Pre-existing Crystals Research Articles

The effects of pre-existing dislocations on the mechanical properties of iron

In atomistic simulations, pre-existing dislocations have been reported to reduce the yield stress compared to the ideal crystals. However, the underlying physics behind yield stress reduction is still unrevealed, which hammers the design of advanced materials. Here, large-scale molecular dynamics simulations are carried out to investigate the influence of pre-existing dislocations on the mechanical properties of body-centered cubic Fe crystals with dislocation, twinning, and phase transformation-dominated deformation mechanisms. The results suggest that the overestimated yield stress of all the crystals is significantly reduced by increasing dislocation numbers and obtaining closer flow stress on the uniform plastic deformation stage. This reduction in yield stress can be attributed to the lower thermo-dynamical driving force required to activate existing dislocations in pre-existing dislocation crystals than that to nucleating new dislocations in ideal crystals. Furthermore, pre-existing dislocations inhibited the phase transformation-dominated deformation process, but the twinning/dislocation-dominated deformation process still exhibited its original deformation mechanism.

Crystallization and time-dependent changes in betaine-urea-water natural deep eutectic solvents

The ability to predict eutectic compositions resulting in a liquid (natural) deep eutectic solvent [(NA)DES] at room temperature or below widens their applicability range. This study examines the effect of the components’ molar ratio and water content on the stability and supramolecular network of betaine-urea-water (BUW) NADES. It also explores time-dependent changes in the solvents and possible methods to determine the stability. After being prepared at elevated temperatures (≥70 °C), some BUW crystallized upon cooling or seeding with pre-existing crystals, creating orthorhombic betaine hydrate crystals. BUW ratios yielding liquid NADES were determined by proton nuclear magnetic resonance (1H NMR), identifying a solid–liquid boundary. Moreover, BUW conductivity decreased continually over time, suggesting a dynamic state of the solvents. Our results indicate that betaine crystallization depends on water and urea concentrations, supporting the reported synergistic effect. Additionally, this work demonstrates 1H NMR as a method to predict BUW ratios forming liquid NADES.

Effects of the Gas-Atomization Pressure and Annealing Temperature on the Microstructure and Performance of FeSiBCuNb Nanocrystalline Soft Magnetic Composites.

FeSiBCuNb powders prepared by the gas atomization method generally exhibit a wide particle size distribution and a high degree of sphericity. In addition, the correspondingly prepared nanocrystalline soft magnetic composites (NSMCs) perform good service stability. In this paper, effects of the gas-atomization pressure and annealing temperature on the microstructure and soft magnetic properties of FeSiBCuNb powders and NSMCs are investigated. The results show that the powders obtained by a higher gas-atomization pressure possess a larger amorphous ratio and a smaller average crystallite size, which contribute to the better soft magnetic performance of the NSMCs. After being annealed at 550 °C for 60 min, the NSMCs show a much better performance than those treated by the stress-relief annealing process under 300 °C, which indicates that the optimization of the soft magnetic properties resulting from the precipitation of the α-Fe(Si) nanocrystalline largely overwhelms the deterioration caused by the grain growth of the pre-existing crystals. In addition, the annealed NSMCs prepared by the powders with the gas-atomization pressure of 4 MPa show the best performance in this work, μe = 33.32 (f = 100 kHz), Hc = 73.08 A/m and Pcv = 33.242 mW/cm3 (f = 100 kHz, Bm = 20 mT, sine wave).

Effect of pre-existing crystals and melt homogeneity on the decompression-induced crystallization of hydrous rhyodacite magma

