2,412 research outputs found
A low temperature X-ray single-crystal diffraction and polarised infra-red study of epidote
Full text linkThe effects of low-temperature on the crystal structure of a natural epidote [Ca1.925Fe0.745Al2.265Ti0.004 Si3.037O12(OH), a = 8.8924(7), b = 5.6214(3), c = 10.1547(6) angstrom and beta = 115.396(8)degrees at room conditions, Sp. Gr. P2(1)/m] have been investigated with a series of structure refinements down to 100 K on the basis of X-ray single-crystal diffraction data. The reflection conditions confirm that the space group is maintained within the T-range investigated. Structural refinements at all temperatures show the presence of Fe3+ at the octahedral M(3) site only [%Fe(M3) = 70.6(4)% at 295 K]. Only one independent proton site was located and two possible H-bonds occur, with O(10) as donor and O(4) and O(2) as acceptors. The H-bonding scheme is maintained down to 100 K and is supported by single crystal room-T polarised FTIR data. FTIR Spectra over the region 4,000-2,500 cm(-1) are dominated by the presence of a strongly pleochroic absorption feature which can be assigned to protonation of O(10)-O(4). Previously unobserved splitting of this absorption features is consistent with a NNN influence due to the presence of Al and Fe3+ on the nearby M(3) site. An additional relatively minor absorption feature in FTIR spectra can be tentatively assigned to protonation of O(10)-O(2). Low-T does not affect significantly the tetrahedral and octahedral bond distances and angles, even when distances are corrected for "rigid body motions". A more significant effect is observed for the bond distances of the distorted Ca(1)- and Ca(2)-polyhedra, especially when corrected for "non-correlated motion". The main low-T effect is observed on the vibrational regime of the atomic sites, and in particular for the two Ca-sites. A significant reduction of the magnitude of the thermal displacement ellipsoids, with a variation of U-eq (defined as one-third of the trace of the orthogonalised U-ij tensor) by similar to 40% is observed for the Ca-sites between 295 and 100 K. Within the same T-range, the U-eq of the octahedral and oxygen sites decrease similarly by similar to 35%, whereas those of the tetrahedral cations by similar to 22%.</p
The dual relationship between God's creative purposes and the nature of sin and evil in Karl Barth's account of das Nichtige : in dialogue with the monist account of Alvin Plantinga
No full textJohn Hick argues for a two-fold typology of Christian theodicies, namely,
those which offer monist accounts of good and evil and those which offer dualist
accounts. Neither approach, he goes on to argue, is compatible with the basic claims
of Christian thought. On the one hand, monism risks denying the distinction between
good and evil by incorporating evil into the unitary intentionality of the one sovereign
God. Dualist accounts, on the other, risk undermining the sovereignty of God by
affirming the existence of evil as that which conflicts with God’s good (and singular)
will. Hick’s typology presents us, therefore, with the option of either affirming the
full sovereignty of God and denying the truly malevolent nature of evil, or affirming
God’s opposition to evil but then undermining the full sovereignty of God.
Two immensely influential Christian thinkers, namely, Karl Barth and Alvin
Plantinga, are considered as a means of testing this claim. Barth, who is the primary
focus, tends toward a dualistic understanding of good and evil whereas Plantinga
toward a more monistic understanding. Hick’s typology, however, fails to serve their
differing understandings of good and evil adequately. An alternative analysis of this
distinction is proposed drawing on their distinctive understandings of the relationship
between sin and evil and God’s creative purposes. This leads to an analysis of the
conditions under which it is possible to affirm the truly malevolent nature of evil and
God’s full sovereignty. It is contended that Barth’s approach offers a consistent means
of affirming God’s radical opposition to evil while also affirming his full sovereignty
Book review: El Sistema: orchestrating Venezuela’s youth, by Geoffrey Baker
No full textBook review of: El Sistema: orchestrating Venezuela’s youth, by Geoffrey Baker.
