Heterogeneous Crystals Research Articles

Anomalous crystal optics of heterogeneous crystals

Various approaches to analysis of the characteristics of optically heterogeneous crystals (with the characteristic thicknesses of layering lamellae exceeding the light wavelength) are considered in the example of layered two-component heterogeneous structures. In the simplest cases, the quantitative information on the layering structure may be obtained from analysis of numerical characteristics such as the moments of the appearance of visible isogyres or the positions of isotropic compensation points. In more complicated cases, it may be reasonable to use the program specially written by the authors for constructing conoscopic figures of heterogeneous crystals. Finally, the heterogeneity of complex crystals may be characterized proceeding from the qualitative estimate of the degree of distortion of their conoscopic figures.

Crystal Growth Studies in a Lime-Aluminosilicate Glass System

The crystallization of wollastonite-type, (Ca, Mg, Fe)SiO 3 , in the system of Al 2 O 3 -CaO-Fe 2 O 3 -K 2 O-MgO-MnO 2 -Na 2 O-P 2 O 5 -SiO 2 -TiO 2 has been studied over the temperature range of 1148-1348 K. A linear growth rate ofsurface crystallization of the wollastonite-type has been observed for the glass system. The crystal growth rate was measured as a function of temperature in a platinum crucible contact regime where heterogeneous crystal has been found to occur. The beginning of the crystallized-layer growth was preceded by a considerable induction period. The activation energy barrier for crystallization of the wollastonite-type at large undercoolings was determined to be 222.20 kJ.mol - 1 .

Compositional heterogeneity of olivine in thermally metamorphosed serpentinite from Southwest Japan

Compositionally heterogeneous crystals of olivine occur in thermally metamorphosed serpentinites from Southwest Japan. They have a variation in forsterite (Fo) content up to 10 mol% within a specimen. The chemical heterogeneity is the result of two causes: the intermingling of metamorphic neoblasts with relict crystals of primary olivine, and the compositional variation within the neoblasts themselves. The metamorphic olivine in each specimen shows a bimodal distribution of Fo content with a highly magnesian group (Fo93-98, varying among specimens) and a relatively ferroan one (Fo85-94). Textural relationships and the variation of NiO and FeO contents between the two groups of olivine suggest that the heterogeneity results from the local involvement of magnetite and awaruite in dehydration reactions of serpentine at the initial stages of metamorphism

In situmonitoring of powder reactions in percolating solution by wet-cell X-ray diffraction techniques

This note describes how the kinetics of powder reactions in percolating solution can be studied by X-ray diffraction using a wet-cell flow-through reaction chamber. The device can be routinely moved between diffractometer and controlled laboratory (pressure, temperature) conditions with the ease of a standard powder holder. Short-termin situmeasurements and long-termquasiin situmonitoring of dissolution and crystallization reactions are possible with a minimum of sample preparation and little disturbance of the system. Measuring time-dependent changes in the concentration of crystalline reactants and products provides information for quantifying reaction kinetics and for determining dissolution and crystal growth mechanisms. Results can be compared with changes in solution chemistry of the collected eluate, enabling a more complete reconstruction of heterogeneous crystal–solution reactions in open systems.

Systematic analysis of K-feldspar 40Ar/39Ar step heating results II: relevance of laboratory argon diffusion properties to nature

We examine a database containing the results of 40Ar/39Ar step-heating experiments performed on 194 basement K-feldspars to recover thermal history information. Qualitative examination of 40Ar/39Ar systematics reveals that about half of the K-feldspars examined are sufficiently well behaved to be suitable for thermal history analysis. Correlation algorithms are developed to quantitatively assess the degree to which age and 39Ar release spectra are compatible with the same volume diffusion process. Upon applying these methods, we find that 65% of all samples yield correlation coefficients in excess of 0.8, whereas roughly 40% give values above 0.9. We further compare the observed correlation behavior with that predicted from the multidiffusion domain model and find good agreement for samples with correlation coefficients above 0.9. In contrast, hydrous phases unstable under in vacuo heating and K-feldspars with highly disturbed age spectra yield poorly correlated age and diffusion properties. The high degree of correlation exhibited by the majority of K-feldspars we have analyzed validates extrapolation of experimentally determined diffusion properties to conditions attending natural Ar loss within the crust. Despite this, a significant number of basement K-feldspars analyzed by the step-heating method yield 40Ar/39Ar systematics that are clearly problematic for thermal history analysis. We numerically explore the effects of low-temperature alteration of K-feldspar on thermochronological analysis and identify a range of conditions under which information is progressively lost. Finally, we demonstrate the insensitivity of thermal history calculations to detailed knowledge of the diffusion mechanism by introducing the heterogeneous diffusion model. We find that the multidiffusion domain approach can successfully recover imposed thermal histories from heterogeneous diffusion-type crystals and conclude that most details of the interpretive model employed are of secondary importance. The only requirement for recovering thermal histories from K-feldspar 40Ar/39Ar step-heating results is that argon loss proceeds by volume diffusion and that laboratory argon release adequately mimics the natural diffusion boundaries and mechanisms—a requirement implicitly met by those samples exhibiting high degrees of correlation.