Abstract Decompression-induced crystallization is an important process that controls the behavior of volcanic eruptions because it strongly affects magma rheology and degassing behavior in the shallow parts of volcanic conduits. Several decompression experiments have been performed to understand and model the crystallization processes; however, the effect of superheating (i.e., heating above the liquidus temperature for a definite period of time) before decompression has not been elucidated, despite the proposal of its importance in previous cooling experiments. As the superheating influences the number of pre-existing crystals and melt homogeneity, it is expected to control decompression-induced crystallization. In this study, we investigated the effects of pre-existing crystals and melt homogeneity on crystallization during the decompression of rhyodacitic magma at a temperature of 900 °C. The magma studied herein has a liquidus temperature of ~920 °C. Five starting materials were prepared via heating at different super-liquidus temperatures (940, 970, 1050, and 1300 °C) and a sub-liquidus temperature (900 °C) using an internally heated pressure vessel and a cold-seal pressure vessel, respectively. Decompression experiments using these starting materials were conducted from 130 to 30 MPa at decompression rates of 5, 20, and 100 MPa h–1. When the melt was completely homogenized (at 1050 and 1300 °C), no crystals were formed at 100 MPa h–1 and the small amounts of crystals heterogeneously formed along the capsule wall were found at 5 and 20 MPa h–1. At the same decompression rate, the number density of plagioclase formed during decompression increased as the superheating temperature decreased from 970 to 900 °C, despite the higher number densities of pre-existing crystals before decompression in the samples with lower superheating. Such finding indicates that nucleation occurs easily when the number density is initially high. This result is inconsistent with the idea that nucleation occurs when supersaturation is sufficient to overcome the energy barrier for nucleation, and the growth of pre-existing crystals decreases supersaturation. In contrast, the results of our experiments can be explained by considering that higher superheating results in a more homogeneous melt structure with few pre-crystal clusters, which are growth sites, and ultimately the suppression of nucleation. Based on these results, we conclude that pre-existing crystals and melt homogeneity strongly affect the crystal texture formed by decompression. For application to natural systems, the high number density of microlites found in natural samples may be due to heterogeneous nucleation caused by the presence of pre-crystal clusters and other mechanisms. Furthermore, the superheating of magma in a reservoir caused by the injection of high-temperature mafic magma may influence the crystal texture during magma ascent and, hence, control the explosivity of the eruption.

Phase-Field Modeling of Crystal Growth on the Surface and in the Volume of Na2O-2CaO-3SiO2 Glass under Periodic Temperature Conditions.

We performed modeling of two- and three-dimensional phase-field methods (2D- and 3D-PFM) for crystal growth on the surface and in the volume of Na2O-2CaO-3SiO2 glass to investigate crystal growth behaviors under periodic temperature conditions. In this study, the periodic temperature conditions were set to 993 K for 180 s and 873 K for 252 s repeatedly. Phase-field mobilities, L S and L B, were determined to compare with the experimental surface crystal growth rate, u S, and the volume crystal growth rate, u B, at 873-1023 K. 2D-PFM with L S and L B reproduced quantitatively the temperature dependence of u S and u B. The parameters of L S and L B were consistent with those of 11 kinds of silicate melts, considering the surface and bulk diffusion coefficients. 3D-PFM simulated the single- and multinucleated crystal growth behaviors: the single-nucleated crystal simulation revealed that a ring was formed around the pre-existing crystal by heterogeneous nucleation. These radii obtained by 3D-PFM were comparable to the experimental values. The multinucleated crystal simulation revealed the contact and interaction between the crystals, e.g., new crystal rings could not be formed at the contacting region. In random nucleation, the 3D-PFM simulation demonstrated the crystal shape of the multinucleated crystals under periodic temperature conditions. It was comparable to the experimental photographs obtained by Yuritsyn et al.

Formation Mechanism of Flower-like Polyacrylonitrile Particles.

Flower-like polyacrylonitrile (PAN) particles have shown promising performance for numerous applications, including sensors, catalysis, and energy storage. However, the detailed formation process of these unique structures during polymerization has not been investigated. Here, we elucidate the formation process of flower-like PAN particles through a series of in situ and ex situ experiments. We have the following key findings. First, lamellar petals within the flower-like particles were predominantly orthorhombic PAN crystals. Second, branching of the lamellae during the particle formation arose from PAN's fast nucleation and growth on pre-existing PAN crystals, which was driven by the poor solubility of PAN in the reaction solvent. Third, the particles were formed to maintain a constant center-to-center distance during the reaction. The separation distance was attributed to strong electrostatic repulsion, which resulted in the final particles' spherical shape and uniform size. Lastly, we employed the understanding of the formation mechanism to tune the PAN particles' morphology using several experimental parameters including incorporating comonomers, changing temperature, adding nucleation seeds, and adjusting the monomer concentration. These findings provide important insights into the bottom-up design of advanced nanostructured PAN-based materials and controlled polymer nanostructure self-assemblies.

