Mechanisms, Growth Rates, and Morphologies of Gas Hydrates of Carbon Dioxide, Methane, and Their Mixtures

Mechanisms of growth and dissociation, growth rates, and morphology of gas hydrates of methane, carbon dioxide, and two CH4:CO2 mixtures (80:20 and 30:70 nominal concentration) were studied using using high resolution images and very precise temperature control. Subcooling and a recently proposed mass transfer-based driving force were used to analyze the results. When crystal growth rates did not exceed 0.01 mm/s, all systems showed faceted, euhedral crystal habits at low driving forces. At higher driving forces and growth rates, morphologies were different for all systems. These results solve apparent contradictions in literature about the morphology of hydrates of methane, carbon dioxide, and their mixtures. Differences in the growth mechanism of methane-rich and carbon dioxide-rich hydrates were elucidated. It was also shown that hydrate growth of methane, carbon dioxide, and their mixtures proceed via partial dissociation of the growing crystal. Temperature gradients were used to dissociate hydrates at specific locations, which revealed a most interesting phenomenon: On dissociation, carbon dioxide-rich hydrates propagated onto the bare substrate while drawing water from the opposite side of the sample. Furthermore, it was shown that an abrupt change in morphology common to all systems could be correlated to a change in the slope of growth rate data. This change in morphology was explained by a shift in the crystal growth mechanism.

Application of the Fractal Dimension Calculation Technique to Determine the Shape of Selected Monchepluton Intrusion Crystals (NE Fennoscandia)

Thirty-eight samples of minerals from Paleoproterozoic Layered PGE Intrusion Monchepluton, located in NE Fennoscandia, were tested. An automated computational technique was used which involved counting the sides superimposed on vectorized graphics using separated crystal boundaries. The results were obtained for olivine, orthopyroxene, clinopyroxene, and plagioclase. On this basis, an interpretation of the mineral box fractal dimension was made, along with an interpretation of its nature in the rock in which it was found. The performed calculations were applied to the sampling positions, and maps of changes in fractal dimensions were prepared. The nature of the minerals studied was correlated with the type of rocks present in Monchepluton. Then, the fractal dimensions were scaled to a percentage scale determining the mean value for the euhedral crystal as 100%, and a map was obtained representing the advancement of secondary processes after summing these data for all four investigated minerals. This method was analyzed and its advantages and limitations shown.

Gold in the Farallones Block of the Shale-Hosted, Clastic-Dominated Castellanos Zinc-Lead Deposit (Northwest Cuba)

The Zn-Pb ores of the Castellanos shale-hosted, clastic-dominated deposit in northwest Cuba average nearly 1 g/t Au, with local maximum concentrations up to 34 g/t Au. This deposit is stratiform with respect to the bedding in the host black shales and shows a bottom to top zoning of ore assemblages made up of a stockwork underlying the main orebody, a basal pyrite-rich zone and a disseminated to massive Zn-Pb ore zone capped by a discontinuous, thin barite-rich zone. Petrographic data and textural relations allow distinguishing five textural types of pyrite (framboidal Py I, colloform Py IIa, euhedral Py IIb, massive Py IIc and banded colloform Py III) successively formed during ore deposition. The main Zn-Pb ore formed after the crystallization of disseminated, sedimentary framboidal pyrite (Py I) in black shales by the superimposition of several crystallization events. The crystallization sequence of the main ore-forming stage evolved from the precipitation of colloform sphalerite and pyrite (Py IIa) with skeletal galena and interstitial dolomite-ankerite to similar ore assemblages but showing subhedral to euhedral crystal habits (Py IIb) and interstitial calcite-rich carbonates. This stage ended with the development of massive pyrite (Py IIc), mainly occurring at the base of the stratiform orebody. A late fracturing stage gave way to the development of a new generation of colloform banded pyrite (Py III) just preceding the crystallization of early barite. Au is mainly concentrated in pyrite showing variable contents in the different textural types of pyrite and a bottom to top enrichment trend. Minimum contents occur in massive pyrite (Py IIc) from the basal pyrite-rich zone (0.18 ppm Au average), increasing in pyrite IIa (from 0.29 to 2.86 ppm Au average) and in euhedral pyrite (Py IIb) (from 0.82 to 9.02 ppm Au average), reaching maxima in colloform banded pyrite (Py III) formed just before the crystallization of early barite at the top of the orebody. Au enrichment in pyrite correlates with that of Sb (0.08–4420 ppm), As (0.7–35,000 ppm), Ag (0.03–1560 ppm) and to a lesser extent Cu (3–25,000 ppm), Ni (0.02–1600 ppm) and Mn (0.6–5030 ppm). Au deposition should have taken place by oxidation and, probably cooling, of reduced (H2S-dominated) fluids buffered by organic matter-rich black shales of the host sedimentary sequence. The input of such reduced fluids in the ore-forming environment most probably occurred alternating with that of the main oxidized fluids which leached Zn and Pb from the large volume of sandstones and siltstones making up the enclosing sequence, thus being responsible for the precipitation of the majority Zn-Pb ore. Supply of Au-carrying reduced fluids might progressively increase over the course of ore formation, reaching a maximum at the beginning of the late fracturing stage. This evolution of Au supply is consistent with the early crystallization of barite since Ba can also only be transported at low temperature by highly reduced fluids. These results highlight the potential of medium-sized, shale-hosted, clastic-dominated deposits to contain economic (by product) Au amounts and show that ore-forming fluids can change from oxidized (SO42+ dominated) to reduced (H2S-dominated), and vice versa, throughout the evolutionary history of a single deposit.

