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Despite the industrial implications and worldwide abundance of gas hydrates, the formation mechanism of these compounds remains poorly understood. We report direct molecular dynamics simulations of the spontaneous nucleation and growth of methane hydrate. The multiple-microsecond trajectories offer detailed insight into the process of hydrate nucleation. Cooperative organization is observed to lead to methane adsorption onto planar faces of water and the fluctuating formation and dissociation of early hydrate cages. The early cages are mostly face-sharing partial small cages, favoring structure II; however, larger cages subsequently appear as a result of steric constraints and thermodynamic preference for the structure I phase. The resulting structure after nucleation and growth is a combination of the two dominant types of hydrate crystals (structure I and structure II), which are [inked by uncommon [5.sup.12][6.sup.3] cages that facilitate structure coexistence without an energetically unfavorable interface. 24 March 2009; accepted 10 September 2009 Published online 8 October 2009; 10.1126/science.1174010 Include this information when citing this paper.

Ferrihydrite is a key reactive nanoparticle in the Earth surface environment that regulates nutrient and metal cycling in marine, lacustrine, and groundwater settings; yet its genesis is unknown and there is no mechanistic explanation of its nanostructural nature. We develop a model, based on established aqueous precipitation theory, for rough-surface phases growing by accretion of monomers under fast diffusion conditions. The model is entirely defined in terms of a minimal set of necessary microscopic (molecular-scale) characteristic parameters. It is applied to ferrihydrite where the needed microscopic parameters are constrained by some of the mineral's known physico-chemical properties. Our model qualitatively reproduces the main nanostructural properties and known precipitation kinetics of ferrihydrite, predicting that ferrihydrite will be nanometric in size, will have a narrow particle size distribution (standard deviation width equal to a fraction of the average particle size), and will typically precipitate from supersaturated conditions within times ranging from a fraction of a second to several days, under normal environmental-proxy conditions. The model also suggests that different metastable structures of ferrihydrite having similar rough surfaces would kinetically compete during nucleation and growth to coexist in the final product, as observed by Janney et al. (2000a, 2001). Keywords: Ferrihydrite, precipitation, nucleation, oxyhydroxide, nanoparticle

High-quality three-dimensional photonic crystal (PC) heterostructures were fabricated using the modified self-assembly method, and their structural and optical properties were analyzed. Results suggest that the optical quality of heterostructures formed by depositing bigger particles on small ones is superior to that of heterostructures formed by stacking smaller particles on big ones, due to the rough interface effects in the latter structure. The roughness of the interface in the latter structure can be largely improved by introducing a thin two-dimensional planar defect layer into the PCs, and significant progress in the quality of the heterostructures is achieved. The important role of the thin planar defect layer in the quality of the heterostructures was also verified by numerical simulations. OCIS codes: 220.4000, 160.5293, 230.5298, 250.6715.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.11.014 Byline: Alessandro Mura (a), Peter Wurz (b), Herbert I.M. Lichtenegger (c), Helmold Schleicher (d), Helmut Lammer (c), Dominique Delcourt (e), Anna Milillo (a), Stefano Orsini (a), Stefano Massetti (a), Maxim L. Khodachenko (c) Keywords: Mercury; Atmospheres; dynamics Abstract: In this study we compare the sodium exosphere observations made by Schleicher et al. [Schleicher, H., and 4 colleagues, 2004. Astron. Astrophys. 425, 1119-1124] with the result of a detailed numerical simulation. The observations, made during the transit of Mercury across the solar disk on 7 May 2003, show a maximum of sodium emission near the polar regions, with north prevalence, and the presence of a dawn-dusk asymmetry. We interpret this distribution as the resulting effect of two combined processes: the solar wind proton precipitation causing chemical alteration of the surface, freeing the sodium atoms from their bounds in the crystalline structure on the surface, and the subsequent photon-stimulated and thermal desorption of the sodium atoms. While we find that the velocity distribution of photon desorbed sodium can explain the observed exosphere population, thermal desorption seems to play a minor role only causing a smearing at the locations where Na atoms are released on the dayside. The observed and simulated distributions agree very well with this hypothesis and indicate that the combination of the proposed processes is able to explain the observed features. Author Affiliation: (a) Istituto di Fisica dello Spazio Interplanetario-CNR, Rome, Italy (b) Physics Institute, University of Bern, Bern, Switzerland (c) Space Research Institute, Austrian Academy of Sciences, Graz, Austria (d) Kiepenheuer-Institut fuer Sonnenphysik, Freiburg, Germany (e) CETP-CNRS, Saint Maurice des Fosses, France Article History: Received 29 April 2008; Revised 23 October 2008; Accepted 10 November 2008