Molecular dynamics simulation of seeded crystal growth in glass

The mechanism of non-congruent growth of a crystal from glass has been sought using molecular dynamics simulations. Specifically, as a model of this process, the growth of a lithium niobate (LiNbO 3 ) crystal seed sandwiched between two lithium niobosilicate (LNS) glass slabs has been simulated as a function of time and temperature. The growth of pre-existing crystal is strongly affected by the orientation of crystal seed, temperature, and the SiO 2 concentration in the surrounding LNS glass matrix. The orientation of LiNbO 3 seed surface that has inherently larger interplanar distance results in a relatively slower crystal growth. The addition of SiO 2 to LNS system significantly decreases the crystal growth, which primarily occurs in the region devoid of Si. The suppressive effect of SiO 2 on growth rate can be traced to the existence of defect complex comprising of Si substituted at the Nb site and a nearby Nb vacancy. • Crystal growth is observed in interfacial structures containing lithium niobosilicate glasses and lithium niobate crystals. • Crystal orientation with large interplanar space results in high crystal growth rate. • High SiO 2 content suppresses the formation of ordered structures. • [Si Nb -V Nb ] defect complex exists in LiNbO 3 crystal at low SiO 2 concentration.

Assessing glass‐ceramic homogeneity and nucleation self‐correlation by crystallization statistics

AbstractIn this work, we implemented and tested a statistical method to evaluate the microstructural uniformity of partially crystallized glasses employing the aggregation index (R‐index), a parameter derived from the Poisson distribution function, which has mainly been used in Ecology studies. Since the crystal nucleation rate strongly depends on the chemical composition, the spatial crystal distribution in glass‐ceramics can be used to infer their chemical homogeneity. We also tested the hypothesis, advanced by some authors, of preferential secondary nucleation close to preexisting crystals (even in chemically homogeneous samples). To this end, we conducted a nearest‐neighbor statistical analysis of the spatial crystal distribution in partially crystallized Li2Si2O5 and Ba5Si8O21 glasses, used as model materials, by inspecting optical micrographs obtained at different magnifications. The resultant R‐indexes indicate a very high degree of homogeneity of the crystal number distribution, reflecting the uniform distribution of the chemical elements in the parent glasses. Moreover, the results for both glasses refute the suggestion that crystal nucleation is self‐correlated. These outcomes allow us to suggest the R‐index as a valuable and easily implemented tool to evaluate the chemical homogeneity of glasses that undergo internal nucleation, such as those used for glass‐ceramics.

Gradient microstructure evolution under thermo-mechanical coupling effects for a nickel-based powder metallurgy superalloy－Dynamic recrystallization coexist with static recrystallization

In order to withstand special working conditions, it is of great significance to manufacture turbine disks with a gradient microstructure (disk core with fine and uniform grains and disk edge with coarse grains) along the radial direction. A new and effective forming way-gradient hot deformation is proposed, and a gradient structure from the edge to the center can be obtained. In the present work, the gradient deformation behavior, dynamic recrystallization kinetics, and microstructural evolution of a nickel-based powder metallurgy superalloy are studied by thermal gradient compression. Dynamic recrystallization kinetics of this superalloy is established. Recrystallization fraction increases from 72.6 %–93.6 % because of the gradient deformation degree. Microstructure changes significantly from the center to the edge along the radial direction, and average grain size ranges from 6.65 μm to 15.62 μm. There is a big difference between experimental value and calculated data of recrystallization fraction, which indicates that dynamic and static recrystallization take place in different areas simultaneously. Static recrystallization can coexist with dynamic recrystallization in the trapezoid sample by gradient hot deformation. The difference between experimental and calculated results is the static recrystallization, and the maximum values of static recrystallization fraction is around 30 %. A great number of dynamic recrystallization grains distribute along pre-existing grain boundary, second phases and twin crystal. The main means of static recrystallization nucleation is the bulge of original grain boundaries.

Comparison of Microwave Short Time and Oven Heating Pretreatment on Crystallization of Raisins.

Crystalline material can develop on the surface of raisins during storage and transport, affecting the final acceptability of the product. In this work, a mild thermal pretreatment was applied to raisins to melt the pre-existing crystals and the effect of such thermal treatments on the development of crystals over a storage period was investigated. The raisins selected for this study were of the Thompson seedless variety from one Chilean company. The thermal pretreatment of raisins at 50 °C and 70 °C for 20 min in an oven and microwave (800 W) irradiation for 15 s resulted in a reduction in the percentage of crystallized raisins (w/w) from more than 50% in the control samples to less than 10% after 35 days of storage at 15 and 25 °C in a 57% relative humidity environment. The results showed that some textural parameters, such as cohesiveness and chewiness, were not affected by thermal treatment and were independent of storage temperature.

