Melt Separation Research Articles

Crystallization and Dielectric Properties of Ferroelectric Glass-Ceramics Containing Sodium Niobate Crystals

Ferroelectric glass-ceramics of the sodium-niobate-aluminum-silicate (NaO-Nb2O5-SiO2+Al2O3) system, with various amounts of ZrO2 added, were synthesized through melt casting followed by controlled crystallization. The crystallization behavior, microstructure, and dielectric performances of the glass-ceramics were investigated. The addition of 1 mol% ZrO2 to the glass-ceramics improved the glass forming ability of the phase separated melts and supported the formation of transparent glass-ceramics after precipitation of the NaNbO3 ferroelectric crystals. The presence of ZrO2 also promoted a well-defined microstructure which presented as an improvement in the dielectric constant of the glass-ceramic heat treated at 750°C for 1 h. Dipole polarization was responsible for the dielectric response of the glass-ceramics, but due to structural heterogeneities coupled with high migration of the mobile species, a complicated dielectric relaxation mechanism was also found.

Liquid separation in Cu–Zr–Ag ternary alloys

While cooling beyond a certain temperature limit (TSEP) the Cu–Zr–Ag ternary melt of the separated field in the ternary diagram decomposes into an Ag-rich liquid (L1) and another liquid rich in Cu and Zr (L2) both under equilibrium and non-equilibrium conditions. The master alloy samples were melted and cast in a wedge-shaped copper mold, in order to obtain a gradient of cooling rate. The shape of separated melt, its size and distribution were investigated. The boundaries of the liquid ternary miscibility gap have been enlarged owing to high cooling rate. The results confirm that the volume fraction of the separated phases has increased due to higher cooling rate. Three microstructure types can be distinguished during the liquid separation process as a function of L1.

Spinodally decomposed patterns in rapidly quenched Co–Cu melts

The Co–Cu system is analyzed in the region of the metastable miscibility gap with separation of the undercooled melt into the Co-rich and Cu-rich liquids. Phase separation of undercooled and quenched samples of the Co50Cu50 melt are investigated experimentally using an electromagnetic levitation technique, quenching on a Pb-solder-coated copper chill substrate and splat-quenching methods. It is found that quenching of the liquid samples with cooling rate ≳106Ks-1 leads to a freezing of splats having the microstructure of spinodally decomposed liquids. The composition of the Co-rich phase measured by transmission electron microscopy is Co71.7Cu28.3 and that of the Cu-rich phase is Co26.8Cu73.2. These compositions are inside the spinodal region and close to the calculated spinodal boundary in the phase diagram of the Co–Cu system at temperatures below T≈1450K. Experimental results are compared with predictions of computational modeling using a model of fast spinodal decomposition.

Backward tracking of gas chemistry measurements at Erebus volcano

Erebus volcano in Antarctica offers an exceptional opportunity to probe the dynamics of degassing – its behavior is characterized by an active lava lake through which sporadic Strombolian eruptions occur. Here, we develop a framework for interpreting contrasting degassing signatures measured at high temporal resolution, which integrates physical scenarios of gas/melt separation into a thermodynamic model that includes new volatile solubility data for Erebus phonolite. In this widely applicable framework, the measured gas compositions are backtracked from surface to depth according to physical templates involving various degrees of separation of gas and melt during ascent. Overall, explosive signatures can be explained by large bubbles (gas slugs) rising slowly in equilibrium from at least 20 bars but at most a few hundred bars in a magmatic column closer to the stagnant end‐member than the convecting end‐member. The span of explosive signatures can be due to various departure depths and/or slug acceleration below a few tens of bars. Results also reveal that explosive gases last equilibrated at temperatures up to 300°C colder than the lake due to rapid gas expansion just prior to bursting. This picture (individual rise of gas and melt batches from a single, potentially very shallow phonolitic source) offers an alternative to the conclusions of previous work based on a similar data set at Erebus, according to which differences between quiescent and explosive gas signatures are due to the decompression of two deep, volatile‐saturated sources that mixed to various degrees (phonolite at 1–3 kbar and basanite at 5–8 kbar).