New York, NY: Oxford University Press, 2014; ISBN: 9780199341559
($35.00)Publisher PD
The structure of P21/c (Ca0.2Co0.8)CoSi2O6 pyroxene and the C2/c - P21/c phase transition in natural and synthetic Ca, Mg, Fe2+ pyroxenes
Full text linkThe P21/c synthetic (Ca0.2Co0.8)CoSi2O6 pyroxene was synthesized by slow cooling from melt at high pressure. Single crystals suitable for X-ray diffraction were obtained and refined. The results were compared to those of C2/c pyroxenes along the series CaCoSi2O6-Co2Si2O6. Strong similarities in the crystal chemical mechanism of the transition with the synthetic CaFeSi2O6-Fe2Si2O6 and CaMgSi2O6-Mg2Si2O6 pyroxenes, both at an average and local level are apparent.The results, examined together with two new refinements of pigeonite in the ureilites ALHA77257 and RKPA80239 and of a set of natural and synthetic C2/c and P21/c pyroxenes, show that the average cation radius in the M2 site is the driving force for the phase transition from C2/c to P21/c . The longest M2-O3 distances and the O3-O3-O3 angles follow the same trend, dictated2only by the ionic radius in M2, in either synthetic or natural pyroxenes, regardless of the ionic radius of the M1 cations. The transition also affects the difference between bridging and non-bridging oxygen atoms and the extent of tetrahedral deformation, whereas the M1-O, M2-O1 and M2-O2 distances are unaffected by the transition and are determined only by the ionic radius of the bonding cation. The structural changes between the ionic radius and the high temperature C2/c and P21/c transitions are similar, and different from the high-pressure transition.The analysis of natural and synthetic pyroxenes shows that the transition with composition occurs in strain free pyroxenes for a critical radius of 0.85 Å. Increasing strain stabilizes the P21/c structure to higher temperature and larger cation radius.Finally, our results show that the monoclinic P21/c Ca-poor clinopyroxene, i.e the pigeonite mineral, crystallizes only at conditions where the structure is HT-C2/c, and changes to the P21/c symmetry during cooling
Development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse-grained quartz veins
Full text linkThe microstructure and crystallographic preferred orientation (CPO) of quartz were quantified in 17 samples of natural monomineralic tabular veins. The veins opened and were deformed, up to shear strain γ > 15, in a small temperature window (about 25°C) above 500°C, as established by Ti-in-quartz thermometry. The veins filled a set of fractures within the Adamello tonalite (southern Alps, Italy) and localized homogeneous simple shear during postmagmatic cooling. The local (square millimeter scale) and bulk (square centimeter) CPO were investigated by computer-integrated polarization microscopy (CIP) and X-ray texture goniometry. Weakly deformed veins (WDV: γ <1) consist of millimeter- to centimeter-sized crystals with a strong CPO showing a c-axis girdle slightly inclined, mostly with the shear sense, to the foliation (XY) plane and a strong maximum close to the lineation (X). Moderately deformed veins (MDV: 2 <γ <3) consist of elongated nonrecrystallized ribbon grains and most have a CPO showing a strong Y maximum of c axes some with weak extension into a YZ girdle. Strongly deformed veins (SDV: γ = 4 to 15) are pervasively to completely recrystallized to fine (34-40 μm grain size) aggregates with a strong CPO similar to that of MDV. The slip systems during plastic deformation were dominantly prism 〈a〉 with subordinate rhomb and basal 〈a〉 slip. Recrystallization occurred rather abruptly for 3 <γ <4. In contrast to dislocation creep experiments in quartz (and other minerals), a steady-state recrystallized fabric is achieved at early stages of deformation (γ ≈ 4) as there is no evidence, with increasing strain, of strengthening of the CPO, of rotation of the fabric skeleton, or of change in grain size. WDV represent weakly deformed relicts of veins with an initial CPO believed to have developed during crystal growth but unsuitably oriented for prism 〈a〉 slip during subsequent shear. MDV and SDV appear to derive from veins different from WDV, where the vein crystals grew with orientation favorable for prism 〈a〉 slip. The relationship between the initial growth CPO and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism 〈c〉 and prism 〈a〉 slip, with the temperature of growth causing growth of crystals well oriented for slip. The initial CPO of veins, from which quartz mylonites are commonly derived, plays a critical role in the fabric evolution. The strong growth- and strain-induced CPOs of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal 〈a〉 slip would have been dominant
Major element zonation following rapid heating of homogeneous glass in wire-loop experiments_full dataset
No full textData used for the publication "Major element zonation following rapid heating of homogeneous glass in wire-loop experiments" [https://doi.org/10.1016/j.chemer.2025.126383]. Data obtained as part of a systematic investigation of magmatic degassing process in analogue lunar magma, funded by the Leverhulme Trust through Research Project Grant RPG-2021-015, and the Carnegie Trust through PhD Scholarship award PHD01069
Massive volcanic domes on Venus and the mobilisation of crystal mush: insights from the Troodos Ophiolite, Cyprus
No full textVenus is widely regarded as Earth’s ‘sister’ planet, given similarities in size, mass,
density, chemical composition, and also their proximity. However, there are also striking differences between the two planets. The surface of Venus is dominated by volcanic and tectonised volcanic terrains. Volcanism on Venus is, presumably, largely plume-related due to the absence of evidence for plate tectonic processes. Under the extreme high temperatures and pressures of the Venusian surface, lava flows can extend for hundreds to thousands of kilometres. Steep-sided domes are among the most starkly discernible volcanic landforms on the surface of Venus, given their gigantic size (e.g., 0.12–1.73 km in height, and 8.3–61.8 km in diameter) and peculiar shape, compared to other volcanic features. Those domes are believed to be volcanic edifices, characterised by steep margins, and relatively smooth, flat upper surfaces, exhibiting a circular shape in plan view. Given their distinct morphology, they are important in (1) understanding the range of magmatic processes operating on Venus, and (2)
elucidating geological evolution of planets lacking plate tectonics.