Identifying hydrothermal quartz vein generations in the Taiyangshan porphyry Cu-Mo deposit (West Qinling, China) using cathodoluminescence, trace element geochemistry, and fluid inclusions

Porphyry-style mineralization is characterized by superposition of multiple generations of quartz growth, punctuated by dissolution and juxtaposed within a single vein. Such superposition inevitably obscures temporal relationships between quartz formation, alteration events, and fluid inclusion assemblages (FIAs), and consequently complicates the interpretation of fluid origin and evolution. Very detailed information of these relationships recorded within hydrothermal quartz from the Taiyangshan porphyry Cu-Mo deposit (NW China) is obtained using integrated cathodoluminescence (CL) imaging, trace element analyses, and fluid inclusion petrography.The CL characteristics range from bright blue-CL quartz (Q1) associated with early high-temperature potassic alteration, to dark blue-CL quartz (Q2) broadly coinciding with metal precipitation, to post-ore dull red-CL fine-grained, anhedral, and recrystallized quartz (Q3) subgrains. Early bright blue-CL quartz (Q1) crystals, generally at the edges of stockwork veinlets close to wallrock, are characterized by oscillatory growth zoning, and exhibit alignment of primary, equant-shaped, brine-bearing FIAs along the banded growth zones. Later dark blue-CL quartz (Q2) crystals are generally euhedral in shape and grow with c-axes pointing inward to the vein center. They exhibit well-defined, but rarely preserved, growth zones truncating Q1 and crystal terminations projecting into pyrite and chalcopyrite in the vein center although they are not spatially associated with sulfides. These vein features clearly indicate that Q2 crystals grow into an open cavity or fracture during the ore precipitation. This is also evident by coexisting, secondary, irregular-shaped, liquid-rich inclusions and vapor-rich inclusions along fractures transecting Q1 oscillatory growth bands. The coexistence of hypersaline and vapor-rich inclusions moreover provides unequivocal evidence for phase separation during mineralization. Dull red-CL quartz (Q3) crystals appear to coincide with or follow sericitic alteration, and cut and encompass preexisting Q1 and Q2 crystals. They spatially associated with sulfides. The Q3 grains contain rare well-defined FIAs, but show scattered, secondary, irregular-shaped, low-salinity aqueous inclusions with small- to medium-sized vapor bubbles (10–30 vol%), as well as complex fluid inclusions that have migrated to the margins of anhedral Q3 subgrains due to quartz dissolution and recrystallization.Formation of early blue-CL Q1 and Q2 crystals at moderate temperatures are consistent with enrichment of Ti in the quartz lattice compared to neighboring younger Q3. The Q2 crystals are typified by elevated concentrations of Al and K concentrations when compared to Q1 and Q3. The substitution of Ti4+ for Si4+ in the tetrahedral sites of quartz leads to an increased CL intensity from Q1, to Q2, and then to Q3 corresponding to a decrease in quartz precipitation temperature. Ore deposition, recorded by integration of CL textures, FIAs petrography, and trace element variations in Q2, in the Taiyangshan porphyry Cu-Mo deposit must have occurred over a significant change in physicochemical conditions. This suggests that phase separation due to fluid cooling/pressure drops during the transition from lithostatic to hydrostatic regimes is critical to porphyry mineralization. We further propose a complex hydrothermal quartz vein formation history involving multiple fluid inclusions under varying pressure and temperature conditions.

Controllable formation of unusual homocrystals in poly(L-lactic acid)/poly(D-lactic acid) asymmetric blends induced by the constraining effects of pre-existing stereocomplexes

Crystallization in confined environments usually induces polymers showing complicated crystallization kinetics and unusual crystalline structure. Beyond the typical confined polymer systems, pre-existing crystals can also exert confinement effects on the subsequent crystallization of polymorphic or multi-component polymers; this, however, is not well understood at present. Herein, poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA, abbreviated as L/D) asymmetric blends with various PDLA fractions (f D = 0.02–0.5) are chosen as a model system and the effects of pre-existing stereocomplexes (SCs) on the crystallization kinetics and polymorphic structure are investigated. It is found that unusual β-form homocrystals (HCs) of poly(lactic acid) can be formed in an asymmetric L/D blend, which are strongly influenced by the molecular weights (MWs) of the used polymers, L/D mixing ratio, thermal treatment temperature (T max) and crystallization temperature (T c). The formation of β-HCs is preferred in asymmetric L/D blends with low and medium MWs, medium f D (0.1–0.2), medium T max (170–200°C), and low T c (70–110°C). The metastable β-HCs reorganize into the more stable α-HCs via melt recrystallization in the heating process. It is proposed that the β-HC formation stems from the constraining effects of pre-existing SCs; this constraining effect is governed by the content of pre-existing unmelted SCs in the thermally treated samples.

