2,921 research outputs found
Iodate in calcite, aragonite and vaterite CaCO3: Insights from first-principles calculations
No full textCIF files providing structure outputs from ab initio calculations of calcite, aragonite and vaterite with and without substituted iodine.Published asX F Feng, S A T Redfern (2018) Iodate in calcite, aragonite and vaterite CaCO3: Insights from first-principles calculations. Geochimica et Cosmochimica Act
Memory effect of a mechanical anomaly related to ferroelastic domain switching in rhombohedral lead zirconate titanate ceramics
No full textCan Wang, Simon A. T. Redfern, Maren Daraktchiev, and Richard J. Harrison\ud
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ United Kingdom\ud
An anomaly in the temperature dependent mechanical properties of a lead zirconate titanate ferroelectric ceramic has been observed by dynamic mechanical analysis. The anomaly, seen as a rise in modulus, accompanied by a decrease in internal friction, occurs in the low-temperature phase below TC. The temperature of the anomalies varies systematically with the applied forces and the anomaly does exhibit a memory effect. The corresponding static bending deformation, mainly from remnant strain by ferroelastic domain switching, is analyzed, and a critical remnant strain value for triggering the anomaly is obtained. The anomaly is thought to be induced by pinning and depinning of domain walls. The results confirm that the memory effect and the occurrence of the anomaly are controlled by ferroelastic domain switchin
Water incorporation in synthetic and natural MgAl2O4 spinel
No full textThe solubility and incorporation mechanisms of water in synthetic and natural MgAl2O4 spinel have been investigated in a series of high-pressure/temperature annealing experiments. In contrast to most other nominally anhydrous minerals, natural spinel appears to be completely anhydrous. On the other hand, non-stoichiometric Al-rich synthetic (defect) spinel can accommodate several hundred ppm water in the form of structurally-incorporated hydrogen. Infrared (IR) spectra of hydrated defect spinel contain one main O-H stretching band at 3343-3352 cm(-1) and a doublet consisting of two distinct O-H bands at 3505-3517 cm(-1) and 3557-3566 cm(-1). IR spectra and structural refinements based on single-crystal X-ray data are consistent with hydrogen incorporation in defect spinel onto both octahedral and tetrahedral O-O edges. Fine structure of O-H bands in IR spectra can be explained by partial coupling of interstitial hydrogen with cation vacancies, or by the effects of Mg-Al disorder on the tetrahedral site. The concentration of cation vacancies in defect spinel is a major control on hydrogen affinity. The commercial availability of large single crystals of defect spinel coupled with high water solubility and similarities in water incorporation mechanisms between hydrous defect spinel and hydrous ringwoodite (Mg2SiO4) suggests that synthetic defect spinel may be a useful low-pressure analogue material for investigating the causes and consequences of water incorporation in the lower part of Earth's mantle transition zone. (c) 2009 Elsevier Ltd. All rights reserved. <br/
Influence of metal transition ions doping in MgTi2O5 (karrooite): colour and order-disorder behaviour
No full textMemory effect of a mechanical anomaly related to ferroelastic domain switching in rhombohedral lead zirconate titanate ceramics
No full textCan Wang, Simon A. T. Redfern, Maren Daraktchiev, and Richard J. Harrison
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ United Kingdom
An anomaly in the temperature dependent mechanical properties of a lead zirconate titanate ferroelectric ceramic has been observed by dynamic mechanical analysis. The anomaly, seen as a rise in modulus, accompanied by a decrease in internal friction, occurs in the low-temperature phase below TC. The temperature of the anomalies varies systematically with the applied forces and the anomaly does exhibit a memory effect. The corresponding static bending deformation, mainly from remnant strain by ferroelastic domain switching, is analyzed, and a critical remnant strain value for triggering the anomaly is obtained. The anomaly is thought to be induced by pinning and depinning of domain walls. The results confirm that the memory effect and the occurrence of the anomaly are controlled by ferroelastic domain switchin
Optical phonons, OH vibrations, and structural modifications of phlogopite at high temperatures: An in-situ infrared spectroscopic study
