High-pressure stability, structure and compressibility of Cmcm -MgAl2O4: an ab initio study     

M. Catti 《Physics and Chemistry of Minerals》2001,28(10):729-736

 Quantum-mechanical solid-state calculations have been performed on the highest-pressure polymorph of magnesium aluminate (CaTi₂O₄-type structure, Cmcm space group), as well as on the low-pressure (Fd3ˉm) spinel phase and on MgO and Al₂O₃. An ab initio all-electron periodic scheme with localized basis functions (Gaussian-type atomic orbitals) has been used, employing density-functional-theory Hamiltonians based on LDA and B3LYP functionals. Least-enthalpy structure optimizations in the pressure range 0 to 60 GPa have allowed us to predict: (1) the full crystal structure, the pV equation of state and the compressibility of Cmcm-MgAl₂O₄ as a function of pressure; (2) the phase diagram of the MgO–Al₂O₃–MgAl₂O₄ system (with exclusion of CaFe₂O₄-type Pmcn-MgAl₂O₄), and the equilibrium pressures for the reactions of formation/decomposition of the Fd3ˉm and Cmcm polymorphs of MgAl₂O₄ from the MgO + Al₂O₃ assemblage. Cmcm-MgAl₂O₄ is predicted to form at 39 and 57 GPa by LDA and B3LYP calculations, with K ₀=248 (K′=3.3) and 222 GPa (K′=3.8), respectively. Results are compared to experimental data, where available, and the performance of different DFT functionals is discussed. Received: 31 January 2001 / Accepted: 16 May 2001  相似文献   

Can the Earth's dynamo run on heat alone?   总被引：1，自引：0，他引：1  

David Gubbins  Dario Alfè  Guy Masters  G. David Price  M. J. Gillan 《Geophysical Journal International》2003,155(2):609-622

  相似文献   

Characterization of trace Nd and Ce site preference and coordination in natural melanites: a combined X-ray diffraction and high-energy XAFS study     

S. Quartieri  F. Boscherini  J. Chaboy  M.C. Dalconi  R. Oberti  A. Zanetti 《Physics and Chemistry of Minerals》2002,29(7):495-502

 Natural melanite garnets from carbonatitic rocks have been studied by a multitechnique approach based on both experimental (chemical, diffractometric and spectroscopic) methods and full-multiple scattering calculations of the X-ray absorption near-edge structure. In particular, the site location and geometry of trace amounts of neodimium (from 176 to 1074 ppm) and cerium (791 ppm) in natural garnets have been studied by fluorescence-detected X-ray absorption fine-structure spectroscopy (XAFS) at high energy. The measurements, done at both Nd K (43569 eV) and Ce K (40443 eV) edges, demonstrate that, in all the samples, the trace elements are located in the dodecahedral X site and not in ill-defined defects. The local geometry around the two rare-earth elements is compatible with their ionic radius and is compared with that of Ca, the major element at the X site, as determined by single-crystal X-ray diffraction data. This work represents the first example of direct investigation of trace-level REE coordination in natural garnets, and confirms the great relevance for the Earth Sciences of the use of fluorescence XAFS at high energy. Received: 7 September 2001 / Accepted: 15 April 2002  相似文献   

The relation between ore-deposit formation and subduction: Mass balance considerations     

G. P. Glasby 《Island Arc》1996,5(4):396-406

Abstract Mass balance calculations indicate that the potential supply of metals resulting from subduction of oceanic crust and sediment far exceeds that required for mineralization in island ares. Subduction of oceanic crust is more important than that of sediment in supplying these metals and the subduction of manganese nodules makes only a negligible contribution. The nature and extent of mineralization in island ares therefore depend on the nature, geometry and rate of subduction. In the Pacific, two types of subduction system occur: the high-stress (Chilean) type, characterized by shallow subduction and the formation of porphyry copper deposits in andesitic ares, and the low-stress (Mariana) type, characterized by deep subduction and the formation of Kuroko deposits in the back-arc basin, and porphyry copper and epithermal gold deposits in the island are. Kuroko deposits are analogous to the epithermal deposits but are formed from saline hydrothermal fluids under much higher hydrostatic pressure.  相似文献   

Forced vibrations of a beam elastically restrained against rotation and carrying a spring–mass system     

D. V. Bambill  C. A. Rossit 《Ocean Engineering》2002,29(6):912-626

An exact solution for the title problem is obtained in closed form fashion in the case of a Bernoulli–Euler beam. It is assumed that the exciting force is applied to the mass which is elastically mounted on the beam. The mathematical model constitutes a first order approximation to a motor or engine elastically mounted on a structural element. The operation of the machine generates a transverse, sinusoidally varying force. The problem is of basic interest in mechanical, naval and ocean engineering systems from the point of view of the determination of dynamic displacements and stresses; sound radiation calculations, etc. The present problem arose in connection with the mounting of an engine on a structural beam in a small naval vessel and when excessive vibrational level was noted. This study was undertaken in order to understand the physical problem and to correct the mechanical situation  相似文献   

Theoretical aspects of grade-tonnage calculations     

G. M. Philip  D. F. Watson 《Geotechnical and Geological Engineering》1985,3(2):149-154

Conclusions A theoretical approach to grade-tonnage calculations shows that methods based on accumulations are incorrect. Strictly correct calculations must involve multiplication of the various spatial functions, rather than the multiplication of measurements made at locations. If, however, polygonal interpolation is employed to estimate the spatial functions involved, then the results of the two approaches are identical. If any other interpolation procedure is used, then differences become obvious. Where trends are present in the data being computed, accumulation methods are likely to introduce serious errors.  相似文献   