Carbonation reactivity enhancement of γ-C2S through biomineralization

Most of the calcium silicates exhibit high carbonation reactivity, and thus start to be utilized in the production of low carbon concrete due to the growing pressure of carbon mitigation in the cement industry. In this study, powdered γ-C2S, with limited hydraulicity, is preconditioned in a suspension consisting of urease-producing bacterium and urea. This biomineralization process induces the precipitation on the γ-C2S surface of calcite crystals with spherical shape, the size of which is dependent on the urea suspension concentration. The effect of the pre-existing calcite crystals on the carbonation behavior, mechanical and micro-morphology of γ-C2S compacts is investigated. Results indicate the compressive strength are significantly improved with the pre-existing calcite crystals, and the increasing percentage is 43.66 % at 30 min carbonation and 0.25 mol/L urea concentration. The strength enhancement is found to be associated with the pre-existing calcite crystals acting as nucleation seeds which accelerate the carbonation at the early stage. In addition, the pre-existing calcite seeds induce the preferential formation of calcite in the crystalline carbonation products of γ-C2S, which may be beneficial to the enhancement of the carbonated products.

Effect of stretching on the crystallization of un-crosslinked ethylene/propylene(/diene) random copolymers

The phase behavior of ethylene/propylene(/diene) (EP(D)M) co- (and ter-)polymers characterized by ethylene content in the range 64–78 wt%, and comparatively low level of crystallinity, in the range 1–15%, is analyzed during stretching, through collection of wide angle X-ray scattering data. All samples experience crystallization by effect of stretching, but whereas for the samples with ethylene content ≥71% crystallinity starts increasing already after yielding, the nearly amorphous sample, with 64 wt% ethylene content, experiences crystallization only at strain above a threshold (≈170%). In all cases, a high degree of orientation of the amorphous phase is achieved already at low deformations. It is shown that for the samples with high ethylene content, the pre-existing crystals undergo initial fragmentation and/or mechanical melting at low deformations, and successive oriented recrystallization at higher deformations. The role of oriented amorphous phase, along with the role of pre-existing crystals as nuclei able to foster further crystallization is highlighted.

Supersaturation Nucleation and Growth of Plagioclase: a numerical model of decompression-induced crystallization

Supersaturation Nucleation and Growth of Plagioclase (SNGPlag) is a numerical model that predicts the nucleation and growth of plagioclase crystals in a decompressing magma as a function of time. The model is written in Matlab, but is available as a standalone compiled program. SNGPlag uses the MELTS webservice to determine equilibrium plagioclase mode, for a user-defined magma composition, as a function of pressure and temperature. User inputs include decompression path, the presence and size distributions of antecrysts and phenocrysts, and crystal shape. At each time step, the model evaluates the difference between the calculated crystallinity and equilibrium crystallinity for a given pressure and temperature to determine the degree of supersaturation, which then sets plagioclase nucleation and growth rates. Growth rates are used to grow the existing crystals whereas nucleation adds new crystals. SNGPlag produces results that can be compared to quantitative textures in natural volcanic rocks, including total crystallinity, microlite number density, microlite crystal size distribution, the characteristic size of microlite crystals, as well as a time series of crystallinity. Model results are consistent with the established crystallization theory. As expected, microlite crystallinity increases as decompression rate slows. Decompression path greatly affects microlite textures. For the same average decompression rate, single-step paths have higher crystallinities and microlite number densities than multi-step decompressions, which are in turn more crystalline than continuous paths. Pre-existing crystals damp microlite crystallization, as these crystals provide a substrate to accommodate crystal growth and thus reduce supersaturation. The size distribution and volume fraction of these pre-existing crystals determines the magnitude of the damping. SNGPlag predicts that melt composition and temperature also exert important controls. Higher temperatures and higher silica contents both reduce microlite crystallization. In comparison with the previous studies of decompression rate based on microlite crystallization experiments, SNGPlag generally predicts minimum decompression rates that are up to three-to-four times slower. The difference is likely because those studies applied single- or multi-step decompression experiments to simulate natural magma ascent, which may be better represented by continuous decompression pathways or series of continuous decompression intervals punctuated with pauses. Previous studies also fail to account for the effects of phenocrysts or antecrysts on microlite nucleation and growth.