No full textThe thermal behavior of optical phonons and OH vibrations of phlogopite (a trioctahedral mica) was examined at temperatures up to 1000 K using in situ infrared spectroscopy. The results showed that with increasing temperature, O–K bands in phlogopite exhibited a relatively strong variation in frequency in a manner similar to those in muscovite. The work revealed that different types of OH bands (fundamentals and combinations) have very different thermal behavior or temperature dependence, and their absorption coefficients are commonly not constant on heating. OH combination bands that are associated with summation processes of multi-phonon interactions commonly show a decrease in their intensities on heating, but in contrast combination bands due to difference processes generally exhibit an increase. This means that temperature dependencies of their absorption coefficients need to be considered when using the Beer-Lambert law to determine or estimate OH contents or hydrogen concentrations at high temperatures. The results showed a structural anomaly associated with a discontinuity in the temperature derivative of the wavenumber of Al–O and Si–O vibrations and O–H stretching near 600 K. However, framework-related phonon modes in the FIR and MIR regions do not suggest a break of the original monoclinic structural symmetry in the investigated temperature region. The complex changes are attributed to temperature-induced alteration of local configuration involving TO4 tetrahedra and a possible change of the orientation of OH dipoles, in addition to a previously reported distortion of MO6 octahedra. Increasing temperature to 1000 K also causes partial dehydroxylation, as evidenced by the disappearance of the OH band near 3623 cm−1 and the decrease in OH band height and area of other OH bands. The study did not record the formation of H2O inside phlogopite as a result of partial dehydroxylation. The work offers new data and findings that have important implications in understanding the complex structural modifications and the behavior of phonon modes and the thermal stability of hydroxyls on approaching the dehydroxylation, as well as the way hydrogen is released from micas at high temperatures. Our data also show that phologpite becomes less transparent with increasing temperature suggesting a change of radiative properties and ability to transmit heat, which could be of interest for modeling thermal-transmission in crustal rocks
The Group Selection Debate and ALife: Weak Altruism, Strong Altruism, and Inclusive Fitness (abstract)
No full textModels of the evolution of social behaviour are often framed in terms of either multi-level selection or inclusive individual fitness theory. Although both of these descriptions correctly predict changes in gene frequency (where group fitness is defined as the average individual fitness of the group members), it is still a hotly contested issue as to which provides a faithful description of the underlying causal processes at work. Furthermore, the type of model analysis used reflects the philosophical bias of the author. It is important for Alife researchers to be aware of this issue when evaluating or presenting models of social evolution, for many authors simply claim as a matter of fact that their model works via multi-level or (inclusive) individual selection, without acknowledging the alternative perspective. In this talk, two particular areas of ongoing contention between multi-level and individual selectionists will be illustrated, using examples from the Alife literature. The first of these concerns the evolution of weakly altruistic traits. These are behaviours that provide a whole-group benefit at some cost to the actor. Crucially, however, the cost to the actor is more than offset by its share of the group benefit, such that the lifetime number of offspring of the actor is increased. In a recent paper West et al. (2007, J. Evol. Biol., 20, p.415) have advocated that the evolution of such traits can be adequately explained in terms of direct fitness benefit, thus avoiding the need to invoke selection at the group level. However, this explanation hides the fact that weak altruists suffer a relative fitness disadvantage within every group. Indeed, the local attractor within any one group is the extinction of weak altruists. Therefore, the behaviour cannot spread unless groups compete and groups with more weak altruists are fitter than those with less. While the individualist methodology correctly predicts if the behaviour will evolve, it obscures the mechanistic explanation. This suggests that models couching the evolution of social behaviour in terms of individual benefit should be analysed to determine whether group structure is playing any causal role in the evolutionary dynamics. The second issue to be addressed by this talk concerns the evolution of strong altruism, i.e., behaviours where there is a reduction in the lifetime number of offspring of the actor. For such behaviours to evolve there must be a correlation in interactions, such that the recipients of an altruist’s help tend to be altruists themselves. This correlation frequently occurs in nature through the limited dispersal of kin, and is usually modelled by inclusive fitness equations that contain no notion of group fitness. However, the underlying mechanism is that kin groups with more altruists outcompete those with less. Once this is realised, it becomes apparent that other assortative group formation mechanisms can in principle produce the same effect. Appealing to kinship is therefore simply invoking one kind of assortative grouping. This talk will further elaborate on these points, including definitions of a group, and consider claims about the strength of group selection
Forecasting banknotes