Minerály skupiny columbitu a mikrolitu v granitovom pegmatite pri Liešťanoch: prvý výskyt vzácnoprvkovej Nb-Ta mineralizácie v Strážovských vrchoch (Slovenská republika)

Accessory minerals of columbite and microlite groups were identified in granitic pegmatite dike intruded into parental Carboniferous (~350 Ma) leucogranites of the crystalline basement of the Tatric Unit, Central Western Carpathians. The pegmatite is situated on E slope of Bystrý Hill near Liešťany village, the Strážovské vrchy Mts., Slovakia. Primary columbite-(Fe) forms euhedral crystal (~3 mm across) with diffuse internal zoning reflecting a relatively small compositional variations: Mn/(Mn + Fe) = 0.40 - 0.45 and Ta/(Ta + Nb) = 0.21 - 0.24. Secondary anhedral domains of Ta-rich columbite-(Fe) to tantalite-(Fe) (≤200 μm) with Mn/(Mn + Fe) = 0.45 - 0.47 and Ta/(Ta + Nb) = 0.45 - 0.62 partly replace primary columbite-(Fe) along crystal rims. Moreover, secondary subhedral crystals of microlite-group minerals (≤25 μm) form fracture fillings in columbite-(Fe). The microlites show uniform high Ta/(Ta + Nb) ratio (0.77 - 0.80) and U content (7.7 - 10.2 wt.% UO2; 0.18 - 0.21 U apfu) but different contents of F, Ca, Na and Pb: central parts locally show fluorcalciomicrolite composition (~2 wt.% F, ~9.5 wt.% CaO, 2.2 - 2.7 wt.% Na2O), whereas main microlite mass forms zero-valent-dominant microlite with inclusions (≤8 μm) of Pb-rich zero-valent-dominant microlite (16.8 - 19.7 wt.% PbO; 0.46 - 0.56 Pb apfu). Textural relationships and chemical compositions of Nb-Ta minerals indicate primary magmatic origin of columbite-(Fe) and post-magmatic (early subsolidus to late hydrothermal) formation of secondary Ta-rich columbite-(Fe) to tantalite-(Fe) and microlite-group minerals.

Synneusis: does its preservation imply magma mixing?

The Ghansura Felsic Dome (GFD) occurring in the Bathani volcano-sedimentary sequence was intruded by mafic magma during its evolution leading to magma mixing. In addition to the mafic and felsic rocks, a porphyritic intermediate rock occurs in the GFD. The study of this rock may significantly contribute toward understanding the magmatic evolution of the Ghansura dome. The porphyritic rock preserves several textures indicating its hybrid nature, i.e. that it is a product of mafic-felsic magma mixing. Here, we aim to explain the origin of the intermediate rock with the help of textural features and mineral compositions. Monomineralic aggregates or glomerocrysts of plagioclase give the rock its characteristic porphyritic appearance. The fact that the plagioclase crystals constituting the glomerocrysts are joined along prominent euhedral crystal faces suggests the role of synneusis in the formation of the glomerocrysts. The compositions of the glomerocryst plagioclases are similar to those of plagioclases in the mafic rocks. The results from this study indicate that the porphyritic intermediate rock formed by the mixing of a crystal-rich mafic magma and a crystal-poor felsic melt.

Synthesis of by the Flux Method

has been successfully synthesized using Bi2O3–B2O3 eutectic flux. In particular, we succeeded in synthesizing a low-temperature-phase crystal (α-) at 1073 K as well as high-temperature-phase crystal (β-). The morphology of α- and β- particles prepared by the flux method is a euhedral crystal. In contrast, the morphology of particles prepared by solid state reaction differs: α- is aggregated and β- is necked. Ultraviolet-visible diffuse reflectance spectra indicate that the absorption edge is at a longer wavelength for β- than for α- with β- absorbing light of wavelengths up to nearly 400 nm.

Facet crystallography by electron diffraction in the SEM

The study of fracture in engineering materials often involves an analysis of the crystallography of the fracture surface. In particular, the question is often asked, "What, if any, low index plane corresponds to the plane of a particular fracture surface facet?" To determine the crystallographic plane of a surface facet, it is necessary to determine the orientation of the grain and the orientation of the facet plane relative to the grain. For example, if a euhedral crystal of known orientation is fractured, an optical reflection goniometer can be used to measure the angles between a facet and known crystal faces in order to deduce the direction of the facet normal. Laue x-ray diffraction patterns taken from well aligned facets can also be analyzed to determine the orientation of the crystal normal to the facet. In many engineering materials, the facets are small, usually as a result of a small grain size in the material, and it becomes impractical to use these techniques.

Gradient Metamorphism, Zonal Weakening of the Snow-pack and Avalanche Initiation

A co-ordinated study involving both a field investigation in Yellowstone National Park and a laboratory investigation was undertaken to evaluate the process of temperature-gradient metamorphism on the mechanical properties of snow. Both parts of the investigation showed that, when subjected to a negative temperature gradient, low-to-medium density snow metamorphoses first into a fine-grained anhedral depth hoar before finally acquiring a fully developed, and stronger, euhedral crystal structure. Measurements indicated that the subhedral snow was the weakest stage in the metamorphic process and that, while strength may drop to as low as 10% of the original strength, the material stiffness decreased by less than 50% Also, it was observed that the location of weakest snow was usually the point of a local maximum density and largest temperature gradient, thus suggesting a relationship between metamorphic state and thermal conductivity.