Core Formation and Mantle Differentiation on Mars

Geochemical investigation of Martian meteorites (SNC meteorites) yields important constraints on the chemical and geodynamical evolution of Mars. These samples may not be representative of the whole of Mars; however, they provide constraints on the early differentiation processes on Mars. The bulk composition of Martian samples implies the presence of a metallic core that formed concurrently as the planet accreted. The strong depletion of highly siderophile elements in the Martian mantle is only possible if Mars had a large scale magma ocean early in its history allowing efficient separation of a metallic melt from molten silicate. The solidification of the magma ocean created chemical heterogeneities whose ancient origin is manifested in the heterogeneous 142Nd and 182W abundances observed in different meteorite groups derived from Mars. The isotope anomalies measured in SNC meteorites imply major chemical fractionation within the Martian mantle during the life time of the short-lived isotopes 146Sm and 182Hf. The Hf-W data are consistent with very rapid accretion of Mars within a few million years or, alternatively, a more protracted accretion history involving several large impacts and incomplete metal-silicate equilibration during core formation. In contrast to Earth early-formed chemical heterogeneities are still preserved on Mars, albeit slightly modified by mixing processes. The preservation of such ancient chemical differences is only possible if Mars did not undergo efficient whole mantle convection or vigorous plate tectonic style processes after the first few tens of millions of years of its history.

Melt separation phenomena in CoNiCuAlCr high entropy alloy containing silver

The effects of Ag additions to AlCoCrCuNi or AlCoCrCuFeNi high entropy (HE) alloys are described. It is shown that Ag additions to these HE alloys results in liquid phase separation (LPS) into two melts, namely, an AlCoCr(Fe) rich liquid (L1) and a second liquid enriched in Ag and Cu (L2). Further, since the L2 has a higher density and a lower melting temperature, it tends to flow to the bottom of the arc melted buttons due to liquid flow (a) along the outer surface of the unmelted L1 and (b) through the interdendritic channels. In addition to the primary macroscopic LPS, secondary (within the different primary liquids) melt separation was observed in the two melts: the upper L1 layer contained L2 spheres which tended to sink to the fusion line while the L2 melt layer found at the bottom of the buttons contained L1 spheres which tended to float to the top of the L2 layer. The L1 microstructure and microhardness resembles that of an Ag-free AlCoCrCuFeNi HE alloy. In spite of the melt separation, relatively simple microstructures were obtained from the two different liquids; these are described in light of the current understanding of this class of alloys. Finally, the possibility of metastable LPS in a Cu-rich HE alloy using electron beam surface melting, which is known to produce significant dynamic undercoolings on the order of 150°C, was investigated but was not observed.

The behavior of noble-metal admixtures during fractional crystallization of As- and Co-containing Cu–Fe–Ni sulfide melts

To study the behavior of macrocomponents and admixtures during the fractional crystallization of sulfide melts and the influence of As on noble metals in this process, we performed a quasi-equilibrium directional crystallization of melt of composition (at.%): Fe—35.5, Ni—4.9, Cu—10.4, and S—48.3, with admixtures of Pt, Pd, Rh, Ru, Ir, Au, Ag, As, and Co (each 0.1 at.%), which imitates the average (by Cu contents) compositions of massive ores at the Noril'sk Cu-Ni deposits. The following sequence of phase formation from melt has been established: mss (zone I) / mss + iss (zone II) / iss (zone III) (mss is (FezNi1–z)S1+δ, iss is (FexCuyNi1–x–y)zS1–z); it corresponds to the distribution of main elements along the sample (primary zoning). Distribution curves for macrocomponents in zones I and II of the sample were constructed, as well as the dependencies of their partition coefficients (k) between solid solutions and sulfide melt on the fraction of crystallized melt. The secondary (mineral) zoning resulted from subsolidus phase transformations has been revealed. Five subzones have been recognized: mss + cp (Ia) / mss + cp + pn (Ib) / mss + pc + pn (IIa) / mss + pc + pn + bn (IIb) / pc + bn + pn + unidentified microphases (III). Admixture species in the sample were studied: (1) admixtures dissolved in primary solid solutions and in main minerals resulted from solid-phase transformations and (2) admixtures forming their own mineral phases. The partition coefficients of Co, Rh, and Ru (mss/L), Ru, Ir, and Rh (mss/cp), and Co, Rh, and Pd (mss/pn) were determined. Minerals of noble metals have been recognized: Pt3Fe, PtFe, Au, (Ag,Pd), (Au,Pt), Ag, Ag3Cu, Au3(Cu,Ag,Pd,Pt), etc., and the regularities of their distribution in the sample have been established. It is shown that some noble-metal admixtures are prone to interact with As. Mineral arsenides and sulfoarsenides of noble metals produced during fractional crystallization have been recognized: PtAs2, Pd3As, (RhAsS), (IrAsS), and (Ir,Rh)AsS. The discovered drop-like inclusions of noble-metal arsenides suggest the separation of the initial sulfide-arsenide melt into two immiscible liquids. By indirect features, the micromineral inclusions are divided into primary, crystallized from melt, and secondary, produced in solid-phase reactions. The results of study are compared with literature experimental data obtained by the isothermal-annealing method and with the behavior of noble metals and As during the formation of zonal massive orebodies at the Noril'sk- and Sudbury-type deposits.

