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S. A. T. Redfern E. Salje A. Navrotsky 《Contributions to Mineralogy and Petrology》1989,101(4):479-484
The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy G as a function of the order parameter Q and temperature T: G 1/2a(T
2c–T)Q
2+1/6CQ
6 with a=24 J·K·mol-1, C = 30 kJ·mol–
T
c = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite. 相似文献
13.
J. J. Reece S. A. T. Redfern M. D. Welch C. M. B. Henderson C. A. McCammon 《Physics and Chemistry of Minerals》2002,29(8):562-570
The partitioning of Fe and Mn between the large M(4) site and the octahedral sites, M(1,2,3) in the amphibole structure has been investigated in two natural manganogrunerites of compositions Ca0.1Mn1.9 Mg1.25Fe2+ 3.56Fe3+ 0.38Si7.81O22(OH)2 and Ca0.24Mn1.57 Mg2.27 Fe2+ 2.76Fe3+ 0.32Si7.84O22(OH)2. The long-range cation distribution in the two samples has been elucidated by in situ neutron powder diffraction revealing that Mn is preferentially ordered onto M(4) ? M(2) >M(1) >M(3) in both samples. Partitioning of Mn from M(4) into the octahedral sites begins at 350 °C, with site exchange energies of ?16.6 kJ mol?1 and ?14.9 kJ mol?1, in samples containing 1.90 and 1.57 Mn apfu, respectively. Mössbauer and infrared spectroscopy have been used to study the samples at room temperature, and Mössbauer data agree well with the diffraction results, confirming that high-temperature cation distributions are retained during cooling. The fine structure in the hydroxyl-stretching region of the IR absorption spectra has been used to discuss qualitatively the site occupancies of the coordinating M(1)M(3)M(1) triplet, linked by O(3). On the basis of such modelling, we conclude that a degree of local clustering is present in both samples. 相似文献
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15.
A system is described which makes best use of the high quantum efficiency and high count rate capability of avalanche photodiodes
for high time resolution observations of optical pulsars. The use of three APDs allows simultaneous photometry of the target
and a reference star, and the monitoring of the sky background. By minimising the optical components in the light path the
optical efficiency of the system is maximised. The TRIFFID (Shearer, A., Stappers, B., O'Connor, P., Golden, A., Strom, R.,
Redfern, M., Ryan, O.: Science 301, 493–495 (2003)) and OPTIMA (Straubmeier, C., Kanbach, G., Schrey, F.: Exp. Astron. 11, 157–170 (2001)) have shown that fibre-fed APD arrays can produce excellent results. This, new, system was used on the 6m
BTA in November 2003 – results on the Crab pulsar are presented. 相似文献
16.
James Lovell-Kennedy Emmanuel Roquette Stefan Schröder Jonathan Redfern 《Basin Research》2023,35(1):187-213
Over 10,000 published detrital zircon ages have been reprocessed (applying a +10% normal and reverse concordance range) and analysed to understand the evolution of the detrital zircon record of north-west Africa during the Phanerozoic. Using dissimilarity and clustering analysis, shifts in detrital zircon populations allow interpretation of the evolution of source regions and source to sink systems throughout the Phanerozoic within the West Gondwana superfan. Previous thermochronology and field studies conducted across north-west Africa indicate significant and sustained shifts in source regions in Meso-Cenozoic times which are not recorded in the detrital zircon geochronology record. This discrepancy is most notable for Mesozoic to modern source to sink studies focused on the evolution of the Atlasic rift and opening of the Atlantic and Tethyan Oceans to the west and north respectively. Our results indicate a high degree of similarity between samples from Cambrian times onwards due to successive phases of sediment recycling. This highlights the need to integrate detrital zircon analysis with other techniques to provide confident reconstruction of sediment routing systems across Morocco. This systematic review also reveals the ubiquitous occurrence of Mesoproterozoic zircons within Moroccan sediment. No basement of this age is known from north-west Africa—often described as the ‘Mesoproterozoic Gap’, which was thought to be a diagnostic feature of sediment derived from the West African Craton. However, zircons of this age form 7% of all analysed zircons and are present in sediments from at least 700 Ma. The presence of this population is interpreted as strongly diagnostic of provenance from either the Amazonian Craton or the Eastern Gondwana Orogen within Central Africa. Their presence in the Moroccan detrital record from the Neoproterozoic onwards raises questions about the position of the West African Craton in the Proterozoic, and for the spatial extent of Mesoproterozoic orogeny within north Africa. 相似文献
17.
