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11.
Martin T. Dove David A. Keen Alex C. Hannon Ian P. Swainson 《Physics and Chemistry of Minerals》1997,24(4):311-317
Neutron total scattering measurements from powdered samples of cristobalite have been used to determine the local structure in both the tetragonal and cubic phases. The results for the cubic phase show directly that the Si–O bonds are tilted at an angle of around 17° to the unit cell [111] direction. It is striking that the structure of β-cristobalite over the range 5–10 Å is closer to that of silica glass than α-cristobalite, which suggests that the local structure of β-cristobalite is not likely to consist of domains with the structure of α-cristobalite. The measurements show a small thermal expansion of the Si–O bonds over the temperature range 570–950 K. 相似文献
12.
Jane Dove 《Geology Today》2016,32(2):70-74
Postage stamps are useful in promoting geological heritage. This concept is explored through an analysis of British and New Zealand stamps, which portray a variety of different types of geological heritage. Coastal, glacial, fluvial and limestone scenery all feature, together with volcanic landforms such as the Giant's Causeway. Other stamps celebrate the work of historically significant figures who have contributed to geology, such as Charles Darwin. The field of palaeontology is represented in stamps, for example, featuring dinosaurs. Others portray economic activities and engineering achievements associated with geology such as oil and gas exploration. Industrial heritage is depicted by stamps featuring, for example, abandoned tin mines, while building stones are represented in stamps of ancient monuments, bridges, cathedrals and domestic architecture. Perhaps subjects for future stamp issues in the UK might draw upon the recently published 100 top geosites list. 相似文献
13.
The ordering of Al and Si in gehlenite is considered using computer simulation. The enthalpy of ordering ΔH per 2Al+2Si atoms is found to be 0.52 eV. It is dominated by the nearest neighbour interaction, but the analysis is carried out to fifth neighbours. The nearest neighbour interaction differs significantly from that for other materials. The structure does not have a connected network of ordering sites, which mainly accounts for the unobservably low transition temperature for Al/Si ordering. Two alternatives are given for the likely ordering pattern. 相似文献
14.
Martin T. Dove Ian P. Swainson Brian M. Powell Donald C. Tennant 《Physics and Chemistry of Minerals》2005,32(7):493-503
Neutron powder diffraction studies of calcite on heating towards the orientational order–disorder phase transition show that
the phase transition is not a simple analogue of an Ising-like transition, but more similar to a rotational analogue of Lindemann
melting. The transition is precipitated by the librational amplitude of the carbonate molecular ions exceeding a critical
value rather than a result of a statistical entropy of ‘wrong’ orientations. Using tested interatomic potentials the single-particle
orientational potential and nearest-neighbour orientational interactions have been calculated. 相似文献
15.
Alfonso Hernández-Laguna Elizabeth Escamilla-Roa Vicente Timón Martin T. Dove C. Ignacio Sainz-Díaz 《Physics and Chemistry of Minerals》2006,33(10):655-666
Quantum mechanical calculations based on the density functional theory (DFT) are used to study the crystal structures of dioctahedral 2:1 phyllosilicates. The isomorphous cation substitution is investigated by exploring different substitutions of octahedral Al3+ by Mg2+ or Fe3+, tetrahedral substitution of Si4+ by Al3+, and different interlayer cations (IC) (Na+, K+, Ca2+, and Mg2+). Samples with different kinds of layer charges are studied: only tetrahedrally charged, only octahedrally charged, or mixed octahedral/tetrahedral charged. The effect of the relative arrangements of these substitutions on the lattice parameters and total energy is studied. The experimental observation of segregation tendency of Fe3+ and dispersion tendency of Mg2+ in the octahedral sheet is reproduced and explained with reference to the relative energies of the octahedral cation arrangements. These energies are higher than those due to the IC/tetrahedral and IC/octahedral relative arrangements. The tetrahedral and octahedral substitutions that generate charged layers also tend to be dispersed. The octahedral cation exchange potentials change with the IC-charge/ionic radius value. 相似文献
16.
The main driving force behind Al/Si ordering in tetrahedral framework aluminosilicates is nearest-neighbour Al/Al avoidance.
Computer simulation is used to explore the direct consequences of such Al/Al avoidance. The main result is that the order-disorder
transition temperature T
c
falls dramatically as the concentration x of Al in the structure is reduced, and if the only interactions are those associated with nearest-neighbour Al/Al avoidance,
T
c
becomes zero for x less than some critical value x
c
, where x
c
=0.31 for the feldspar framework and x
c
=0.34 for cordierite. Also a large degree of short range order is found above T
c
. Both results differ radically from the standard Bragg-Williams model. Plots of entropy and enthalpy of ordering are given
as functions of x and T, which may be used to interpret experimental data or for extrapolation into ranges of x and T inaccessible to experiment.
Received: 14 May 1997 / Revised, accepted: 2 June 1997 相似文献
17.
Computer modelling and theoretical analysis are used to explain the nearly zero and slightly negative coefficients of thermal
expansion in β-quartz well above the α-β phase transition temperature. Quartz was selected for study as an archetypal material with a framework structure of stiff
units, namely SiO4 tetrahedra, linked through shared oxygen atoms as very flexible hinges. The contributions of the soft mode, the Vallade mode,
the TAz phonon branch and the phonon spectrum as a whole are discussed in detail. The results fully support and illustrate a recent
theory of the negative contribution to thermal expansion in framework structures. It is a geometrical effect due to the rotation
of the tetrahedral units, folding together as they vibrate. The very rapid increase in the lattice parameters for about 20 K
above the transition temperature is well accounted for within quasiharmonic theory, and is therefore not evidence for critical
fluctuations or fluctuating patches of α
+, α
− structure.
Received August 14 1997 / Revised, accepted January 26 1998 相似文献
18.
19.
M. T. Dove K. D. Hammonds V. Heine R. L. Withers Y. Xiao R. J. Kirkpatrick 《Physics and Chemistry of Minerals》1996,23(1):56-62
Calculations of the rigid unit mode (RUM) spectrum of the high-temperature phase of SiO2 tridymite are used to explain the patterns of diffuse scattering seen in transmission electron microscopy experiments. These results show that RUM's can occur with wave vectors on curved surfaces in reciprocal space rather than being confined to symmetry points, lines or planes. The fact that the calculations reproduce the detail seen in the diffuse scattering provides a striking nontrivial confirmation of the validity of the rigid unit mode model. 相似文献
20.
We have measured the temperature dependence of the cell parameter of cubic β-cristobalite up to 1300° C by high-precision X-ray powder diffraction. The thermal expansion coefficient decreases on heating, until above 1000° C the cell parameter is virtually constant in value. We discuss this change in the thermal expansion with reference to the behaviour of low-frequency rigid unit modes and fluctuations associated with the α-β phase transition. 相似文献