Landslide analysis in Apennine chain areas     

F. Rapisarda 《Landslides》2007,4(1):75-83

  相似文献   

Dehydration of clastic sediments in subduction zones: Theoretical study using thermodynamic data of minerals     

Ying Li  Hans‐Joachim Massonne  Arne Willner  Hong‐Feng Tang  Cong‐Qiang Liu 《Island Arc》2008,17(4):577-590

Pseudosections for two sediments and one basalt calculated in the system K₂O–Na₂O–CaO–MgO–FeO–Fe₂O₃–Al₂O₃–TiO₂–SiO₂–H₂O for the P–T range 10 to 35 kbar, 300 to 900°C give useful insights into the amount of H₂O released from oceanic crust in subduction zones. In cold subduction zones (20 kbar–300°C to 35 kbar–500°C) hydrous minerals storing 3 to 4 wt% H₂O are still present in metasediments at depths of 120 km. In the same environment, metabasite releases 1 wt% H₂O in the depth range 100 to 120 km, but 4.5 wt% H₂O is transported to greater depths. In hot subduction zones (300°C hotter than the cold subduction zone at 100 km depth), dehydration events of metasediments in the depth range 50 to 80 km correspond to the breakdown of chlorite and paragonite. In the calculations no further water is released at greater depths because the modal content of phengite, the only hydrous mineral phase at these depths, remains almost constant. For the same P–T path, metabasite shows continuous dehydration between 40 and 80 km releasing almost 3 wt% H₂O. At 120 km depth less than 0.4 wt% of H₂O remains. In an average modern subduction zone (～6°C/km) most dehydration of sediments occurs at depths of 70 to 100 km and that of basalts at depths of 80 to 120 km. Only 1.3 wt% H₂O in metasediments and 1.6 wt% H₂O in metabasalt has the potential to be subducted to depths greater than 120 km. The dehydration behavior of sediments concurs with the generally held idea that subduction zone fluids are most effectively transported to great depths by cold subduction. In hot subduction zones, such as those characteristic of early Earth, most H₂O carried by oceanic crust is liberated at depths less than 120 km and, thus, would not contribute to island‐arc magmatism.  相似文献   

Consistency of the activity–composition models of Holland,Green, and Powell (2018) with experiments on natural and synthetic compositions: A comparative study     

Marcos García-Arias 《Journal of Metamorphic Geology》2020,38(9):993-1010

The recent publication of new activity–composition models by Holland, Green and Powell (2018; Journal of Petrology 59 : 881–900), with a melt model calibrated for source compositions ranging from peridotitic to granitic, opens the door to the modelling of multiple petrogenetic processes at supersolidus conditions in which the composition of the melt phase changes considerably, without having to change the melt model. This melt model is also the first one using the internally consistent thermodynamic databases published by T. Holland and R. Powell that contains TiO₂ and Fe₂O₃, further expanding the application of this model to more realistic geological scenarios. The accompanying mineral models are also the first in containing some minor elements, like TiO₂ in garnet and K₂O in clinopyroxene. Consequently, it is relevant to test the applicability of these new models to a large P–T–X range of conditions before they can be used in full. Thermodynamic calculations made with the software Perple_X using these models were compared to experimental results, namely the modal proportions and the composition of the melt and several mineral solution phases. The experiments chosen for the comparative study covered a wide range of source compositions (from mafic to felsic), pressure (from 0.3 to 2.1 GPa), temperature (from 700 to 1,150°C) and total and added water content (structural water: 0.15–1.48 wt%; added water: 0–8 wt%; total water: 0.15%–8.15%). The results indicate that the extended melt model reproduces well the composition of the experimental melts, with an inverse correlation between component amount and fit: the best match is found for SiO₂ (−0.8% on average) and the worst match is found for those elements with the lowest amounts, TiO₂ and MgO (+241% and +235% on average, respectively; values indicate calculated minus experimental, times 100 and divided by experimental). The TiO₂ content in the melt model increases dramatically with increasing pressure, from +90% for P < 1.5 GPa to +593% for P > 1.5 GPa. No comparison was made on the Fe₂O₃ content, as the published iron contents of the experimental melts were always reported as FeO_t. In some cases, there is a substantial mismatch in the modal proportions between experiments and calculations, with the reactant phases less abundant and product phases more abundant in the calculations, an effect that is attributed to kinetic effects in the experiments and to the selected clinoamphibole model. Finally, the extended melt model was compared to the tonalitic melt model of Green et al. (2016; Journal of Metamorphic Geology, 34 : 845–869). Both melt models produce very similar results for SiO₂, Al₂O₃, Na₂O and K₂O, with slightly better results for the tonalitic melt model in FeO_t and MgO and for the extended melt model in CaO. No comparison is made on TiO₂ because the tonalitic melt model does not include this component. In summary, the new activity–composition models represent a significant contribution to thermodynamic calculations on the evolution of siliceous magmas where their composition, temperature and pressure changes substantially.  相似文献   

Solving the kriging problem by using the Gram-Schmidt orthogonalization     

Marek Kacewicz 《Mathematical Geology》1991,23(1):111-118

This paper presents an approach to solve the kriging problem, defined in terms of projections, by using Gram-Schmidt orthogonalization. The Gram-Schmidt orthogonalization allows us to find an optimal approximationY* in then-dimensional subspaceH _n of any vectorY element of a Hilbert spaceH. This approach requiresO(n ³) multiplication operations to obtain an orthogonal basis, andO(n ²) multiplications needed to calculate kriging solution for a given point.  相似文献