Significance of chromian spinels from the mantle sequence of the Andaman Ophiolite, India: Paleogeodynamic implications

The mantle section of the Andaman Ophiolite between Rutland Island in the south and north Andaman in the north has been studied. The restitic peridotite of Rutland Island is comprised of depleted harzburgite to clinopyroxene-bearing harzburgite whereas that in middle and north Andaman is mostly less-depleted, lherzolite-dominated mantle. Chromian spinels from the mantle section of the Andaman Ophiolite belong to four major groups: Group-1 — spinels from Rutland chromitite pods, characterized by high Cr# [=Cr/(Cr+A1) atomic ratio] (~0.71); Group-2 — spinels from north Andaman chromitite pods with two subgroups, 2A — characterized by high Cr# (0.70–0.79) and 2B — characterized by medium Cr# (0.48–0.51) and high Mg# [=Mg/(Mg+Fe2+) atomic ratio] (0.74–0.77); Group-3 — residual accessory grains from Rutland peridotite, characterized by medium Cr# (0.37–0.55) and low Mg# (0.41–0.54); and Group-4 — residual accessory grains from middle and north Andaman peridotites, characterized by low Cr# (0.09–0.23) and high Mg# (0.71–0.81).Group-1 chromian spinels of Rutland Island and Group-2A of north Andaman are likely to have crystallized from boninitic melts, formed by relatively high degrees of partial melting, whereas Group-2B varieties from north Andaman are from tholeiitic melts, formed by lower degrees of melting. The chemistry of the residual accessory chromian spinels (Groups 3 and 4) suggests that the mantle peridotites of Rutland Island towards the south are similar to fore-arc peridotites of suprasubduction zone environments whereas those of north Andaman are less depleted. This spatial and/or temporal directional change in spinel compositions may reflect variations linked to the melting history where the same sliver of oceanic mantle underwent different styles of melting in different tectonic settings at different points in time. The coexistence of both chromitite types, with high- and low-Cr spinels, in the same area from north Andaman possibly reflects temporal variations of separate melt intrusions produced through specific melting stages. It is inferred that a similar geodynamic setting to the present-day also existed during the Cretaceous period with a fore-arc at Rutland Island and a back-arc towards the north at north Andaman.

Mixing of crystal mushes and melts in the genesis of the Bogács Ignimbrite suite, northern Hungary: An integrated geochemical investigation of mineral phases and glasses