Despite their significance, there remains debate regarding the formation mechanisms of these domes. Physical models suggest that Venusian steep-sided domes require eruption of highly viscous liquids to explain their morphologies. This has led several authors to suggest that they represent eruption of SiO2-rich magmas at relatively low temperatures. However, other authors have argued that radar characteristics of steep-sided domes are inconsistent with eruption of very SiO₂-rich materials, and alternatively suggested that they represent eruption of more mafic liquids. In this scenario, high crystallinities may account for unusually high viscosities evidenced by dome morphology. Bulk viscosity of magmas depends on many factors, including composition (e.g., SiO₂ content and extent of polymerisation), volatile content, crystal content, and most importantly temperature. The overall aim of this study is to assess physical models of Venusian steep-sided dome formation by assessing the validity and implications of constraints on lava viscosity which they provide.
Work is divided into two Sections and four chapters. In Section 1 (Chapter 2), I model magma fractionation to calculate the full range of liquid compositions and bulk viscosity of magmas, which is compared to the viscosity thresholds from physical models. A key finding from this work is that high crystal contents are required to account for formation of high viscosity lavas on Venus. In Section 2 (Chapter 3 to 5), I conduct fieldwork to characterise a terrestrial analogue of eruption of high crystallinity picritic lavas in the Troodos Ophiolite, Cyprus. Using geochemical data, petrological and thermodynamic modelling, and field
observations, I investigate the formation and eruption mechanisms, parental magma compositions, and viscosities of picrites. Information from this section is then used to further assess steep-sided dome formation on Venus and controls of crystallinity on magma viscosity.
In Chapter 2, I implement thermodynamic modelling using the rhyolite-MELTS model to constrain magma compositions, and magma viscosities based on various viscosity parameterisations, using bulk compositions inferred from Venera 13 (alkaline basalt) and Venera 14 (low alkali basalt) Soviet lander data. Viscosities are then compared to viscosity thresholds from published physical models of Venusian dome formation. Extensive (>85–90%) fractional crystallisation or equilibrium melting processes alone fail to produce magmas with viscosities required to account for steep-sided domes. The presence of H₂O during equilibrium crystallisation substantially modifies magma composition (e.g., SiO₂), and decreases solidus temperature, contributing to an increase of liquid viscosity. However, this effect is less significant than the direct control of H₂O as a network modifier in lowering viscosity. Instead, our results reveal that crystal contents of >60 vol.% are invariably required to produce sufficiently high magma bulk viscosities. Such high crystallinities probably require eruption of crystal-rich magma followed by surface crystallisation, possibly enhanced by degassing and undercooling.
To further constrain the eruption mechanisms of high crystallinity magma, in Chapter 3, I conduct fieldwork to observe field relations and collect a suite of representative high crystallinity picritic bodies (e.g., lava flows and a dome) from the Margi (Mαρκί, sometimes referred to as Marki) region, Troodos Ophiolite, Cyprus. These picrites represent a series of mafic/ultramafic lava flows, with groundmasses ranging from glassy to holocrystalline/vitrophyric texture and typically containing variable, but very high concentrations of up to cm-sized olivine. The crystal content from different picrite bodies can vary from 36 to 66 vol.%. I describe field relations, petrology, and geochemistry of a number of these picrite bodies, including olivine (+spinel and melt inclusion) compositions, erupted glass compositions and whole rock data. These picrites are discrete units within the upper pillow lava sequence, with crystal contents that imply open-system crystal accumulation. Variations in composition and olivine crystal cargo imply that bodies have discrete compositions, and close proximity of picrites to extensional faults supports a model where extrusion of crystal mush onto the palaeo-seafloor was facilitated by rifting and tapping of small magmatic systems. However, olivine-spinel equilibration temperatures imply minimal re-equilibration, favouring a model of hot crystal mush storage in the crust.
