Period Of Crystal Growth Research Articles

Polyanion-mediated mineralization ? assembly and reorganization of acidic polysaccharides in the Golgi system of a coccolithophorid alga during mineral deposition

Immunolocalization of two highly acidic polysaccharides (PS-1 and PS-2) in a calcifying algaPleurochrysis carterae is described throughout the mineralization process, from before crystal nucleation through the cessation of crystal growth. This unicellular coccolithophorid alga is a useful model for mineralization because it produces calcified scales known as coccoliths in homogeneous cell culture. PS-1 and PS-2 were localized in the crystal coats of mature coccoliths and in electron dense Golgi particles. The polyanions are synthesized in medial Golgi cisternae and co-aggregate with calcium ions into discrete 25 nm particles. Particle-laden vesicles bud from cisternal margins and fuse with a coccolith-forming saccule containing an organic oval-shaped scale which forms the base of the future coccolith. The particles are localized on the base before the onset of mineral deposition and are present in the coccolith saccule throughout the period of crystal (CaCO3) nucleation and growth. During the final phase of coccolith formation, the particles disappear, and the mature crystals acquire an amorphous coat containing PS-1 and PS-2 polysaccharides which remain with the mineral phase after the coccoliths are extruded from the cell. Postulated mechanisms of polyanion-mediated mineralization are reviewed and their relevance to the calcification of coccoliths is addressed.

Growth behavior of L-pyrrolidone-2-carboxylic acid (L-PCA) single crystals

Growth conditions to prepare large single crystals of L-pyrrolidone-2-carboxylic acid (L-PCA), a novel nonlinear optical organic material, have been intensively studied with an attempt for fabrication in optical devices. L-PCA single crystals of good optical quality having dimensions of typically ~ 2.5 × 1.5 ×1.5 cm 3 were grown from aqueous solutions with seed crystals by a slow cooling method. They are chemically stable because molecules are hydrogen bonded to form three-dimensional networks in the crystal state. Further growth conditions to obtain large single crystals in a shorter time have been investigated by considering both mass balance equations and the kinetics for crystal growth. It was found that increasing the cooling rate in the later periods of crystal growth is advantageous for the purpose. The largest crystal was 17.5 g in weight, which was prepared in 6 days.

AN EXPERIMENTAL STUDY ON THE INITIATION AND GROWTH OF FROST FORMATION ON A HORIZONTAL PLATE

An experimental study was made to clarify the fundamental nature of the early stage of crystal growth period of frost formation phenomena. The effect of four dominant parameters on frost formation, namely, plate temperature, air temperature, air humidity ratio, and Reynolds number, are demonstrated through several frost formation properties such as frost height, frost deposition rate, and frost density. The thickness of the frost layer is affected primarily by the air humidity ratio, plate temperature, and air temperature, while the effect of Reynolds number is less significant. High humidity ratio, Reynolds number, and temperature difference between the air stream and the plate all yield high mass deposition rates. Frost density depends primarily on frost surface temperature, besides the other parameters. Vapor diffusion in the frost layer during the crystal growth period was found to be insignificant,

The influence of surfactants and water-soluble polymers on the crystallization of zeolite NaA

The crystallization of zeolite NaA in the presence of various surfactants (sodium dodecyl sulfate, Aerosol OT, cetyltrimethylammonium bromide, Nonidet P40) and of poly(ethylene glycol) was studied and compared to a control crystallization with no additive present. Crystalline-growth curves were determined by quantitative X-ray diffraction studies of samples removed periodically from the hydrothermal system. Crystal-size distributions and linear crystal-growth rates were determined by scanning electron microscopy, and from these data, nucleation profiles were constructed. Changes in the concentrations of soluble aluminum and silicon in the aqueous phase of the hydrothermal system during crystallization were monitored by inductively coupled plasma atomic emission spectroscopy. The additives were shown to lengthen the prenucleation period, shorten the nucleation period, and increase the linear rate of crystal growth. All but Aerosol OT increased the total number of nuclei produced and gave crystals with a reduced mean size and narrower size distribution. These effects may be attributed to an effective reduction in the water content of the hydrothermal system. Aerosol OT reduced the number of nuclei produced and gave larger crystals, an effect that may be attributed to a specific interaction with aluminum species. In general, the aluminum concentration in the aqueous phase was found to decrease dramatically during the period of crystal growth.

Mechanism and kinetics of the dehydration of magnesium oxalate dihydrate

This paper describes the process of random nucleation and subsequent growth of the crystal nucleus, starting by setting up a physical model. A formula for the specific rate of the reaction has been worked out both in the induction period and in the period of increasing reaction rate. Thus the calculation of activation energy has been done for both periods. Then, taking the dehydration of magnesium oxalate dihydrate as an example, the model and formula were both tested. Experiments prove that the nucleation processes comply with the power function rules. In the period of crystal nucleus formation and growth, the activation energy is 82.49 kJ mol −1. In the diffusion period the activation energy becomes larger when α increases. The result is in satisfactory agreement with literature data.

Fluctuation model for compositional heterogeneity in skarn clinopyroxenes.

A model incorporating experimentally determined fluid-crystal cation exchange equilibrium constants is proposed to explain compositional variations in clinopyroxene from Japanese skarns. The proposed “Fluctuation Model” can account for the observed compositional heterogeneity in both individual and aggregate clinopyroxene grains on a thin section scale. The model assumes formation from a hydrothermal solution whose mean concentrations of magnesium, manganese and iron components remained essentially constant throughout crystal growth period at constant temperature and pressure. Compositional heterogeneity developed as a result of local spatial and temporal fluctuations about the mean solution component concentrations. The degree of fluctuation in the proportion of component i in the hydrothermal solution, as defined by the percent coefficient of variation, CVi, is linearly related to the mean mole percent, Mi, of the component in the hydrothermal solution by the equation CVi=0.25×(100-Mi). Various compositional patterns off naturally occurring skarn clinopyroxenes can be closely reproduced by the model.