Recent remobilisation of shallow-level intrusions on Montserrat revealed by hydrogen isotope composition of amphiboles

Ion probe measurements of hydrogen isotopes were made on amphiboles representing different stages of the ongoing eruption of the Soufrière Hills Volcano, Montserrat. The majority (80%) of the andesitic amphiboles show relative intra- and inter-crystal δD homogeneity, with a mean of −38±12‰, consistent with known primary magmatic values. The remainder (20%) of amphiboles show marked δD heterogeneity, with a mean of −6±30‰. The heterogeneous amphiboles have ∼100 μm rims with δD values similar to the homogeneous crystals, but core values which are significantly heavier than primary magmatic values. Early in the eruption 50% of amphiboles are isotopically heterogeneous but post spring 1996 all amphiboles are homogeneous. We interpret these ion probe data in terms of development of a shallow andesitic magma chamber by repeated emplacement of andesitic magma in the shallow crust, over at least the last 100 yr. We suggest such shallow intrusions, emplaced during the volcano–seismic crises preceding the ongoing eruption, solidified. Some regions interacted with hydrothermal fluids and isotopic exchange took place generating the heavy hydrogen isotope signatures in the heterogeneous amphiboles. On onset of the ongoing eruption, such old igneous material was remobilised, with intimate mixing between the new magma batch and the old intrusions indicated by the close proximity of homogeneous and heterogeneous crystals. The isotopically primary magmatic rims on the heterogeneous crystals indicate hydrogen isotope exchange over a period of a few weeks, consistent with timescales of magma ascent at the Soufrière Hills Volcano. Once the old igneous material was flushed out, by spring 1996, the eruption proceeded by extruding material dominated by the new batch of magma. This resulted in a marked increase in extrusion rate.

Occurrence of Bacillus thuringiensis on cured tobacco leaves.

A worldwide survey was conducted to evaluate the frequency and distribution of Bacillus thuringiensis populations on cured tobacco leaves during post-harvest storage. In total, 133 tobacco samples of different types and origins were analyzed. Nine percent of the samples showed the presence of B. thuringiensis, and 24 B. thuringiensis strains were isolated and characterized. The majority of the isolates produced bipyramidal crystals, and three fourths of them showed a second type of crystal protein (cuboidal or heterogeneous crystals). Only three isolates showed the rhomboidal crystal morphology characteristic of the anti-coleopteran B. thuringiensis subsp. tenebrionis. PCR analysis with primers specific for cry1 and cry3 genes revealed eight distinct cry gene profiles. The results of this study indicate that B. thuringiensis is naturally present at low frequency on the phylloplane of cured tobacco leaves and that its distribution is worldwide.

Electrically controlled light scattering effect in the emulsion “liquid crystal - isotropic liquid”

New heterogeneous liquid crystal (LC) system with electrically controlled light scattering is suggested. It is an emulsion of isotropic liquid in the nematic LC. Electro-optical characteristics of such system are reported.

