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Jennifer Linton Alexandra Navrotsky Yingwei Fei 《Physics and Chemistry of Minerals》1998,25(8):591-596
The enthalpies of formation from ilmenite, FeTiO3, and perovskite, CaTiO3, of two intermediate ordered perovskite phases, CaFeTi2O6 and CaFe3Ti4O12, have been measured at 801°C using oxide melt solution calorimetry. These data, in combination with experiments at high pressure
and temperature, indicate that below 1518±50°C CaFe3Ti4O12 is stable at lower pressures (∼7 GPa at 1200°C) than CaFeTi2O6 (∼13 GPa at 1200°C). This relationship should be reversed, and CaFeTi2O6 should become stable at lower pressures than CaFe3Ti4O12, at temperatures above 1518±50°C. These intermediate phases are of petrological interest because they form as a reaction
between two minerals, ilmenite and perovskite, which are commonly associated in kimberlites, and because their pressure-temperature
range of formation overlaps that of origin of kimberlites.
Received: 10 November 1997 / Revised; accepted: 15 January 1998 相似文献
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The Impact Of Extreme Summer Melt on Net Accumulation of an Avalanche Fed Glacier,as Determined by Ground‐Penetrating Radar 
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下载免费PDF全文 Heather Purdie Wolfgang Rack Brian Anderson Tim Kerr Trevor Chinn Ian Owens Matthew Linton 《Geografiska Annaler: Series A, Physical Geography》2015,97(4):779-791
Glacier mass balance is more sensitive to warming than cooling, but feedbacks related to the exposure of previously buried firn and ice in very warm years is not generally considered in sensitivity studies. A ground‐penetrating radar survey in the accumulation area of Rolleston Glacier, New Zealand shows that five years of previous net accumulation was removed by melt from parts of the glacier above the long‐term equilibrium line altitude during a single negative mass balance year. Rolleston Glacier receives a large amount of accumulation from snow avalanches, which may temporarily buffer it from climate warming by providing additional mass that has accumulated at higher elevations, effectively increasing the elevation range of the glacier. However, glaciers reliant on avalanche input may have high sensitivity to climatic variations because the extra mass is concentrated on a small part of the glacier, and small variations in avalanche input could have a large impact on overall glacier accumulation. Further research is needed to better estimate the amount and spatial distribution of accumulation by avalanche in order to quantify the climate sensitivity of small avalanche‐fed glaciers. 相似文献
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Numerical simulations of the helical (m=1) kink instability of an arched, line-tied flux rope demonstrate that the helical deformation enforces reconnection between
the legs of the rope if modes with two helical turns are dominant as a result of high initial twist in the range Φ≳6π. Such a reconnection is complex, involving also the ambient field. In addition to breaking up the original rope, it can form
a new, low-lying, less twisted flux rope. The new flux rope is pushed downward by the reconnection outflow, which typically
forces it to break as well by reconnecting with the ambient field. The top part of the original rope, largely rooted in the
sources of the ambient flux after the break-up, can fully erupt or be halted at low heights, producing a “failed eruption.”