Confined crystallization morphology of poly(ϵ‐caprolactone) block within poly(ϵ‐caprolactone)–poly(l‐lactide) copolymers

AbstractThe confined crystallization of poly(ϵ‐caprolactone) (PCL) block in poly(ϵ‐caprolactone)–poly(l‐lactide) (PCL‐PLLA) copolymers was investigated using differential scanning calorimetry, polarized optical microscopy, scanning electronic microscopy and atomic force microscopy. To study the effect of crystallization and molecular chain motion state of PLLA blocks in PCL‐PLLA copolymers on PCL crystallization morphology, high‐temperature annealing (180 °C) and low‐temperature annealing (80 °C) were applied to treat the samples. It was found that the crystallization morphology of PCL block in PCL‐PLLA copolymers is not only related to the ratio of block components, but also related to the thermal history. After annealing PCL‐PLLA copolymers at 180 °C, the molten PCL blocks are rejected from the front of PLLA crystal growth into the amorphous regions, which will lead to PCL and PLLA blocks exhibiting obvious fractionated crystallization and forming various morphologies depending on the length of PLLA segment. On the contrary, PCL blocks more easily form banded spherulites after PCL‐PLLA copolymers are annealed at 80 °C because the preexisting PLLA crystal template and the dangling amorphous PLLA chains on PCL segments more easily cause unequal stresses at opposite fold surfaces of PCL lamellae during the growth process. Also, it was found that the growth rate of banded spherulites is less than that of classical spherulites and the growth rate of banded spherulites decreases with decreasing band spacing. © 2019 Society of Chemical Industry

Formation of diagenetic siderite in modern ferruginous sediments

Ferruginous conditions prevailed in the world’s deep oceans during the Archean and Proterozoic Eons. Sedimentary iron formations deposited at that time may provide an important record of environmental conditions, yet linking the chemistry and mineralogy of these sedimentary rocks to depositional conditions remains a challenge due to a dearth of information about the processes by which minerals form in analogous modern environments. We identified siderites in ferruginous Lake Towuti, Indonesia, which we characterized using high-resolution microscopic and spectroscopic imaging combined with microchemical and geochemical analyses. We infer early diagenetic growth of siderite crystals as a response to sedimentary organic carbon degradation and the accumulation of dissolved inorganic carbon in pore waters. We suggest that siderite formation proceeds through syntaxial growth on preexisting siderite crystals, or possibly through aging of precursor carbonate green rust. Crystal growth ultimately leads to spar-sized (>50 μm) mosaic single siderite crystals that form twins, bundles, and spheroidal aggregates during burial. Early-formed carbonate was detectable through microchemical zonation and the possible presence of residual phases trapped in siderite interstices. This suggests that such microchemical zonation and mineral inclusions may be used to infer siderite growth histories in ancient sedimentary rocks including sedimentary iron formations.

Morphological Analysis of Archetypal Calcite Cement

Archetypal cements from their deposition to the time they are sampled preserve a record of subsurface fluid evolution that can cover many millions of years. Their morphological development after nucleating on fissure walls is simulated by graphical models showing maturation from isolated to competitive to parallel growth. Graphical models are not available for the cementation of pores that are more complex and diverse. The reconstruction of cement growth in pores is accomplished here using multiple cathodoluminescent (CL) growth zones. Sediment pores are filled in two stages separated by a cementation threshold associated with a rapid drop in permeability. Pre-threshold cement is centripetal ordered, regularly distributed, and nurtured by a megapore network. Post-threshold cement is disordered, sporadically distributed, and nurtured by labile micropore connections. The crystallographic form of euhedral crystals at cement's growing front has environmental significance while the anhedral shape of aggregate crystals it leaves behind is controlled by geometrical selection. Growth of seeded cement is divided into epitaxial and mantle stages; syntaxial when these two are not identified. The epitaxial stage begins with a seed at multiple points then coalesces, simplifies, and morphs towards the Wulff or equilibrium surface. The extent of epitaxial growth is correlated with the size of crystals forming pore walls. Increments of mantle growth can be initiated over the whole surface of a pre-existing crystal, or can be restricted to its edges and/or corners. Restricted seeding is often linked with changes of crystal habit resulting in the reorientation of the crystal's fastest growth direction and the direction of maturation. Some impingement intercrystalline boundaries have offset growth zones explained by dissolution; such a self-cannibalization process is new. The existing explanation that some enfacial junctions are due to pauses in growth is confirmed by the arrangement of growth zones around triple points, but a new type is recorded formed from continuously growing crystals whose intercrystalline boundaries become modified. The presence of the cement's internal growth surfaces allow its development to be reconstructed, providing the foundation for further cement studies; however, one example is presented where the disproportionate filling of pores in adjacent millimeter-thick layers is obscure because its crystals lack internal growth surfaces.