The 16.4Ma old Bogács Ignimbrite, located south of the Bükk Mountains, northern Hungary, was formed during a silicic ignimbrite flare-up in the Pannonian Basin that occurred from 20Ma to 13Ma. It comprises two main units, a lower, variably welded pumiceous and an upper, scoriaceous pyroclastic flow unit. Bulk chemistry of the juvenile clasts indicates a gradual change of geochemical character with an upward decreasing SiO2 content through the stratigraphic section. A detailed in-situ major and trace element investigation of the main mineral phases and glasses combined with petrogenetic model calculations reveals complex magma reservoir processes. Based on the major and trace element variability, six juvenile clast types were distinguished and each contain fresh glass fractions with distinct compositions. The mineral assemblage consists of plagioclase, orthopyroxene, biotite with minor and variable amounts of quartz, amphibole, ilmenite, zircon and allanite. The anorthite content of the plagioclases varies from 20 to 90mol%, whilst the Enstatite content of orthopyroxenes covers also a wide range from 40 to 90mol%. This large compositional variation can be detected even in single crystals. This extreme geochemical variability can be explained by mixing of crystal mush bodies evolved from both basaltic and more silicic magmas. The calcic plagioclases (An=80–90mol%) and magnesian orthopyroxenes (En=70–90mol%) clearly indicate the role of primitive mafic magmas in the growth of the silicic magma reservoir, even though no basaltic volcanic activity was associated with the Miocene silicic volcanism in the Pannonian basin. The prolonged crystallization in the mushy sills resulted in compositionally different residual melt fractions that moved upwards and accumulated in separated melt pods at the roof of the magma reservoir. Intermittent intrusions of mafic and intermediate magmas into this silicic magma system could have resulted in thorough stirring of the crystal mush bodies and the melt pods, leading to eruptive products having compositionally heterogeneous glass and mineral assemblage.

Thermorheological properties of poly (ε‐caprolactone)/polylactide blends

AbstractBlends of poly (ε‐caprolactone) (PCL)/polylactide (PLA) were prepared by solution‐casting method to study their thermal and rheological properties. Differential scanning calorimetry thermographs have shown two separate melting peaks in the blends, which are indicative of immiscible structure at all compositions. Scanning electron microscopy images show droplet morphology of PCL into PLA matrix up to 40 wt% of PCL. Above this concentration, the co‐continuous morphology starts to appear, which becomes again droplet morphology for blends with concentration of PCL higher than about 60 wt%. The viscoelastic properties of the various blends were investigated using rotational rheometry. The enhancement of the elastic modulus of blends at small frequencies at which terminal zone behavior is expected, is a signature behavior of immiscible systems due to the presence of interface and contribution to the stress from interfacial tension. Two emulsion models were used to predict the viscoelastic properties of the blends from the corresponding properties of their pure components that led to the determination of the interfacial tension of PCL/PLA in agreement with experimental findings. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Investigation of the phase separation of Al-Bi immiscible alloy melts by viscosity measurements

The viscosity dependence of immiscible alloys, such as Al96Bi4, Al93Bi7, Al90Bi10 and pure Bi on temperature was measured using a torsional oscillation viscometer. The variation in viscosity with temperature deviated from the Arrhenius equation near the critical temperature. The temperature dependence of the viscosity near the critical temperature could be fitted using an amended viscosity equation based on the critical dynamics theory and gas-like property of metal melts. For the amended viscosity, the activation energy is a function of temperature Ev(T) = E0 + TScf. The change in molecular degree of order is reflected by the configuration entropy, Scf. For the pure Bi melt, during the solidification process, the molecular degree of order of the melt increased and achieved long range order. For the Al-Bi immiscible alloys, the molecular degree of order increased and achieved phase separation.

Microstructure and phase selection in supercooled copper alloys exhibiting metastable liquid miscibility gaps

The influence of both bulk supercooling and cooling rate on the microstructure and phase selection during solidification of Cu–Co, Cu–Co–Fe, and Cu–Nb alloys exhibiting metastable liquid miscibility gaps were investigated using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. Containerless electromagnetic levitation was used to achieve large bulk supercoolings in the specimens. Supercooling of these alloys below a certain temperature resulted in metastable separation of the melt into two liquids, a Cu-lean (Co, Co + Fe, or Nb enriched) melt (L1) and a Cu-rich melt (L2). Usually, the microstructure of the phase-separated alloys consisted of spherulites corresponding to one of the phase-separated liquids embedded in a matrix corresponding to the other. The microstructure and phase selection are found to depend on factors such as: alloy composition, supercooling level, whether the material was dropped before or after recalescence, and the cooling rate during solidification. The following results were observed: (1) solidification of metastable e-Cu with enhanced Co (or Co + Fe, or Nb) solubility; (2) partitionless solidification of the L1 and L2 liquids; (3) spinodal decomposition of the supercooled liquid, and (4) secondary melt separation. The results are discussed and related to current solidification theories regarding solidification paths for the different conditions examined. The miscibility gap boundaries for the different alloys were determined and compared with those reported in the literature.