The application of high‐resolution seismic geomorphology, integrated with lithological data from the continental margin offshore The Gambia, northwest Africa, documents a complex tectono‐stratigraphic history through the Cretaceous. This reveals the spatial‐temporal evolution of submarine canyons by quantifying the related basin depositional elements and providing an estimate of intra‐ versus extra‐basinal sediment budget. The margin developed from the Jurassic to Aptian as a carbonate escarpment. Followed by, an Albian‐aged wave‐dominated delta system that prograded to the palaeo‐shelf edge. This is the first major delivery of siliciclastic sediment into the basin during the evolution of the continental margin, with increased sediment input linked to exhumation events of the hinterland. Subaqueous channel systems (up to 320 m wide) meandered through the pro‐delta region reaching the palaeo‐shelf edge, where it is postulated they initiated early submarine canyonisation of the margin. The canyonisation was long‐lived (ca. 28 Myr) dissecting the inherited seascape topography. Thirteen submarine canyons can be mapped, associated with a Late Cretaceous‐aged regional composite unconformity (RCU), classified as shelf incised or slope confined. Major knickpoints within the canyons and the sharp inflection point along the margin are controlled by the lithological contrast between carbonate and siliciclastic subcrop lithologies. Analysis of the base‐of‐slope deposits at the terminus of the canyons identifies two end‐member lobe styles, debris‐rich and debris‐poor, reflecting the amount of carbonate detritus eroded and redeposited from the escarpment margin (blocks up to ca. 1 km3). The vast majority of canyon‐derived sediment (97%) in the base‐of‐slope is interpreted as locally derived intra‐basinal material. The average volume of sediment bypassed through shelf‐incised canyons is an order of magnitude higher than the slope‐confined systems. These results document a complex mixed‐margin evolution, with seascape evolution, sedimentation style and volume controlled by shelf‐margin collapse, far‐field tectonic activity and the effects of hinterland rejuvenation of the siliciclastic source. 相似文献
18.
S. Karpov G. Beskin A. Biryukov V. Debur V. Plokhotnichenko M. Redfern A. Shearer 《Astrophysics and Space Science》2007,308(1-4):595-599
The fine structure and the variations of the optical pulse shape and phase of the Crab pulsar are studied on various time
scales. The observations have been carried out on 4-m William Hershel and 6-m BTA telescopes with APD photon counter, photomultiplier
based 4-channel photometer and PSD based panoramic spectrophotopolarimeter with 1 μs time resolution in 1994, 1999, 2003 and
2005–2006 years. The upper limit on the pulsar precession on Dec 2, 1999 is placed in the 10 s–2 hr time range. The evidence
of a varying from set to set fine structure of the main pulse is found in the 1999 and 2003 years data. No such fine structure
is detected in the integral pulse shape of 1994, 1999 and 2003 years.
The drastic change of the pulse shape in the 2005–2006 years set is detected along with the pulse shape variability and quasi-periodic
phase shifts.
This work has been supported by the Russian Foundation for Basic Research (grant No 04-02-17555), Russian Academy of Sciences
(program “Evolution of Stars and Galaxies”), by the Russian Science Support Foundation, and by INTAS (grant No 04-78-7366). 相似文献
19.
Patrick Collins Gillian Kyne David Lara Michael Redfern Andy Shearer Brendan Sheehan 《Experimental Astronomy》2013,36(3):479-503
Many astronomical objects emit polarised light, which can give information both about their source mechanisms, and about (scattering) geometry in their source regions. To date (mostly) only the linearly polarised components of the emission have been observed in stellar sources. Observations have been constrained because of instrumental considerations to periods of excellent observing conditions, and to steady, slowly or periodically-varying sources. This leaves a whole range of interesting objects beyond the range of observation at present. The Galway Astronomical Stokes Polarimeter (GASP) has been developed to enable us to make observations on these very sources. GASP measures the four components of the Stokes Vector simultaneously over a broad wavelength range 400–800 nm., with a time resolution of order microseconds given suitable detectors and a bright source - this is possible because the optical design contains no moving or modulating components. The initial design of GASP is presented and we include some preliminary observational results demonstrating that components of the Stokes vector can be measured to $<1$ % in conditions of poor atmospheric stability. Issues of efficiency and stability are addressed. An analysis of suitable astronomical targets, demanding the unique properties of GASP, is also presented. 相似文献
20.
E. J. Palin M. T. Dove S. A. T. Redfern A. Bosenick C. I. Sainz-Diaz M. C. Warren 《Physics and Chemistry of Minerals》2001,28(8):534-544
The nature of Al–Si ordering across the tetrahedral sites in muscovite, K2Al4(Si6Al2O20)(OH)4, was investigated using various computational techniques. Values of the atomic exchange interaction parameters J
l
were obtained. From these parameters, a two-dimensional Al–Si ordering scheme was deduced. The transition temperature T
c for this two-dimensional ordering is 1900 K. There are several possible ordering schemes in three dimensions, based on different
stacking sequences of ordered sheets of tetrahedral sites. Monte Carlo simulations of both two-dimensional and three-dimensional
ordering were performed, but in the three-dimensional simulation only the two-dimensional ordering is seen, implying that
three-dimensional ordering is too slow to be attained during the timescale of the simulation. The effect of the three-dimensional
interactions is to raise the two-dimensional ordering temperature to 2140 K. From the three-dimensional Monte Carlo simulation,
the frequency of occurrence of 4Si0Al, 3Si1Al, 2Si2Al and 1Si3Al clusters was determined, which match those inferred by 29Si MAS–NMR measurements reasonably well. In fact, the match suggests that the cation ordering seen in experiments corresponds
to a configuration with considerable short-range order but no long-range order, similar to a state that is at a temperature
just above an ordering phase transition.
Received: 28 August 2000 / Accepted: 12 March 2001 相似文献