The real crystal structure of fukalite, [Ca.sub.4][Si.sub.2][O.sub.6][(OH).sub.2]([CO.sub.3]), was solved by means of the application of order-disorder (OD) theory and was refined through synchrotron radiation diffraction data from a single crystal. The examined sample came from the Gumeshevsk skarn copper porphyry deposit in the Central Urals, Russia. The selected crystal displays diffraction patterns characterized by strong reflections, which pointed to an orthorhombic sub-structure (the 'family structure' in the OD terminology), and additional weaker reflections that correspond to a monoclinic real structure. The refined cell parameters are a = 7.573(3), b = 23.364(5), c = 11.544(4) (Angstrom], [beta] = 109.15(1)[degrees], space group [P2.sub.1]/c. This unit cell corresponds to one of the six possible maximum degree of order (MDO) polytypes, as obtained by applying the OD procedure. The derivation of the six MDO polytypes is presented in the Appendix (1). The intensity data were collected at the Elettra synchrotron facility (Trieste, Italy); the structure refinement converged to R = 0.0342 for 1848 reflections with 1 > 2[sigma](I) and 0.0352 for all 1958 data. The structure of fukalite may be described as formed by distinct structural modules: a calcium polyhedral framework, formed by tobermorite-type polyhedral layers alternating along b with tilleyite-type zigzag polyhedral layers; silicate chains with repeat every fifth tetrahedron, running along a and linked to the calcium polyhedral layers on opposite sides; and finally rows of [CO.sub.3] groups parallel to (100) and stacked along a. Keywords: Crystal structure, fukalite, calcium carbonate silicate, OD structure

A detailed description of the interlayer site in trioctahedral true micas is presented based on a statistical appraisal ofcrystal-chemical, structural, and spectroscopic data determined on two sets of trioctahedral micas extensively studied by both X-ray diffraction refinement on single crystals (SCXRD) and X-ray absorption fine spectroscopy (XAFS) at the potassium K-edge. Spectroscopy was carried out on both random powders and oriented cleavage flakes, the latter setting taking advantage of the polarized character of synchrotron radiation. Such an approach (AXANES) is shown to be complementary to crystal-chemical investigation based on SC-XRD refinement. However, the results are not definitive as they focus on few samples having extreme features only (e.g., end-members, unusual compositions, and samples with extreme and well-identified substitution mechanisms). The experimental absorption K-edge (XANES) for potassium was decomposed by calculation and extrapolated into a full in-plane absorption component ([[sigma].sub.[parallel]]) and a full out-of-plane absorption component ([[sigma].sub.[perpendicular to]). These two patterns reflect different structural features: [[sigma].sub.[parallel]] represents the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets; [[sigma].sub.[perpendicular to] is associated with multiple-scattering interactions entering deep into the mica structure, thus also reflecting interactions with the heavy atoms (essentially Fe) located in the octahedral sheet. The out-of-plane patterns also provide insights into the electronic properties of the octahedral cations, such as their oxidation states (e.g., [Fe.sup.2+] and [Fe.sup.3+]) and their ordering (e.g., trans- vs. cis-setting). It is also possible to distinguish between F- and OH-rich micas due to peculiar absorption features originating from the F vs. OH occupancy of the 04 octahedral site. Thus, combining crystal-chemical, structural, and spectroscopic information is shown to be a practical method that allows, on one hand, assignment of the observed spectroscopic features to precise structural pathways followed by the photoelectron within the mica structure and, on the other hand, clarification of the amount of electronic interactions and forces acting onto the individual atoms at the various structural sites. Keywords: Crystal structure, X-ray absorption spectroscopy, XAFS, XRD, correlation, assignment

Computer modeling of microstructure development was used to determine whether competition for space among growing crystals modifies the crystal-size distribution (CSD) predicted by the crystallization kinetics. Microstructures were modeled with prisms, plates, and cuboids, respectively. In all cases, the true CSDs calculated from crystal volumes in the microstructure corresponded closely with the linear ideal CSDs predicted by crystallization equations indicating that grain impingements did not significantly modify the predicted CSD information. Crystal intersection widths and lengths were measured in 2-dimensional slices through the microstructures to test if the CSD information could be recovered. For prisms and plates, the recovered CSDs compared favorably with the true CSDs, but cuboids yielded mixed results depending on their shapes and need further study. For prisms, the recovered CSDs were linear and for plates slightly curvilinear. These results indicate that rocks with recovered, curvilinear CSDs should be interpreted cautiously as indicators of complex crystallization histories, and that petrographic examination should have precedence in such interpretations. Keyword: Crystal-size distribution, CSD, microstructure, computer modeling