Diffusion and solubilities of Rh, Ru and Ir in olivine and spinel

The true solubilities of highly siderophile elements in silicate and oxide minerals may be difficult to measure because of their low abundances and the tendency of these elements to occur as minute inclusions or “micronuggets”. In order to mitigate this latter problem, we present a new approach that makes use of the solid-state diffusion of such elements into pre-existing crystals. The diffusion of Rh, Ru and Ir was measured in San Carlos olivine, and synthetic monocrystalline forsterite and MgAl2O4-spinel, at atmospheric pressure, 1300 to 1400 °C and oxygen fugacities from air to that of the quartz-fayalite-magnetite (FMQ) oxygen buffer. The activities of all main components were controlled in each experiment by solid-state buffer assemblages: SiOR2R and MgO in olivine experiments were controlled by forsterite (olivine) + periclase (magnesiowustite) (fo + per or sco + mw), or forsterite (olivine) + protoenstatite (fo + en or sco + opx). Activities of AlR2ROR3R and MgO in the spinel experiments were buffered by spinel + periclase (sp + per) or spinel + corundum (sp + cor) mineral assemblages. The diffusion profiles were measured by LA-ICP-MS in both traverse and drilling modes, and by NanoSIMS. Good agreement was obtained between the methods with spatial resolution significantly higher in LA-ICP-MS drilling mode and NanoSIMS at 0.2 μm and 0.1 μm, respectively, compared to 5 μm by LA-ICP-MS in the traverse mode. Diffusion coefficients were more than five times slower, and solubilities five times lower, in the low MgO activity experiments compared with those carried out at high MgO activity at the same temperature and fO2. Our results show that at 1400PoPC and FMQ + 1.5 the maximum solubility of Rh in olivine in equilibrium with metal is 0.7 ppm in high SiOR2R activity mineral associations (those with orthopyroxene) and <0.3 ppm in low SiOR2R activity associations (those with magnesiowustite). Solubility of Rh in forsterite obtained at the same conditions was 0.2 ppm for the enstatite association and 0.17 ppm for the periclase association. Concentrations of Ir in olivine buffered with magnesiowustite were close to those buffered with orthopyroxene: 0.11 ppm and 0.12 ppm, respectively, at 1300 °C and FMQ + 0.1 The Ru concentration in olivine was 0.21 ppm in the orthopyroxene association at FMQ + 6.6 (in air). Solubilities of platinum group elements (PGEs) in spinel show a strong dependence on both fO2 and aSiO2: 150 ppm of Ir at FMQ + 1.5 compared to 1 ppm at FMQ + 6.6 for the cordierite assemblage, and 7 ppm for the periclase assemblage. Conclusions from this study show that olivine is unlikely to play a role in fractionating any of the compatible platinum group elements during partial melting of the mantle.

Epitaxial Crystallization Behavior of Poly(butylene adipate) on Orientated Poly(butylene succinate) Substrate.

The crystallization behavior of poly(butylene adipate) (PBA) in the sheared PBS/PBA blend, as well as on highly orientated poly(butylene succinate) (PBS) substrate, was studied by means of DSC, POM, Raman microscopy, and XRD. The results showed that the pre-existing orientated PBS crystals exhibit a very strong nucleation ability toward PBA as reflected by the increased crystallization temperature and the occurrence of heteroepitaxy and transcrystallization of PBA on the PBS substrate. The epitaxial crystallization of PBA on the PBS substrate results in the formation of α-form PBA crystals in any crystallization conditions.