100,491 research outputs found
Population status of Anchusa littorea. A Critically Endangered species of Sardinia (Italy)
No full textHigh pressure and high temperature behaviour of alkali-halide fluorite CaF2 for technological applications
No full textFluorite (CaF 2 , space group Fm 3 m ) is an alkali-earth halide mineral with important and manyfold technological applications and for these means several experimental and theoretical investigations were performed to characterize the structural, electronic, optical, and elastic properties. However, a detailed knowledge of the thermodynamics and thermoelastic properties of fluorite in a wide temperature and pressure range is still missing. In this work, density functional theory simulations using the hybrid B3LYP functional and all-electron Gaussiantype orbitals basis sets were employed to model these properties between 0 - 1000 K and from 0 GPa to 7 GPa. The calculated PVT equation of state parameters were V 0 T = 42.278(7) & Aring; 3 , K 0 T = 92.14(7) GPa, K ' 0 T = 3.56(2) using a 3rd-order Birch-Murnaghan formulation, alpha 0 = 7.84(2) x 10 -5 K -1 and alpha 1 = 2.17(3) x 10 -5 K -1/2 from a modified Holland and Powell thermal equation of state, and a variation of the bulk modulus with temperature partial derivative K 0 T / partial derivative T = - 0.0160(1) GPa K -1 . The quality of the theoretical results was assessed by comparison with the few available data reported in the scientific literature, finding a general good agreement and extending the knowledge on this important technological material
Conservation and restoration of indigenous plants to improve community livelihoods: the Useful Plants Project
No full textPreliminary results on the conservation of Lamyropsis Microcephala (Moris) Dittrich & Greuter (Asteraceae), a threatened endemic species of the Gennargentu massif, Sardinia (Italy)
No full textLamyropsis microcephala (Moris) Dittrich & Greuter (Asteraceae) is a perennial species, narrow endemic of Sardinia (Italy) and known only in two sites of the
Gennargentu massif. The aims of this study are to investigate characteristics of the ecology and biology of the species which might be important for its conservation and reassess its conservation status. Here we show the preliminary results of this study, with a focus on the abundance and distribution of the species, its reproductive capacity and the levels of threats. From the data collected in the field it appears that the population had been
fragmented into two subpopulations. The total population covered an area of around 12 hectares and consisted of around 2500 individuals. The reproductive biology of the species, in particular the low production of fertile seeds and absence of seedlings (recruitment) in the field, seems to be one of the main biological causes of the threatened status of the species. Increases in extensive grazing and tourism, linked to the skiing and trekking activities, have been recognized as the major anthropogenic threats to the species. The preliminary results of this study confirm the conservation status of L. m-icrocephala as Critically Endangered
Density functional investigation of the thermophysical and thermochemical properties of talc [Mg3Si4O10(OH)2]
No full textThe knowledge of the P, T behavior of talc is very important in mineralogical–petrological and geophysical research fields because talc can be considered a hydrous phase that can recycle water into the Earth’s mantle and also an important mineral in both industrial and technological applications. However, very few works have been presented to fully characterize the thermodynamic properties of this mineral, especially at atomic scale. In a previous work, we modeled the structural and mechanical properties of talc using the B3LYP-D* hybrid density functional, which included a correction for the dispersive forces and all-electron Gaussian-type orbital basis sets. The results were in good agreement with single-crystal X-ray and neutron diffraction experimental data. Here, we extend the investigation to the thermochemical and thermophysical properties of talc using the same density functional approach and the quasi-harmonic approximation, providing the thermal equation of state, the heat capacity and the entropy of the mineral at different P, T conditions
Density functional investigation of the thermo-physical and thermo-chemical properties of 2M(1) muscovite
No full textIn the present study, we computed the thermo-chemical and thermo-physical properties of the 2M(1) polytype of muscovite in the 0-10 GPa and 0-900 K ranges, using the hybrid DFT/B3LYP-D* density functional, corrected to take into account dispersive forces, and by using the quasi-harmonic approximation. The bulk modulus K-TO of muscovite, its first derivative K', and the unit-cell volume at zero pressure V-0 at 298.15 K, calculated using a third-order Birch-Mumaghan equation of state, were K-T0 = 59.93 GPa, K' = 7.84, and V-0 = 940.6 angstrom(3). Our theoretical data are in good agreement with previous experimental results obtained by X-ray diffraction. Thermal bulk moduli, K-T, thermal expansion coefficients, al', and heat capacity at different P-T conditions are given, which could be useful in both geophysical and technological applications. The results of this kind of analysis can be used in the study of the thermodynamic properties of solid phases at physical conditions that are difficult to obtain during experimental procedures, especially under controlled high pressures and temperatures
Structural, vibrational and thermophysical properties of pyrophyllite by semi-empirical density functional modelling