Coexistence of twisted and untwisted crystals is explained via a model that accounts for the coupling of the entropic and energetic effects of impurities and a supra-lattice-scale structural order parameter. It is shown that twisted impure crystals can be in equilibrium with untwisted purer ones. The model explains how coexistence can occur in agates and other systems under hydrostatic stress. The model implies that untwisted crystals grown under one set of conditions could undergo a phase separation that, when accompanied by an imposed compositional gradient, leads to commonly observed, alternating bands of twisted and untwisted crystals and, when occurring in the absence of an external gradient, mossy patterns of crystal texture can emerge. This phenomenon is not related to anisotropic applied stress. Rather coexistence is a consequence of a compositional segregation/twist phase transition. Since twist coexistence is a compositional equilibrium, it arises from the exchange between bulk phases; hence, the detailed nature of the atomic structure within an interface between twisted and untwisted zones is not relevant. The approach places crystal-twist phenomena within the theory of order/disorder phase transitions. Keywords: Agate, agate banding, free energy model, phase separation, quartz fibers, self-organization, twisted crystals, twisted/untwisted crystal equilibria

The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural Mg[Al.sub.2][O.sub.4] spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions, were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, [V.sub.0], the bulk modulus, [K.sub.T0], and its first pressure derivative, K', were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: [V.sub.0] = 529.32(2) [[Angstrom].sup.3], [K.sub.T0] = 193(1) GPa, K' = 5.6(3) for the ordered sample and [V.sub.0] = 528.39(2) [[Angstrom].sup.3], [K.sub.T0] = 192(1) GPa, K' = 5.4(3) for the disordered one. Complete intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence on the degree of order in the Mg[Al.sub.2][O.sub.4] spinel. Keywords: X-ray single-crystal diffraction, spinel, cation ordering, high pressure

Using two hexagonal samples of ice, polycrystalline and single-crystalline, the electrical treeing has been investigated under an applied impulse voltage. The crystallinity and temperature of the ice play an important role in the events of the treeing. In the single-crystalline ice, the tree in the samples at -25 [degrees]C always progressed along the basal plane of the ice crystal, but at -196 [degrees]C, it progressed along the c-axis. These distinctive patterns of the tree progression were attributed to the crystal axis dependence on the electrical conductivity and the relative permittivity at each temperature. The current accompanying the trees were detected by means of an electro-optic coupling with light emitting diodes and photodiodes. After the large current pulses which occur just after voltage application, small intermittent current pulses were visible in the wave tail of the applied voltage. These intermittent pulses appear to generate by the movement of accumulated charges after the formation of tree channels. The electrical breakdown strength of ice at -25 [degrees]C was large for the electric field parallel to the c-axis of the single-crystalline ice. Index Terms--Hexagonal ice crystals, polycrystalline ice, single-crystalline ice, standard lightning impulse voltage, electrical treeing, discharge current, breakdown probability.

This paper describes a microplasticity model for analyzing the variation of cutting force in ultra-precision diamond turning. The model takes into account the effect of material anisotropy due to the changing crystallographic orientation of workpieces being cut. A spectrum analysis technique is deployed to extract the features of the cutting force patterns. The model has been verified through a series of cutting experiments conducted on aluminum single crystals with different crystallographic cutting planes. The results indicate that the model can predict well the patterns of the cutting force variation. It is also found that there exists a fundamental cyclic frequency of variation of cutting force per revolution of the workpiece. Such a frequency is shown to be closely related to the crystallographic orientation of the materials being cut. The successful development of the microplasticity model provides a quantitative means for explaining periodic fluctuation of micro-cutting force in diamond turning of crystalline materials. [DOI: 10.1115/1.1454108]

Thermodynamics of the growth of epitaxial strained nanocrystals of Ge on Si(001) in relation to multiple shapes, size distribution, metastable species, and transient behavior is reviewed. Relevant models to describe biogenic and solution-phase synthesis, Ostwald ripening and surface stabilization, and thermodynamic interactions of nanocrystals are discussed.

The crystal structure of 1,2-diphenyl-5,7-di-tert-butylspiro(2.5)octa-1,4,7-trien-6-one is examined using X-ray analysis. It was observed that although the C-C-C spiro angle in 3(49.64(7) degrees) is one of the smallest known for carbon atom, it has no effect on the relationship between C=O bond length and the difference between the single and double bond lengths in the cyclohexadienone moiety. These findings show that the neutral 3 is not a good model for the charged bridged vinylidenephenonium as indicated by both structural criteria and NRT analyses.