The helical current sheet associated with the instability is squeezed between the approaching legs, temporarily forming a
double current sheet. The leg – leg reconnection proceeds at a high rate, producing sufficiently strong electric fields that
it would be able to accelerate particles. It may also form plasmoids, or plasmoid-like structures, which trap energetic particles
and propagate out of the reconnection region up to the top of the erupting flux rope along the helical current sheet. The
kinking of a highly twisted flux rope involving leg – leg reconnection can explain key features of an eruptive but partially
occulted solar flare on 18 April 2001, which ejected a relatively compact hard X-ray and microwave source and was associated
with a fast coronal mass ejection. 相似文献
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An approximate method is presented for calculating drift forces on arrays of vertically axisymmetric bodies in a wave tank. It is assumed that the wave scattering properties of an isolated element and of the array in the open sea are known. The procedure described allows the open-sea results to be post-processed to give good estimates of drift forces when a structural array is placed in a wave tank. For the particular case of an array of vertical circular cylinders extending throughout the depth, the method is compared with accurate results from the full linear problem for scattering by the array both when the array is in the tank and when it is in the open sea. The results show how the mean forces on the array when in the tank may differ considerably from those experienced in the open sea. 相似文献
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T. Nieves-Chinchilla A. Vourlidas J. C. Raymond M. G. Linton N. Al-haddad N. P. Savani A. Szabo M. A. Hidalgo 《Solar physics》2018,293(2):25
The magnetic topology, structure, and geometry of the magnetic obstacles embedded within interplanetary coronal mass ejections (ICMEs) are not yet fully and consistently described by in situ models and reconstruction techniques. The main goal of this work is to better understand the status of the internal magnetic field of ICMEs and to explore in situ signatures to identify clues to develop a more accurate and reliable in situ analytical models. We take advantage of more than 20 years of Wind observations of transients at 1 AU to compile a comprehensive database of ICMEs through three solar cycles, from 1995 to 2015. The catalog is publicly available at wind.gsfc.nasa.gov and is fully described in this article. We identify and collect the properties of 337 ICMEs, of which 298 show organized magnetic field signatures. To allow for departures from idealized magnetic configurations, we introduce the term “magnetic obstacle” (MO) to signify the possibility of more complex configurations. To quantify the asymmetry of the magnetic field strength profile within these events, we introduce the distortion parameter (DiP) and calculate the expansion velocity within the magnetic obstacle. Circular-cylindrical geometry is assumed when the magnetic field strength displays a symmetric profile. We perform a statistical study of these two parameters and find that only 35% of the events show symmetric magnetic profiles and a low enough expansion velocity to be compatible with the assumption of an idealized cylindrical static flux rope, and that 41% of the events do not show the expected relationship between expansion and magnetic field compression in the front, with the maximum magnetic field closer to the first encounter of the spacecraft with the magnetic obstacle; 18% show contractions (i.e. apparent negative expansion velocity), and 30% show magnetic field compression in the back. We derive an empirical relation between DiP and expansion velocity that is the first step toward improving reconstructions with possible applications to space weather studies. In summary, our main results demonstrate that the assumed correlation between expanding structure and asymmetric magnetic field is not always valid. Although 59% of the cases could be described by circular-cylindrical geometry, with or without expansion, the remaining cases show significant in situ signatures of departures from circular-cylindrical geometry. These results will aid in the development of more accurate in situ models to reconcile image. 相似文献
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K. Leinenweber J. Linton A. Navrotsky Y. Fei J. B. Parise 《Physics and Chemistry of Minerals》1995,22(4):251-258
An exploratory high-pressure study of the join CaTiO3-FeTiO3 has uncovered two intermediate perovskites with the compositions CaFe3Ti4O12 and CaFeTi2O6. These perovskites have ordering of Ca2+ and Fe2+ on the A sites. Both of these perovskites are unusual in that the A sites containing Fe2+ are either square planar or tetrahedral, due to the particular tilt geometries of the octahedral frameworks. For CaFe3Ti4O12, the structure has been refined from a powder using the Rietveld technique. This compound is a cubic double perovskite (SG Im $\bar 3$ , a = 7.4672 Å), isostructural with NaMn7O12. Fe2+ is in a square-planar A site (similar to Mn3+ in NaMn7O12) with Fe-O = 2.042(3) Å, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Å. Calcium is in a distorted icosahedron with Ca-O =2.635(5) Å. CaFeTi2O6 crystallizes in a unique tetragonal double perovskite structure (SG P42/nmc, a = 7.5157(2), c = 7.5548(2)), with A-site iron in square-planar (Fe-O = 2.097(2) Å) and tetrahedral (Fe-O = 2.084(2) Å) coordination, again with distant second neighbor oxygens near 2.8 Å. Rietveld refinement was also performed for the previously known perovskite-related form of FeTiO3 recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a = 5.1233(1) and c = 13.7602(2). The ordered perovskites appear to be stable at 1215 GPa and CaFe3Ti4O12 is found as low as 5 GPa. Thus these perovskites may be important to upper mantle mineralogy, particularly in kimberlites. These compounds are the first known quenchable perovskites with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites. 相似文献
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