Nucleation, crystal growth and the thermal regime of cooling magmas

Crystallization at the margin of a quiet cooling magma has been studied numerically, taking into account the kinetics of crystallization. The variables are the latent heat value, the growth and nucleation functions, the initial magma temperature, and the thermal contrast between magma and country rock. We have investigated a wide range of values for these parameters corresponding to natural conditions. We show that after a highly transient stage, crystallization tends toward an equilibrium between heat production (latent heat release) and heat loss. Given the small diffusivity of country rocks, latent heat release is the main factor controlling the temperature evolution. In order to minimize the latent heat release, crystallization occurs at a temperature where nucleation is small. This can be close to either the liquidus or the solidus, depending on the initial conditions. The main process controlling crystallization is nucleation and not crystal growth. Nucleation occurs as a series of sharp pulses followed by longer periods of crystal growth. The nucleation pulses give birth to thermal oscillations. These oscillations can be sustained if the interior magma temperature is above the liquidus independently of the heat loss mechanism. We show that the phenomenon occurs on the scale of a few centimeters which corresponds to the inch‐scale layering of many ultrabasic complexes. The model allows us to calculate crystal sizes which are in good agreement with geological observations. The crucial parameters which determine crystal size variations near the margins of igneous bodies are the initial thermal conditions as well as the nucleation and growth functions. In the main cooling regime close to the liquidus, significant size variations can be created by small thermal disturbances.

Growth of a zoned plagioclase porphyroblast in a mylonite

Zoning patterns in a plagioclase porphyroblast in a blastomylonite derived from Lewisian gneiss near Loch Eriboll, northwest Scotland, suggest that the crystal grew during deformation. Discontinuities in the concentrations of CaO, K 2 O, and FeO mark the boundaries of overgrowths and define a core-overgrowth structure. The simultaneous changes in concentration of CaO and K 2 O are used to construct a qualitative chemical potential diagram that reflects changes in a Ca ++/ a 2 H + and a K + / a H + in the coexisting fluid during growth. Increases in these activity ratios are believed to have caused supersaturation of the grain boundary fluids with respect to plagioclase. The repeated periods of growth indicate repeated overstepping of equilibrium, which could have been caused by ion exchange on surfaces newly exposed during deformation. These data fit a model that relates cyclic variations of strain rate in fault zones with cyclic periods of crystal growth, which in turn could have a major influence on fault rock rheology.

セレン半導体の電気的性質について

As amorphous Se is heated, it transforms to metallic Se and to liquid Se at last. The following is the result of our experiments of how the electrical conductivity and photo-conductivity of Se varies with these transformations.At first, we placed a red Se between platinum electrodes as shown in Fig. 1. We heated them to melt the Se and cooled rapidly. Thus we obtained amorphous Se between the electrodes, which were connected to d.c. amplifier. We then measured the electrical conductivity and photoconductivity of Se while heating.The rate of heatihg was 0.8°C/min. These results are shown in Fig. 4.The electrical conductivity of Se varied with temperature, especially it changed suddenly at the transition point of Se.In Fig. 4, we consider that the part (A) is a amorphous phase, the part (D) a period of crystal growth, and the Se perfectly crystallized in (E). Electrical conductivity of Se begins to decrease rapidly at 220°C and recovers again later. We did not measure the electrical conductivity of Se above 320°C.Next, we measured the electrical conductivity as the temperature went down. The change from (G) to (E) was found to be reversible, but that from (D) to (A) was irreversible. From these results, it seems that the metallic Se is more stable than amorphous one at room temperature and amorphous Se is transformed to metallic Se by heating, but the reverse change does not seem to occur.PHOTO-CONDUCTIVITY IN (B), (C) AND (D).The photo-conductivity in each part of (B), (C) and (D) are shown in Fig. 7. The intensity of incident white light was 36080ergs/cm2·sec and the exposure time was 30sec. We took the maximum deflection of galvanometer in 30sec as the value of photo-current.In Fig. 7, σ is the electrical conductivity in the dark and Δσ is the difference in electrical conductivity between the cases in which the electrode was exposed and not exposed.It is interesting to know that the photo-conductivity reaches its maximum at (D).PHOTO-CONDUCTIVITY IN (H).In this part, we noticed some appreciable photo-currents, but our apparatus could not catch any photo-current at any higher temperature than this.

Factors that Affect the Formation of a Crystalline Deposit in Evaporated Milk

Summary The average weight of the calcium citrate crystals found in 300 14.5-oz. cans of crystal-bearing evaporated milk was 0.426g. per can. Many more cans were examined which contained no crystals. The occurrence of crystals could not be correlated with the time during the lactation period of the cow when the milk was produced, the movement or agitation the milk received during storage, the salt composition of the milk as determined analytically, or the pH or viscosity of the milk. Conditions usually favorable to the growth of calcium citrate crystals in evaporated milk were storage at temperatures above 60° F. and direct additions of CaCl 2 and citric acid solutions to the milk before sterilization. The period of maximum crystal growth was during the storage period from 4 to 20 mo. The growth of calcium citrate crystals in several batches of evaporated milk apparently was retarded by addition of disodium phosphate as a stabilizer before sterilization and by storage at low temperatures.