Liquid immiscibility between silicate, carbonate and sulfide melts in melt inclusions hosted in co-precipitated minerals from Kerimasi volcano (Tanzania): evolution of carbonated nephelinitic magma

The evolution of a carbonated nephelinitic magma can be followed by the study of a statistically significant number of melt inclusions, entrapped in co-precipitated perovskite, nepheline and magnetite in a clinopyroxene- and nepheline-rich rock (afrikandite) from Kerimasi volcano (Tanzania). Temperatures are estimated to be 1,100°C for the early stage of the melt evolution of the magma, which formed the rock. During evolution, the magma became enriched in CaO, depleted in SiO2 and Al2O3, resulting in immiscibility at ~1,050°C and crustal pressures (0.5–1 GPa) with the formation of three fluid-saturated melts: an alkali- and MgO-bearing, CaO- and FeO-rich silicate melt; an alkali- and F-bearing, CaO- and P2O5-rich carbonate melt; and a Cu–Fe sulfide melt. The sulfide and the carbonate melt could be physically separated from their silicate parent and form a Cu–Fe–S ore and a carbonatite rock. The separated carbonate melt could initially crystallize calciocarbonatite and ultimately become alkali rich in composition and similar to natrocarbonatite, demonstrating an evolution from nephelinite to natrocarbonatite through Ca-rich carbonatite magma. The distribution of major elements between perovskite-hosted coexisting immiscible silicate and carbonate melts shows strong partitioning of Ca, P and F relative to FeT, Si, Al, Mn, Ti and Mg in the carbonate melt, suggesting that immiscibility occurred at crustal pressures and plays a significant role in explaining the dominance of calciocarbonatites (sovites) relative to dolomitic or sideritic carbonatites. Our data suggest that Cu–Fe–S compositions are characteristic of immiscible sulfide melts originating from the parental silicate melts of alkaline silicate–carbonatite complexes.

The Crustal Magma Storage System of Volcán Quizapu, Chile, and the Effects of Magma Mixing on Magma Diversity

Crystal zoning as well as temperature and pressure estimates from phenocryst phase equilibria are used to constrain the architecture of the intermediate-sized magmatic system (some tens of km 3 ) of Volcan Quizapu, Chile, and to document the textural and compositional effects of magma mixing. In contrast to most arc magma systems, where multiple episodes of open-system behavior obscure the evidence of major magma chamber events (e.g. melt extraction, magma mixing), the Quizapu magma system shows limited petrographic complexity in two large historical eruptions (1846–1847 and 1932) that have contrasting eruptive styles. Quizapu magmas and peripheral mafic magmas exhibit a simple binary mixing relationship. At the mafic end, basaltic andesite to andesite recharge magmas complement the record from peripheral cones and show the same limited range of compositions. The silicic end-member composition is almost identical in both eruptions of Quizapu. The effusive 1846–1847 eruption records significant mixing between the mafic and silicic end-members, resulting in hybridized andesites and mingled dacites. These two compositionally simple eruptions at Volcan Quizapu present a rare opportunity to isolate particular aspects of magma evolution—formation of homogeneous dacite magma and late-stage magma mixing—from other magma chamber processes. Crystal zoning, trace element compositions, and crystal-size distributions provide evidence for spatial separation of the mafic and silicic magmas. Dacite-derived plagioclase phenocrysts (i.e. An 25 – 40 ) show a narrow range in composition and limited zonation, suggesting growth from a compositionally restricted melt. Dacite-derived amphibole phenocrysts show similar restricted compositions and furthermore constrain, together with more mafic amphibole phenocrysts, the architecture of the magmatic system at Volcan Quizapu to be compositionally and thermally zoned, in which an andesitic mush is overlain by a homogeneous dacitic magma that is the source for most of the 1846–1847 and 1932 erupted magmas. Dacite formation is best explained by mineral–melt separation (crystal fractionation) from an andesitic mush, which is inferred to have thermally and compositionally buffered the dacite magma thereby keeping it at relatively low crystallinity (<30 vol. %). The dominant cause of compositional diversity is melt separation. Back-mixing of mush (i.e. crystals with signatures of growth both in the andesitic mush and in the dacite magma) into the overlying dacite magma is rarely observed. Recharge events that increase crystal and magma diversity in the dacite magma are limited to an episode of mafic recharge and mixing just prior to the 1846–1847 eruption, where evidence for magma mixing is present on all scales. Chamber-wide mixing was incomplete (mixing efficiency of ∼0·53–0·85) as flow lobes vary significantly in composition along the proposed mixing array. Estimates of viscosity variations during the course of magma mixing suggest that mixing dynamics and the degree of magma interaction on all scales were established at the beginning of the recharge event.