No full textA central bank’s liquidity forecast is important in ensuring that it supplies the banking system’s need for central bank money. Banknote (or currency in circulation) demand is the largest and for some central banks the most variable component of the liquidity forecast. Accurate forecasting of banknotes is essential in ensuring an accurate liquidity forecast and in turn effective monetary policy implementation. This Handbook discusses these issues and outlines a structural time series state space (STSSS) model which is now used by central banks including the Bank of England and ECB to forecast banknotes (currency in circulation).Forecasting banknotes
Experimental and theoretical constraints on lithium isotope fractionation during brine evaporation and halite precipitation
No full textMarine evaporitic halite, precipitating from the advanced stage of extremely evaporated seawater, is the most common
7 and abundant evaporitic mineral. The extent to which lithium (Li) is incorporated into evaporitic halite during geo8
logical periods of massive halite deposition, and the mechanisms of incorporation, are both unknown. These are each
9 important, however, for the quantification of the isotope effect of the evaporitic sink on δ7Liseawater, and for the poten10
tial use of halite as a recorder of seawater δ7Li. Here, new experimental data are presented for the Li fractionation
11 factor for halite from an isothermal evaporation experiment (25◦C for 100 days) using marine-derived brine, compara12
ble in composition to Phanerozoic CaCl2 oceans. The resultant solids were characterized by X-ray diffraction (XRD)
13 and scanning electron microscopy (SEM) to confirm the mineralogy and crystallinity of the product. Our results show
14 that Li isotopes are not constant during the evaporation process implying that there must be a process that fractionates
15 Li isotopes during halite precipitation. We suggest that this results from Li ionic substitution for sodium (Na) in crys16
talline halite, a scenario supported by ab-initio calculations, but we also show that Li within halite is predominantly
17 present within fluid inclusions. Thus, the Li isotopic composition of halite is controlled by a mixture of Li within the
18 fluid inclusions and Li that is incorporated into the halite crystal. Overall, the brine becomes enriched in 7Li during
19 evaporation due to the preferential incorporation of 6Li into chloride precipitates, and the evolution of δ7Librine is con20
trolled by the precipitated mineral assemblage. The theoretical equilibrium fractionation factor 1000lnα(mineral−f luid)
21 is -7.8 ‰ for Li-halite (LiNa26Cl27) at 25 ◦C at equilibrium, which is at the lower limit of Li isotope fractionation
22 factors. Rayleigh fractionation models were fitted to the experimental data yielding a fractionation factor of -25 ± 3
23 ‰. The difference in Li isotope fractionation factor between brine and structurally bound Li may result from kinetic
24 fractionation effects. This study presents a scenario highlighting that extensive halite deposits, formed over geological
25 time, could potentially impact the δ7Li values of seawater during that period, and place constraints on their utility as
26 an archive for δ7Liseawater
Nanopaleomagnetism of meteoritic Fe-Ni studied using X-ray photoemission electron microscopy
No full textUnder a Creative Commons license.X-ray photoemission electron microscopy (XPEEM) enables natural remanent magnetisation to be imaged with ~30nm resolution across a field of view of 5-20 μm. The method is applied to structural features typical of the Widmanstätten microstructure (kamacite - tetrataenite rim - cloudy zone - plessite) in the Tazewell IIICD iron meteorite. Kamacite lamellae and the tetrataenite rim are multidomain, whereas plessite consists of laths of different phases displaying a range of stable magnetisation directions. The cloudy zone (CZ) displays a complex interlocking domain pattern resulting from nanoscale islands of tetrataenite with easy axes distributed along three possible crystallographic directions. Quantitative analysis of the coarse and intermediate CZ was achieved using a combination of image simulations and histogram profile matching. Remanence information was extracted from individual regions of interest ~400nm wide, demonstrating for the first time the capability of XPEEM to perform quantitative paleomagnetic analysis at sub-micron length scales. The three tetrataenite easy axis orientations occur with equal probability in the coarse and intermediate CZ, suggesting that spinodal decomposition in these regions was not strongly influenced by internal interaction fields, and that they are suitable candidates for future paleomagnetic studies. The fine CZ shows a strong dominance of one easy axis. This effect is attributed to island-island exchange interactions that render the fine CZ unsuitable for paleomagnetic study. Variations in the relative strength (proportion of dominant easy axis) and direction (direction of dominant easy axis) of a paleomagnetic field can be resolved from different regions of the CZ using XPEEM, raising the prospect of obtaining a time-resolved measurement of the active dynamo period in meteorites originating from the upper unmelted regions of differentiated asteroids (e.g. chondrites, pallasites, mesosiderites). © 2014.Open Access funded by Natural Environment Research Council.Peer Reviewe
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