Phase equilibrium and calorimetric study of the transition of MnTiO3 from the ilmenite to the lithium niobate structure and implications for the stability field of perovskite     

Jaidong Ko  Nancy E. Brown  Alexandra Navrotsky  Charles T. Prewitt  Tibor Gasparik 《Physics and Chemistry of Minerals》1989,16(8):727-733

The phase boundary between MnTiO₃ I (ilmenite structure) and MnTiO₃ II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO₃ I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔH^o=5300±1000 J·mol⁻¹, ΔS^o = 1.98 ±1J·K⁻¹· mol⁻¹. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol⁻¹. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate, ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO₃ is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms of crystal structure and lattice vibrations.  相似文献   

Energetics of radiation damage in natural zircon (ZrSiO4)     

Susan Ellsworth  Alexandra Navrotsky  Rodney C. Ewing 《Physics and Chemistry of Minerals》1994,21(3):140-149

Transposed temperature drop calorimetry at 1000 °C was performed on natural zircons (ZrSiO₄) from Sri Lanka that were partially to completely metamict due to α-decay event damage (0.06 to 11.7×10¹⁵ α-decay events/mg). The enthalpy of annealing at room temperature (ΔH_anneal) varies sigmoidally as a function of radiation dose. ΔH_anneal reaches a saturation plateau at radiation doses greater than 5×10¹⁵ α-decay events/mg. The annealing of several samples to a crystalline structure with broadened diffraction peaks does not significantly affect the enthalpy of annealing. The large magnitude of the enthalpy of annealing plateau, ?59±3 kJ/mol, suggests that the damage to the structure is pervasive on the scale of Angstroms, consistent with the loss of mid-range order characteristic of a glass. The energetics are consistent with, but do not require, chemical heterogeneity caused by micro-domains of amorphous SiO₂-rich and ZrO₂-rich regions in the metamict state.  相似文献   

Distribution and migrations of the prawnPalaemon longirostris in the Mira River estuary (southwest Portugal)     

Alexandra Cartaxana 《Estuaries and Coasts》1994,17(3):685-694

Palaemon longirostris was sampled monthly at 12 sites in the Mira River estuary (southwest Portugal) from October 1990 to September 1991. Animals were counted, measured, and sexed. The estuarine distribution of prawns appeared to follow the salinity displacement, the animals being found at river stations during summer and autumn when saline encroachment up the estuary was greater. During winter and spring when freshwater input from the river was greatest, and thus moving the salinity gradient toward the mouth, the prawns migrated downstream to brackish areas. Ovigerous females were collected only from estuarine areas (January to August), suggesting that reproduction does not take place in freshwater habitats. Higher percentages of females than males were generally observed. Females, particularly ovigerous ones, were larger than males.  相似文献   

Variation among fossil chironomid assemblages in surficial sediments of Bodensee-Untersee (SW-Germany): implications for paleolimnological interpretation     

Alexandra Schmäh 《Journal of Paleolimnology》1993,9(2):99-108

In paleolimnology, subfossil head capsules of chironomids play an important role as ecological indicators of lake history. It is important to determine, therefore, whether fossil assemblages are representative of former biocoenoses. There is evidence that headcapsules washed in from other places can make up a significant percentage of the total. As interpretations are usually drawn from the examination of a single core, it is of special interest to know whether a fossil assemblage of a single site properly reflects limnological conditions of the whole lake. This study examined the taxonomic distribution of subfossil chironomids in the surficial sediments of the Bodensee-Untersee, with the aim of assessing the variability in chironomid assemblages. Apparently, most of the head capsules of the profundal fossil assemblages in the Untersee had been washed in from the littoral zone or from the slope. Although the Bodensee-Untersee is a rather large lake, variability is surprisingly low among all samples. Therefore a correct interpretation from a single core may be possible.  相似文献   

Direct measurements of latent heat during crystallization and melting of a ugandite and an olivine basalt     

Rebecca A. Lange  Katharine V. Cashman  Alexandra Navrotsky 《Contributions to Mineralogy and Petrology》1994,118(2):169-181

Step-scanning calorimetric measurements using a Setaram HT1500 calorimeter were performed between 800 and 1400°C on two natural samples: a ugandite from the East African rift and an olivine basalt from the western Mexican arc. Our measurements provide the first in-situ quantitative assessment of enthalpy during melting of initially crystalline natural samples. The distribution of latent heat across the liquidus-solidus intervals of the two samples is distinctly different, reflecting significant variation in the sequence and abundance of mineral phases during melting (clinopyroxene and leucite in the ugandite; olivine, clinopyroxene, and plagioclase in the basalt). Our data further indicate that the common assumption of a uniform distribution of latent heat across the liquidus-solidus interval of a magma is a reasonable approximation for the olivine basalt, but is grossly in error for the ugandite. This is due to cotectic precipitation of leucite and clinopyroxene, leading to a large, disproportionate release of latent heat early in the crystallization sequence. The implication for the thermal history of a crystallizing ugandite magma is that therate of heat loss during conductive cooling will unitially be more rapid than the average rate. The net result will be to produce lower magmatic temperatures after a given cooling interval relative to models assuming a uniform release of latent heat. An additional series of scanning calorimetric experiments were performed at variable rates (1,2 and 3°/min) to evaluate the role of kinetics on the distribution of enthalpy during both melting and crystallization of the ugandite and olivine basalt. The results indicate that clinopyroxene is the most important mineral phase in controlling the shapes of the enthalpy profiles during cooling; this is due to its large enthalpy of fusion and its tendency for sluggish nucleation, followed by rapid crystallization at temperatures that vary with cooling rate. The resolution of the calorimeter (in terms of heat detected per unit time) is also important in determining the shapes of theobserved enthalpy profiles during these rapid scans. Estimates based on the observed calorimetric signal associated with melting of olivine, and the lack of a calorimetric signal during melting of leucite, combined with known enthalpies of fusion for the two phases, indicate detection limits of approximately 0.6–1.2 kJ per 5 min increments.  相似文献   