Evolution of peralkaline silicic complexes: Lessons from the extrusive rocks

Young (<1Ma) volcanic complexes in the Kenya Rift Valley are used to outline recent progress in our understanding of how peralkaline silicic systems evolve. Such systems are all to some degree unique, varying in their structural development, the interplay of petrogenetic complexes, and the range of lithologies. Peralkaline silicic reservoirs vary greatly in size and form (even beneath volcanoes of roughly similar size) but all are basalt-driven, in that basaltic magma is the fundamental source of heat and volatiles in the magmatic system. Fractional crystallisation of basaltic magma is the dominant differentiation mechanism but important contributions are made by magma mixing, remobilisation of crystal mushes and feldspar resorption, exsolution of carbonate phases and various volatile-magma interactions. Peralkaline silicic magmas are water-rich (>4wt.%) and the rhyolitic varieties evolve to temperatures<800°C at oxygen fugacities largely within the range ∆FMQ 0 to −1. They can be categorised as of cold-wet-reduced type. Low viscosities (<104–106Pas) result in rapid growth of volcanic edifices, highly efficient crystal–melt separation and the ubiquitous development of compositionally zoned caps to reservoirs. No peralkaline equivalent of the monotonous intermediate ignimbrites found in some calc–alkaline systems has yet been found.

Numerical investigation of oxygen impurity distribution during multicrystalline silicon crystal growth using a gas flow guidance device

Oxygen is one of the most important types of impurities that can cause thermal donor or light-induced degradation in mc-Si solar cells. The objective of this study is to investigate the effect that installing a gas flow guidance device in a mc-Si crystal-growth furnace would have on the oxygen impurity distribution in the melt during the growth process. The installation of such a gas flow guidance device can enhance the gas flow near the free surface, which would allow the argon to carry a greater amount of evaporated SiO gas outside the furnace. Furthermore, the enhanced motion of the gas flow also improves heat transfer near the free surface, which would make the melt vortex separate more easily. The separated melt vortex, which is located near the central region of the melt-crystal interface, directs any oxygen impurity towards the central region of the melt-crystal interface. This is why the oxygen concentration can be reduced by installing the gas flow guidance device. The effectiveness of the gas flow guidance device depends on the vertical distance between it and the free surface (h) as well as the gap between the crucible sidewall and the tip of the device (d). The effect on the oxygen concentration in the melt is significant when smaller values for h and d are adopted.

Poly(p-dioxanone)–poly(ethylene glycol) network: synthesis, characterization, and its shape memory effect

Multi-shape polymers have recently attracted growing interest on account of their ability to switch between multiple shapes in a predetermined way. In this work, thermally induced shape-memory PPDO–PEG networks with well-defined architecture have been designed and prepared by coupling hydroxyl-telechelic three-arm star-shaped PPDO and PEG diol precursors with HDI in 1,2-dichloroethane. The molecular weight and the feed ratio of the two precursors affected the gel content and the cross-link density of the network. The DSC analysis indicated that increasing the content of a precursor, or its molecular weight, was helpful in enhancing the crystallization degree of its own domains. The testing of dual-shape and triple-shape effects of networks showed that composition and cross-link density played an important role. As a typical sample, PPDO(9k)65–PEG(6k)35 exhibits a triple-shape effect, which possesses two separate and apparent melting endothermic peaks for the PPDO and PEG segments respectively.