The Celli-Flytzanis lattice model of snapping bonds was observed to have a continuum limit in which the displacement w(x,y,t) related to the steady-state propagation with velocity v, the transverse sound velocity being c, satisfies a nonlinear differential equation. The effect of the moving screw dislocation was described significantly by the nonlinear term proportional (partial derivative of w with respect to t)(partial derivative of w with respect to x). Numerical consequences of the solitary wave equation were examined.

A crystal chemistry-based technique for estimating site types and ionizable site densities on a variety of mineral surfaces is detailed. The minerals used included quartz, anorthite, albite, sanidine, sillimanite, andalusite, kaolinite, corundum, hematite, goethite, rutile and periclase, based on their adsorption and dissolution properties.

The thermodynamic characteristics of silicon (diamond cubic phase) were computed utilizing an empirical many-body potential developed by Tersoff based on the principle of bond order. The model provided values which were close to empirical data for properties governed by energetics. The thermal expansion coefficient was less accurate because the model potential was too stiff and incapable of representing properly the volume dependence of the transverse acoustic modes. Also, the sensitivity of the potential to whether each atom remains bonded to only four neighbors showed that the short-range behavior of the potential may require model improvement before being applied to thermomechanical characteristics at high temperatures or large deformations.

The heuristic Kronig-Penney model can be employed to compute the energy bands of a linear crystal where the periodic potential is approximated by a meanderlike one-dimensional potential. It is shown that microcomputers can be utilized in making graphic plots of energy bands in k-space as important physical relationships. Without these plots, either the solution cannot be determined explicitly because of mathematical difficulties or it is oversimplified.

A crystal-structure simulation is done using available experimental chemical data to study crystal chemical response of basalt clinopyroxene to increasing pressure. Clinopyroxene behavior in a magmatic environment is greatly influenced by pressure. An empirical geobarometer, based on relationship of cell volume and M1-site volume, is made using the fact that there is internal consistency of simulation data. Construction and working of the geobarometer are discussed.

Octahedral units (OUs) can be used as the building blocks for the construction of molecular or crystalline models. These models, simply constructed from paper ribbon, are helpful visual aids for teaching stereochemistry and coordination chemistry. The steps for making pyramidal components, OUs, and octahedra are described.

Lattice energy calculations involving atomic electrostatic and van der Waals parameters are made on model two-dimensional crystals of hydrocarbon chains and fluorocarbon chains. The electrostatic parameters for the CF2 groups are obtained from an X-ray study of the deformation electron density of perfluoroglutaramide. The results indicate that empirical lattice energy calculations can clarify the molecular assembling processes in relaxed systems of insoluble amphiphilic molecules.

Three-dimensional models are necessary to fully appreciate solid-state chemistry and crystal structure and yet enough models to accurately portray the various crystal structures are prohibitive in space, time and money. However, a single illuminated model can solve these problems by allowing students to observe multiple cubic system arrangements on one structure without significant set-up time required. More than 1024 structures are possible with minor rewiring. Construction directions and suggestions for using the model are included.

Olefin pom pons are more effective for making different cubic crystal three-dimensional models for chemistry classes than styrofoam balls. They are inexpensive and available in bulk. The pom pons come in a variety of colors and so do not need to be painted. They are durable enough that students can examine them closely and even assemble the different packing arrangements themselves.

Three-dimensional models of crystals can be inexpensively built by chemistry students using square tissue culture Petri dishes with multiwell plates. Two colors of marbles are used to represent the atoms and the 25 compartments in the dishes can be used for the coordinates. A kit costs $10 and includes instructions for building various structures in a three-hour exercise for sophomores. Pictures of models of TiO2 and CsCl are included.

The selective photocoupling reaction occurring in crystalline adduct of tetracyanobenzene (TCNB) with benzyl cyanide was described. The crystal structural conformation suggested that TCNB and BzCN were interacting through weak charge-transfer (CT) forces. The photochemistry between TCBN-BzCN system in the solution was very different from that in the solid state.

Researches on locating a good model of a real crystal reveal that dust particles in ionized plasma can be organized into unique crystals called plasma crystals. This discovery has implications for studies of the crystalline state, reactions in interstellar dust clouds and dust-contamination of newly formed surfaces in microelectronics fabrication. Particle distribution in horizontal planes of this plasma crystal are more orderly than those with a random point distribution in a plane, confirming that this new crystal is a dusty plasma crystal.