Interaction of cations on octahedral and tetrahedral sites in simple spinels: A comment     

Alexandra Navrotsky 《Physics and Chemistry of Minerals》1984,10(4):192-193

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A thermochemical study of glasses and crystals along the joins silica-calcium aluminate and silica-sodium aluminate     

Alexandra Navrotsky  Gilles Peraudeau  Paul McMillan  Jean-Pierre Coutures 《Geochimica et cosmochimica acta》1982,46(11):2039-2047

Enthalpies of solution in molten 2PbO · B₂O₃ at 985 K are reported for series of glasses xCa_0.5AlO₂-(1?x)SiO₂ (O ≤ x ≤ 0.99) and xNaAlO₂-(1?x)SiO₂ (0 ≤ x ≤ 0.56). The data are compared to values for the corresponding crystalline aluminosilicates and to preliminary data for systems containing KAlO₂ and Mg_0.5AlO₂. The enthalpies of mixing of glasses become more exothermic with increasing basicity of the mono- or divalent oxide. The tendency toward immiscibility on the silica-rich side, indicated by the shape of the heat of mixing curve between x = 0 and x = 0.4, is pronounced in the calcium aluminate system, but not in the sodium aluminate system. The shape of the heat of mixing curve, which is roughly symmetrical about x = 0.5, can be rationalized in terms of glass structure by considering essentially random substitution of Si and Al on a continuous three dimensional tetrahedral framework, with stabilization arising from electrostatic interactions between aluminum and the nonframework cation balancing the destabilizing effects arising from perturbation of the aluminosilicate framework by the nonframework cation. These trends are consistent with the variation of physical properties of aluminosilicate melts.  相似文献   

Derivation Of Refractive Index And Temperature Gradients From Optical Scintillometry To Correct Atmospherically Induced Errors For Highly Precise Geodetic Measurements     

Alexandra I. Weiss  Maria Hennes  Mathias W. Rotach 《Surveys in Geophysics》2001,22(5-6):589-596

Refraction effects of optical beams are generally caused by an inhomogeneous propagation medium and are a major source of systematic errors in the precise optical determination of angles and distances in the atmospheric surface layer. In this contribution a method for deriving vertical temperature and refractive index gradients from optical scintillation is presented. Knowledge of these gradients is required for the compensation of atmospherically induced errors for highly precise terrestrial geodetic measurements, like direct transfer and levelling. The advantage of the present optical method is, that temperature and refractive index gradients can be derived as line-averaged values over the propagation path, which is not possible by meteorological point measurements. Field observations have been carried out with a displaced-beam scintillometer over flat terrain and under different atmospheric conditions in order to verify this method. The experiments show, that this method allows to derive accurate correction values for precise terrestrial geodetic measurements.  相似文献   

Application of Generalized Likelihood Uncertainty Estimation (GLUE) at different temporal scales to reduce the uncertainty level in modelled river flows     

Ragab Ragab  Alexandra Kaelin  Muhammad Afzal  Ioanna Panagea 《水文科学杂志》2020,65(11):1856-1871

ABSTRACT

In this study, the distributed catchment-scale model, DiCaSM, was applied on five catchments across the UK. Given its importance, river flow was selected to study the uncertainty in streamflow prediction using the Generalized Likelihood Uncertainty Estimation (GLUE) methodology at different timescales (daily, monthly, seasonal and annual). The uncertainty analysis showed that the observed river flows were within the predicted bounds/envelope of 5% and 95% percentiles. These predicted river flow bounds contained most of the observed river flows, as expressed by the high containment ratio, CR. In addition to CR, other uncertainty indices – bandwidth B, relative bandwidth RB, degrees of asymmetry S and T, deviation amplitude D, relative deviation amplitude RD and the R factor – also indicated that the predicted river flows have acceptable uncertainty levels. The results show lower uncertainty in predicted river flows when increasing the timescale from daily to monthly to seasonal, with the lowest uncertainty associated with annual flows.  相似文献   

The <Superscript>230</Superscript>Th/U dating of sulfide ores in the ocean: Methodical possibilities,measurement results,and perspectives of application     

V. Yu. Kuznetsov  G. A. Cherkashev  V. E. Bel’tenev  A. Yu. Lein  F. E. Maximov  V. V. Shilov  T. V. Stepanova 《Doklady Earth Sciences》2007,417(1):1202-1205

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