorph (commonly found in tubers). This origianl proposal was extensively considered by Wu and Sarko (3,4) who arrived at the conclusion that both B and A types (found in cereals) can be described as right-handed, parallel-stranded double helices, packed in an antiparallel fashion. However, A and B polymorphs differ considerably in the crystalline packing of the duplexes. Current models derived from biochemical data (5,6) rely only on parallel arrangements of molecules. Similar arguments about chain polarity also arise from biosynthetic considerations; it would be more reasonable to expect parallel packing of parallel-stranded double helices since all the chains would have the same polarity. There is therefore a discrepancy between the currently accepted models (7). The present paper is concerned with the threedimensional structure analysis of the A-type polymorph . New crystallographic data were obtained by using two complementary methods: electron diffraction on single, micron-sized crystals (8) and deconvolution of X-ray powder patterns (9) into individual peaks. Wide-angle X-ray diffraction diagrams recorded were similar to those displayed by native cereal starch granules and those corresponding to fibrallar samples. They all can be indexed using an orthorhombic-like unit cell having a volume of -2.2 nrrr'. According to the measured density of 1.48, 12 glucose residues, together with a few water molecules, can be accommodated in the unit cell. Computer modelling of IX (1-4)-linked glucose units indicated that the observed repeat of c = 1.069 nm can either correspond to an extended conformation of a maltotriose unit or to a shallow conformation of a maltoheaxose unit. Therefore, three structural hypotheses had to be tested against their packing constraints: (i) four single three-fold helices (n = 3, h = 0.355 nm); (ii) two six-fold helices (n == 6, h = 0.178 nm); and (iii) two double six-fold helices repeating in 2c = 2.139 nm [in order to restore the observed repeat of 1.069 nm, each double helix has to be parallel stranded, the single strands (n = 6, h = 0.355 nm) being associated to its counterpart through a two-fold axis]. All cases were thoroughly examined and only the latter was Shown to be a valid candidate. Further molecular modelling showed that the conformation at the glycosidic linkage corresponds to the intersectionof n = 6 and h -0.355 nm (left-handedchirality) (Figure I). Such a situation corresponds to one of the stable energy minima (10) and is reminiscent of the observed conformation of a crystalline amylose oligomer (11). Assembling two such parallel single strands through a two-fold symmetry operation permits forma-

Magnesioferrite from the Cretaceous-Tertiary boundary, Caravaca, Spain

Magnesioferrite grading toward magnetite has been identified as a very small but meaningful constituent of the basal iron-rich portion of the Cretaceous-Tertiary (K-T) boundary clay at the Barranco del Gredero section, Caravaca, Spain. This spinel-type phase and others of the spinel group, found in K-T boundary clays at many widely separated sites, have been proposed as representing unaltered remnants of ejecta deposited from an earth-girdling dust cloud formed from the impact of an asteroid or other large bolide at the end of the Cretaceous period. The magnesioferrite occurs as euhedral, frequently skeletal, micron-sized octahedral crystals. The magnesioferrite contains29 ± 11 ppb Ir, which accounts for only part of the Ir anomaly at this K-T boundary layer(52 ± 1 ppb Ir). Major element analyses of the magnesioferrite show variable compositions. Some minor solid solution exists toward hercynite-spinel and chromite-magnesiochromite. A trevorite-nichromite (NiFe 2O 4-NiCr 2O 4) component is also present. The analyses are very similar to those reported for sites at Furlo and Petriccio, Umbria, Italy. On the basis of the morphology and general composition of the magnesioferrite grains, rapid crystallization at high temperature is indicated, most likely directly from a vapor phase and in an environment of moderate oxygen fugacity. Elemental similarity with metallic alloy injected into rocks beneath two known impact craters suggests that part of the magnesioferrite may be derived from the vaporized chondritic bolide itself, or from the mantle; there is no supporting evidence for its derivation from crustal target rocks.

Dolomitization of Lower Paleozoic Burrow-Fillings

ABSTRACT Dolomite in an upper Ordovician sequence composed of the Irene Bay and Thumb Mountain Formations on Devon Island in the Canadian Arctic Archipelago is confined to burrows. Micron-sized dolomite crystals may have formed in burrows contemporaneous with deposition because of seasonal salinity changes in the Irene Bay-Thumb Mountain shelf lagoon. These penecontemporaneous dolomite crystals formed nuclei for the selective precipitation of late diagenetic dolomite from dilute subsurface solutions. Post-lithification crystal growth during late diagenesis caused idiotopic dolomite fabrics with intercrystalline micrite to become more coarsely crystalline xenotopic fabrics with no intercrystalline micrite. Dolomite crystal growth was accompanied by a progressive decrease in strontium and sodium contents and by a lowering of the amount of excess calcium. A possible rule regarding dolomite compositional variations is that high Mg/Ca solution ratios favor precipitation of more stoichiometric dolomite whereas low Mg/Ca solution ratios favour precipitation of calcium-rich dolomite with the exception that solutions of very low salinities will precipitate stoichiometric dolomite regardless of their Mg/Ca solution ratios.