Role of Dislocations in Silver Halide Photography

Dislocations and donor centers have important functions in the optimization of the performance of silver halide emulsion grains. The basic properties of dislocations relevant to photographic sensitivity together with the experimental observations which established these properties are reviewed in this paper. Internal latent image and internal particles of photolytic silver are formed during exposure of AgCl and AgBr crystals, sensitized with Ag2O and Ag2 donor centers, by the separation of Ag atoms along the dislocation lines. The surface sites of termination of dislocations have enhanced reactivity compared with low-energy surfaces. Dissolution, chemical sensitization, and the initiation of surface chemical development occur at higher rates at these sites. These properties led to the concept of the dislocation sensitivity center. Development centers are formed by the combination of Ag atoms with Ag2 latent image growth nucleus precursors at the surfaces of dislocation sensitivity centers. The formation of Ag2 molecules along the subsurface dislocation of the center has to be minimized. Microcrystals of AgCl and AgBr with dislocations introduced during nucleation and initial growth pass into solution at a high rate during a short period of ripening leaving dislocation-free growth nuclei. For stable distributions of dislocations in microcrystals, concentration gradients of halide ions have to be established in heterogeneous crystals. Controlled densities of dislocations are introduced to reduce the elastic strains associated with steep concentration gradients. In double-structure grains with a higher iodide shell and in triple-structure grains with a higher iodide narrow zone, the concentration gradients can be established by the addition of a fine dispersion of Ag(Br,I) particles or of an iodide ion releasing compound. The resulting dislocation distributions are stable because of the immobility of the iodide ions in the crystal. The mechanisms involved in the creation of dislocations in structured tabular microcrystals are discussed.

3-D Imaging of Crystals at Atomic Resolution

ABSTRACTElectron crystallography has now been used to investigate the structures of inorganic materials in three dimensions. As a test of the method, amplitudes and phases of structure factors were obtained experimentally from high resolution images of staurolite taken in a number of different projections. From images in five orientations, a three-dimensional Coulomb potential map was constructed with a resolution of better than 1.4Å. The map clearly resolves all the cations (Al,Si,Fe) in the structure, and all of the oxygen atoms. This method promises great potential for structure determinations of small domains in heterogeneous crystals which are inaccessible to x-ray analysis. Three-dimensional structure determinations should be possible on small domains only approximately 10 unit cells wide, and may resolve site occupancies in addition to atom positions. Given a microscope stage with a suitable range of tilt and enough mechanical stability, the method could also be applied to small crystalline particles larger than about 50Å to 100Å. In addition, it may be possible to apply the method to derive the two-dimensional structure of periodic defects.

3D structure determination from electron-microscope images: electron crystallography of staurolite.

Resolution of better than 2 A has been obtained in many crystals by high-resolution electron microscopy. Although this resolution is sufficient to resolve interatomic spacings, structures are traditionally interpreted by comparing experimental images with contrast calculations. A drawback of this method is that images are 2D projections in which information is invariably obscured by overlap of atoms. 3D electron crystallography, developed by biophysicists to study proteins, has been used to investigate the crystal structure of staurolite. Amplitudes and phases of structure factors are obtained experimentally from high-resolution images (JEOL ARM 1000 at the National Center for Electron Microscopy at LBL), taken in different directions from thin regions where dynamic scattering is minimal. From images in five orientations (containing 59 independent reflections to a resolution of 1.38 A), a 3D electron potential map is constructed which resolves clearly all cations (Al, Si, Fe, including those with partial occupancy) and all O atoms. This method has great potential in crystal structure determinations of small domains in heterogeneous crystals which are inaccessible to X-ray analysis. It is estimated that 3D structure determinations should be possible on regions only about ten unit cells wide and should resolve not only atom positions but also site occupancies. The method is also applicable to space-group determination.