No full textPyrophyllite has a significant role in both geophysics as a hydrous phase, which can recycle water into the Earth’s mantle, and many industrial applications, such as petroleum and civil engineering. However, very few works have been proposed to fully characterize the thermodynamic properties of this mineral, especially at atomic scale. In the present work, we report structural, vibrational, thermochemical and thermophysical properties of pyrophyllite, calculated at the density functional theory level with the hybrid B3LYP functional, all-electron Gaussian-type orbitals and taking into account a correction to include dispersive forces. V(P, T) data at 300 K fit with isothermal third-order Birch–Murnaghan equations of state and yield KT0 = 46.57 GPa, K′ = 10.51 and V0 = 213.67 Å3, where KT0 is the thermal bulk modulus at 0 GPa, K′ is the first derivative and V0 is the volume at zero pressure, in very good agreement with recent experimental results obtained by in situ single-crystal synchrotron XRD. The compressional behaviour is highly anisotropic, with axial compressibility in ratio β(a):β(b):β(c) = 1.218:1.000:4.188. Pyrophyllite bulk modulus, thermal expansion coefficients and heat capacity at different P–T conditions are provided. The results of this kind of analysis can be useful in both geophysical and technological applications of the mineral and expand the high-temperature and high-pressure knowledge of this phase at physical conditions that are still difficult to obtain by experimental means. The simulated vibrational spectrum can also be used as a guideline by other authors in their experimental investigation of pyrophyllite
Structural and elastic behaviour of aragonite at high-pressure: A contribution from first-principle simulations
No full textAragonite (CaCO3, space group Pmcn) is an important mineral for both geological and biological reasons, being one of the phases that recycles carbon in deep Earth conditions and the product of biomineralization of several terrestrial and marine organisms, respectively. Because of its ubiquity, aragonite has been the subject of several investigations to understand its elastic behaviour and stability at different P-T conditions, but the results reported in literature are still very scattered. Aiming at providing further details on this topic, in the present work we determined the structural and elastic properties of aragonite at absolute zero (0 K) within the Density Functional Theory framework, using a posteriori correction to include the weak long-range interactions. The equation of state parameters for this mineral phase, calculated between 0 GPa – 25 GPa, were K0 = 80.2(7) GPa, K’ = 4.37(10) and V0 = 223.00(6) Å3, in good agreement with the bulk modulus calculated from the elastic moduli (KR = 78.49 GPa). The results were compared to previous experimental and theoretical data, finding them in line with some specific studies, and show that some structural features (e.g., the carbonate ion aplanarity) could be related to the mechanism of phase transition to the post-aragonite phase at high pressure. The present work highlights the importance of including van der Waals interactions in the physical treatment of the structural and elastic properties of aragonite, and further extends the knowledge of the behaviour of this mineral as a function of pressure
Seasonality effects on plant phenology and seed ecology in Oritrophium peruvianum (Asteraceae), a threatened tropical alpine species
No full textOritrophium peruvianum (Lam.) Cuatr. (Asteraceae) is a threatened perennial herb, endemic to the tropical alpine zone of the Andean páramos and used for medicinal purposes. In this study, its population structure, phenological patterns and seed viability and germination were analysed. In all of the three investigated populations, the proportion of flowering plants was smaller than that of non-flowering. Size structure differed among populations, with two following a normal and one a left-skewed distribution, with a higher percentage of young plants. Flowering was spread throughout the rainy season, with fruiting intensity highest at the end of the rainy season and at the beginning of the dry season. Seedling emergence was limited and appeared to be independent of the season, indicating opportunities for continuous seedling emergence and mortality throughout the year. Seed viability was higher in seeds collected at the end of the dry season compared with those harvested at the end of the rainy season. Laboratory studies suggest that under natural conditions, germination timing is likely to be independent from the season
Effects of fluorine content on the elastic behavior of topaz [Al2SiO4(F,OH)2]
No full textIn this work, we modeled the structure, the compressional behavior and the physical properties of topaz over six different fluorine contents and a wide range of pressure, using a quantum mechanical approach based on periodic boundary conditions. We adopted the density functional theory using the B3LYP functional and all-electron Gaussian-type orbitals basis sets. An atomic level description of the athermal (T = 0 K) pressure-induced structural modification of topaz is provided. From the compression results we obtained the athermal bulk modulus (KT0), its first derivative (Kâ2) and the athermal volume at zero pressure (V0) by a third-order Birch-Murnaghan equation fit. The results show that KT0increases with fluorine content. The compressional pattern is anisotropic, as observed by the axial compressibility and second-order elastic constants calculations. We observed that the compression involves three different mechanism, polyhedral contraction, polyhedral tilting and hydrogen bonding, all of them influenced, with different extent, by the fluorine content in topaz. Recent experimental results obtained by single-crystal X-ray and neutron diffraction of specific topaz compositions are in very good agreement with our simulations, which further extend the knowledge of the structural and elastic properties of topaz over a wider range of fluorine content
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