Microstructure morphology and solute segregation in non-equilibrium solidification of metastable immiscible Cu50Co50 alloy

Non-equilibrium solidification of undercooled metastable immiscible Cu50Co50 alloy was performed by using glass-fluxing coupled with cyclic superheating method. The evolutions of microstructure morphology and solute segregation were elucidated as a function of initial undercooling. As for the samples undercooled by 62K and 105K, α-Co dendrites form primarily from the homogeneous liquid phase and then break into granular grains during the recalescence process. Moreover, the growth of dendrite changes from the solute-controlled mode to thermal-controlled mode with increasing undercooling, which generates the reduction of trunk size and the increase of solute content. In contrast, dual-layer structure prevails for larger undercoolings, i.e., 188K and 220K, where the strengthened immiscible effect leads to the decrease of solute content in separated phases and the occurrence of second separation. As for the sample with separated melt structure, nucleation triggering was adopted and serious shrinkage cavities can be observed due to the inadequate feeding of residual liquid.

Quality of Brazilian vegetable oils evaluated by (modulated) differential scanning calorimetry

Vegetable oils are increasingly replacing fossil- oil-based polymers, and therefore aimed at being used in polymerization reactions from -20 to 100 � C. Therefore, phase transitions and heat capacities in this temperature range should be well characterized to optimize processing conditions and energy inputs. By using the DSC analysis, only small primary correspondence or divergence between different oil types are seen as a function of their degree of unsaturation, but it does not clearly distinguish detailed features such as shoulder bands related to the separate melting processes of single fatty acid components. By using modulated DSC analysis, the combined analysis of reversing and non-reversing heat signals provides better results. The latter confirms that the melting is not a phys- ical one-step process, but equilibrates between phase transitions and enthalpic reorganizations of the fatty acids that can be monitored separately. The specific heat capacities measured during modulated DSC are somewhat lower than traditional calorimetric measurements, but relate to the degree of unsaturation. The thermal behavior of palm-, soy-, sunflower-, corn-, castor-, and rapeseed-oil is discussed in relation to their composition, by applying a first or second heating scan.

DETAILED ASSESSMENT OF PLATINUM-GROUP MINERALS ASSOCIATED WITH CHROMITITE STRINGERS IN THE MERENSKY REEF OF THE EASTERN BUSHVELD COMPLEX, SOUTH AFRICA

The minerals of the platinum-group elements (PGM) and their associated host minerals from the Merensky Reef at the Two Rivers platinum mine, South Africa, were investigated using an FEI 600F field emission Mineral Liberation Analyzer. Polished sections were prepared from intersections of upper and basal chromitite stringers collected from 12 boreholes drilled into a variety of Merensky Reef facies types. Primary minerals associated with the chromitite stringers, on the scale of a thin section, include orthopyroxene (30–60%), clinopyroxene (~2%), plagioclase (20–40%), and base-metal sulfides (up to 2%), whereas secondary silicates include tremolite (~4%), phlogopite (~1%), chlorite (<1%) and serpentine (<1%). The most abundant base-metal sulfides (BMS) occurring in the Merensky Reef in this area of the Bushveld Complex are pyrrhotite (46% of all BMS), pentlandite (32% of all BMS) and chalcopyrite (22% of all BMS). Abundances of the PGM (in area %, total PGM) are dominated by maslovite (PtBiTe, 29.7%), isoferroplatinum (Pt3Fe, 26.9%), braggite [(Pt,Pd,Ni)S, 13.3%], cooperite [(Pt,Pd,Ni)S, 11.9%] and laurite (RuS2, 8.3%), with michenerite (PdBiTe), sperrylite (PtAs2), stibiopalladinite (Pd5Sb2), geversite (PtSb), platarsite (PtAsS), and a variety of unnamed phases accounting for the remainder. There is a strong association of most PGM with pentlandite and chalcopyrite, but less so with pyrrhotite. Observed PGM–silicate mineral associations include a marked tendency for PGE antimonides, (sulf)arsenides, bismuthotellurides and sulfides to be associated with secondary silicates (in addition to BMS), which is taken as evidence for the redistribution of these PGMs by late fluids. The data presented here are consistent with existing models for mineralization within the Merensky Reef, which involve the separation of a sulfide melt from a silicate liquid, that then scavenged PGE while percolating downward until arrested by impermeable chromitite.