In Chapter 4, based on olivine-hosted melt inclusion (MI) data, I use petrological modelling to constrain parental magma compositions, and petrogenesis of high crystallinity lavas. I also perform forward fractional crystallisation and partial melting using rhyolite-MELTS to constrain conditions under which olivine crystallisation occurred. Results are discussed within the framework of glass (e.g., erupted liquids), olivine and spinel compositional data. The key findings are (1) crystal-rich magmas are likely formed by magma recharging and repetitive fractionation and concentration of olivine crystals within magmatic systems at less than 0.4 GPa (i.e., ~15 km depth); (2) at least 40% of olivine phenocrysts within picrites are in equilibrium with liquids in which they have erupted, and that picrites are likely formed both by fractionation of olivine, and remobilisation of xenocrysts; (3) Erupted picrites represent a mixture of variably evolving melts (closed system fractionation) and olivine from a mush-rich magmatic system, with the average Fo content from different picritic bodies ranging between Fo₈₉ to Fo₉₁; (4) Parental magmas to both erupted liquids and olivines had variable compositions, including both variations in primary compositions and due to variable extents of sulfide saturation during early stages of olivine crystallisation.
In Chapter 5, I perform viscosity calculations based on glass compositions, H2O content, crystallinity, and crystal size distribution or morphology measured directly from picrite samples to estimate the viscosity of picritic flows and lavas in Margi. Although these lavas have very high crystal contents, calculated viscosities are relatively low due to the low viscosity of erupted liquids, which is, in turn, a function of mafic compositions, high water contents and high erupted temperatures. As a result, the picrite dome at Margi is not a good analogue for investigating the eruption mechanisms for Venusian domes, although picritic bodies at Margi do provide insight into the formation and eruptibility of high crystallinity magma. I further compile dome morphologies for all terrestrial lava domes from basaltic to rhyolitic compositions and other extra-terrestrial lava domes. Venusian steep-sided domes have a smaller aspect ratio (height/width) than 90% of terrestrial lava domes, indicating that direct
comparison between terrestrial domes and Venusian steep-sided domes is challenging given (1) their large difference in volumes and (2) the difference in surface environments between the two planets (e.g., surface temperature, pressure, atmosphere compositions, etc). As such, greater emphasis should instead be placed on improving models of dome formation under
Venusian conditions, and especially, on considering the key role of crystallinity, lava cooling rate as well as composition in controlling lava viscosity and dome morphology
Effects of temperature on the crystal structure of epidote: a neutron single-crystal diffraction study at 293 and 1070K
Full text linkThe effects of temperature on the crystal structure of a natural epidote [Ca-1.925 Fe0.745Al2.265Ti0.004Si3.037O12(OH), a = 8.890(6), b = 5.630(4), c = 10.150(6) and beta = 115.36(5)A degrees, Sp. Gr. P2(1) /m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe3+ at the M-3 site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)-H(1)center dot center dot center dot O(2) and O(10)-H(1)center dot center dot center dot O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)-H(1)center dot center dot center dot O(4) is energetically more favourable, as H(1)center dot center dot center dot O(4) = 1.9731(28) , O(10)center dot center dot center dot O(4) = 2.9318(22) and O(10)-H(1)center dot center dot center dot O4 = 166.7(2)A degrees, whereas H(1)center dot center dot center dot O(2) = 2.5921(23) , O(10)center dot center dot center dot O(2) = 2.8221(17) and O(10)-H(1)center dot center dot center dot O2 = 93.3(1)A degrees. The O(10)-H(1) bond distance corrected for "riding motion" is 0.9943 . The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe3+ share the M(3) site along with Al3+ [%Fe(M3)(1,070K) = 74(2)%]. The refined amount of Fe3+ at the M(1) is not significant [%Fe(M1)(1,070K) = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for "rigid body motions". The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)-O(9)-Si(1) of the Si2O7 group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)center dot center dot center dot O(4) and H(1)center dot center dot center dot O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe3+ between different octahedral sites and/or convergent ordering on M(3) are discussed.</p
Geoffrey Robertson on the History of Human Rights
No full textQueen\u27s Counsel, broadcaster and author Geoffrey Robertson has achieved international fame by defending high-profile cases, often representing victims of alleged human rights abuses. Here, at an event organised by Amnesty Australia, he gives a short history of human rights, from the Magna Carta to the present
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