Amoeboid olivine aggregates in the Allende meteorite

Greyish-brown, irregularly-shaped aggregates composed predominantly of olivine make up ~2% of the Allende meteorite by volume. Many of the aggregates are constructed of subspherical lumps of micron-sized crystals of olivine, pyroxene, nepheline and sodalite surrounded by coarsergrained olivine. Rarely, anorthite, spinel and perovskite are also present. The olivine ranges in composition from Fo64 to Fo99. Pyroxenes range from aluminous diopside to hedenbergite to very Al-rich and Ti-Al-rich varieties. The nepheline contains 1.6–2.4% K 2O and 1.6–5.2% CaO but the sodalite is significantly poorer in these elements. The spinel contains 2.1–13.4% FeO. Textural information and oxygen isotopic data suggest that the aggregates are composed of primary, solid condensates from the solar nebula. The perovskite. spinel and Ti-Al-rich pyroxenes are the remains of high-temperature condensates but the olivine compositions and the presence of feldspathoids indicate that some of the grains continued to react with the solar nebular vapor in the temperature range 500–900°K.

Mécanismes de croissance, polytypisme et polymorphisme de la muscovite hydroxylée synthétique

Synthetic OH-muscovite has been hydrothermally synthesized at T = 600 °C and 460 °C under 1000 bar water pressure using different starting mixtures. A systematic microtopographic study of basal faces of micron-sized crystals has been performed by means of replica techniques of conventional electron microscopy. Statistically, growth is found to proceed first through nucleation and layer-by-layer growth processes, subsequently followed by a screw dislocation mechanism. Careful spiral growth pattern studies lead to the determination of the different stacking sequences involved in the exposed ledges of the screw dislocations, i.e. of both periodicities and stacking schemes of the spiral grown polytypes. A pre-ordering of 1M, 2M 1, 3T[222] and 3T[ 2 2 2 ] type is found to occur first during layer-growth before any screw dislocation appears in these crystals, thereby indicating that common polymorphic ordering is muscovite is occuring rather during perfect growth than during dislocation growth. On crystals of 2M 1 initial structure, spirals are more frequently reproducing the 2M 1 structure than generating new polytypes like 3Tc[02 2 ] and 5Tc[0( 22 ) 2]. Structural determinations of polymorphs by means of the microtopographic method are in good agreement with powder X-ray identifications. In the last part of this paper, the nature of stacking faults in 1M and 2M 1 sequences, the role of the stacking sequence on the crystal habit, as well as the nature of growth twins in synthetic muscovite are emphasized. With the present experimental data in mind, the earlier reported growth theories on mica polymorphism and polytypism are discussed.

Experimental Vadose and Phreatic Cementation of Skeletal Carbonate Sand

ABSTRACT Some processes of and diagenesis in carbonate rocks have been simulated in the laboratory, using CO2-charged water to leach a bed of carbonate sand. Precipitation of calcite cement in a second sand body, above and below an artificial water table, was induced by CO2 evasion. Magnesian calcite was preferentially leached from the source bed, and calcite was precipitated as cement in the second sand unit. More cement was produced in the vadose zone than in the phreatic zone. Cement formed in the zone is composed of micron-sized crystals, and occurs as abundant rim cement and grain contact cement. Crystal size increases with distance from grain boundaries. Rounding of intergranular void spaces is common. Cement formed in the zone is more coarsely crystalline (50 microns) than in the and abuts directly against original skeletal grain boundaries. The texture of the cement closely resembles textures observed in cementation of skeletal calcarenites in Bermuda.

Dedolomitization fabric in peels; a possible clue to unconformity surfaces

Dedolomitization is usually a surface phenomenon related to weathering surfaces and hence is commonly found below unconformities. These unconformities may be ancient ones. Recognition of dedolomite in subsurface may suggest a possible unconformity. Thin sections and stained peels of dedolomite under plane polarized light may reveal a crystalline limestone devoid of ghosts of dolomite rhombs. However, if exactly the same area that has been examined under plane polarized light is viewed under crossed nicols the dolomite ghosts may become evident. These ghosts are diagnostic of dedolomitization. Peels are normally black under crossed nicols because the acetate is isotropic. Etching may bring out solubility differences between calcite crystals making up the matrix of the dedolomite and those in the original rhombs. During peel preparation, the micron-sized crystals occupying the original rhombs may become attached to the surface of the peel and hence show up as an anisotropic substance demarcating the dolomite rhombs as ghosts. Not all peels of dedolomite show dolomite rhombs under crossed nicols.

Polymeric growth from the solid state

Trioxane (CH 2O) 3 is partly transformed to polyoxymethylene (CH 2O) ∞ by irradiating with 1 MeV electrons and warming below the melting point. Polymer fibres along and growing parallel to the polar axis of the matrix nucleate on oriented micron-sized crystals which are inhomogeneously distributed. These occur preferentially on {00.1} sub-grain boundaries and among clusters of tiny bubbles left after irradiation, but tend not to occur around preexisting polymer and near crystal-hole boundaries. The technique of boundary decoration has been used to reveal details of strain patterns in indented crystals. By irradiating only small areas and observing the pattern of polymerization in and around these, clear evidence is found for an intermediary migrating through distances ∼1 mm which must be present with a minimum threshold concentration for the formation of fibrous polymer. At high doses, radiation damage tends to heal out.

Crystal-induced myocarditis and pericarditis

A patient exposed for years to tricalcium phosphate in the dust of a dentifrice died from alveolar adenocarcinoma. Micron-sized crystals of the salt were found in normal, inflamed, granulomatous and neoplastic areas in the viscera and in the mildly inflamed myocardium and pericardium. Excepting neoplastic changes, similar but severer lesions were induced experimentally in rats after ingestion of massive amounts of tricalcium phosphate, alpha quartz, barium sulfate, bismuth subcarbonate, and magnesium trisilicate. Particles of those substances appeared systemically and in the congested myocardium and pericardium with or without inflammation.