Micro-analysis of sulfur-isotope ratios and zonation by laser microprobe

The University of Wisconsin laser microprobe/mass spectrometer system combines high spatial resolution with precise, accurate, and rapid analysis of sulfur-isotope ratios. Spot sizes of 100–200 μm are routine and much higher spatial resolution may be possible. Analytical precision ranges from 1 σ = ±0.15%. (pyrite, pyrrhotite, sphalerite) to 1 σ = ±0.43%.. (galena, chalcopyrite). Comparison of laser microprobe and conventional combustion analyses indicates that laser δ 34S values are approximately equal to or lower than combustion values for these minerals, and an empirical correction of −0.1‰ (sphalerite) to +1.4%. (pyrrhotite) is required. Variations in δ 18O of SO 2, laser power density, and burn duration affect the magnitude of this correction; thus, consistent technique is critical. Natural samples were analyzed by laser microprobe to evaluate the extent of sulfur-isotope ratio zonation. Coexisting sulfide mineral pairs from the Rua Cove Mine, Alaska, show that isotopic equilibrium exists in the stockwork feeder zone, while disequilibrium characterizes coexisting pairs from the overlying, more rapidly cooled massive sulfide blanket. Conventional sulfur isotope extraction data on these samples are not valid due to the fine-grained, intergrown nature of the sulfide phases, which precludes satisfactory mineral separation. Isotopically homogeneous sphalerite and pyrite crystals were found in samples from the amphibolite facies Balmat Mine, while isotopically heterogeneous pyrite crystals were found in a sample from the upper greenschist facies Sullivan Mine. Sulfur-isotope zonation up to 2.9%. in 200 μm was found in banded sphalerite blacksmoker chimney material from the Bayda Mine (Oman). Retrograde sulfides from the Marcy Anorthosite Massif vary by 4.2%. over <1 cm, and finely disseminated sulfides from an amphibolite-granulite facies transition zone sample, southern India, are in disequilibrium across the 25 cm facies transition. These results show that sulfur isotope homogeneity may be attained in the cores of annealed high-grade metamorphic minerals while lower temperature, retrograde, or hydrothermally deposited minerals are often isotopically zoned due to temporal variations in fluid chemistry and temperature.

Microhardness and surface free energy in linear polyethylene: The role of entanglements

The microhardness of a series of melt crystallized samples of linear polyehtylene was investigated in a wide range of molecular weights. The x-ray long period was analyzed to study the variation of the hardness-derived constantb as a function of molecular weight (Mη). It is pointed out thatb offers a measure of the hardness depression due to the finite thickness of the lamellar crystals. The data obtained show that the increase and final leveling-off (forMη 200 000) ofb withMη parallels the concurrent increase of the surface free energy, as derived from DSC experiments. Results are discussed using the concept og chain folded lamellae as thermodynamically stable non-homogeneous microphases. Comparison of experimental and calculated data supports the view that the number of molecular entanglements, segregated onto the defective surface boundary of the heterogeneous crystals influence the shearing mechanism within the “mesocrystals” and thereby control the yield behavior of the material.

Precipitation kinetics; elementary processes

The paper contains the results of our previous investigations on the crystallization of ZnC 2O 4 · 2H 2O and a more detailed study of the behaviour of the solid phase based on the evolution of the crystal size distribution during the process. The overall supersaturation range (2.9 ⪕ S ⪕ 100) consists of three subranges which are characterized by different combinations of the elementary crystallization processes: (a) rapid growth of heterogeneous primary crystals accompanied by secondary processes (breaking and secondary nucleation) at low supersaturations ( S ⪕ 4 at pH = 2 and S ⪕ 20 at pH = 5); (b) molecular growth of primary crystals at medium supersaturations; (c) coagulation growth of homogeneous primary crystals followed by “pulsing” and formation of crystalline “coagulates” at high supersaturations ( S ⪖ 14 at pH = 2 and S ⪖ 75 at pH = 5).

The Determination of Melting Points in Human Fats

Summary.The “melting point” of fats generally means the upper limit of the temperature range through which the melting of the fat takes place, or, when determined according to empirical methods, a point very near this.When the melting point of human fats were determined according to the common methods the results were not reproducible.Therefore the melting process was investigated by following the changes during the melting process in some samples of human fat. It was found that the difficulties in the determinations on human fats were due to the characteristic that even several degrees below the real clear point only a very small fraction of solids is present. A fat as e. g. coconut oil has 3 per cent solids at 0.3° C beiow the upper limit of the melting range. In a human fat the solid fraction of 3 per cent was present at 6° C below the upper limit of the melting range. This means that the viscosity of this fat is low, even at a considerable distance below the “clear point”. Therefore the empirical “constants” which are characterized by a certain low viscosity of the sample, viz. the softening point, the slipping point etc. are found far from the clear point and varying much with small variations in the external conditions.When a sample of human fat to be used for melting point determination solidifies at 0° C heterogeneous crystals separate, which can produce large errors in the observed melting point. To avoid this the sample must be rapidly cooled down to an extremely low temperature (–70° C). In this way the fat solidifies as a microcrystalline mass which makes possible a relatively exact observation of the clear point. It was, by comparing with the dilatometric method, demonstrated that the clear point determined according to the author's procedure is really identical with the thermodynamically fixed upper limit of the melting range.The tendency of the fats to form unstable modifications with low melting points is discussed. Further it is demonstrated that srom a thermodynamical point of view the melting point and the folidifying point are identical.