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1.
The aim of this study is to present an automatic approach for olive tree dendrometric parameter estimation from airborne laser scanning (ALS) data. The proposed method is based on a unique combination of the alpha-shape algorithm applied to normalized point cloud and principal component analysis. A key issue of the alpha-shape algorithm is to define the α parameter, as it directly affects the crown delineation results. We propose to adjust this parameter based on a group of representative trees in an orchard for which the classical field measurements were performed. The best value of the α parameter is one whose correlation coefficient of dendrometric parameters between field measurements and estimated values is the highest. We determined crown diameters as principal components of ALS points representing a delineated crown. The method was applied to a test area of an olive orchard in Spain. The tree dendrometric parameters estimated from ALS data were compared with field measurements to assess the quality of the developed approach. We found the method to be equally good or even superior to previously investigated semi-automatic methods. The average error is 19% for tree height, 53% for crown base height, and 13% and 9% for the length of the longer diameter and perpendicular diameter, respectively.  相似文献   
2.
Single-crystal X-ray diffraction experiments with SiO2 α-cristobalite reveal that the well-known reversible displacive phase transition to cristobalite-II, which occurs at approximately 1.8 GPa, can be suppressed by rapid pressure increase, leading to an overpressurized metastable state, persisting to pressure as high as 10 GPa. In another, slow pressure increase experiment, the monoclinic high-pressure phase-II was observed to form at ~1.8 GPa, in agreement with earlier in situ studies, and its crystal structure has been unambiguously determined. Single-crystal data have been used to refine the structure models of both phases over the range of pressure up to the threshold of formation of cristobalite X-I at ~12 GPa, providing important constraints on the feasibility of the two competing silica densification models proposed in the literature, based on quantum mechanical calculations. Preliminary diffraction data obtained for cristobalite X-I reveal a monoclinic unit cell that contradicts the currently assumed model.  相似文献   
3.
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4.
MgSiO3 akimotoite is stable relative to majorite-garnet under low-temperature geotherms within steeply or rapidly subducting slabs. Two compositions of Mg–akimotoite were synthesized under similar conditions: Z674 (containing about 550 ppm wt H2O) was synthesized at 22 GPa and 1,500 °C and SH1101 (nominally anhydrous) was synthesized at 22 GPa and 1,250 °C. Crystal structures of both samples differ significantly from previous studies to give slightly smaller Si sites and larger Mg sites. The bulk thermal expansion coefficients of Z674 are (153–839 K) of a 1 = 20(3) × 10?9 K?2 and a 0 = 17(2) × 10?6 K?1, with an average of α 0 = 27.1(6) × 10?6 K?1. Compressibility at ambient temperature of Z674 was measured up to 34.6 GPa at Sector 13 (GSECARS) at Advanced Photon Source Argonne National Laboratory. The second-order Birch–Murnaghan equation of state (BM2 EoS) fitting yields: V 0 = 263.7(2) Å3, K T0 = 217(3) GPa (K′ fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: α a  = 8.2(3) and α c  = 10.68(9) (10?6 K?1); β a  = 11.4(3) and β c  = 15.9(3) (10?4 GPa). Hydration increases both the bulk thermal expansivity and compressibility, but decreases the anisotropy of structural expansion and compression. Complementary Raman and Fourier transform infrared (FTIR) spectroscopy shows multiple structural hydration sites. Low-temperature and high-pressure FTIR spectroscopy (15–300 K and 0–28 GPa) confirms that the multiple sites are structurally unique, with zero-pressure intrinsic anharmonic mode parameters between ?1.02 × 10?5 and +1.7 × 10?5 K?1, indicating both weak hydrogen bonds (O–H···O) and strong OH bonding due to long O···O distances.  相似文献   
5.
The crystal structures of natural jadeite, NaAlSi2O6, and synthetic kosmochlor, NaCrSi2O6, were studied at room temperature, under hydrostatic conditions, up to pressures of 30.4 (1) and 40.2 (1) GPa, respectively, using single-crystal synchrotron X-ray diffraction. Pressure–volume data have been fit to a third-order Birch–Murnaghan equation of state yielding V 0 = 402.5 (4) Å3, K 0 = 136 (3) GPa, and K 0 ′  = 3.3 (2) for jadeite and V 0 = 420.0 (3) Å3, K 0 = 123 (2) GPa and K 0 ′  = 3.61 (9) for kosmochlor. Both phases exhibit anisotropic compression with unit-strain axial ratios of 1.00:1.95:2.09 for jadeite at 30.4 (1) GPa and 1:00:2.15:2.43 for kosmochlor at 40.2 (1) GPa. Analysis of procrystal electron density distribution shows that the coordination of Na changes from 6 to 8 between 9.28 (Origlieri et al. in Am Mineral 88:1025–1032, 2003) and 18.5 (1) GPa in kosmochlor, which is also marked by a decrease in unit-strain anisotropy. Na in jadeite remains six-coordinated at 21.5 (1) GPa. Structure refinements indicate a change in the compression mechanism of kosmochlor at about 31 GPa in both the kinking of SiO4 tetrahedral chains and rate of tetrahedral compression. Below 31 GPa, the O3–O3–O3 chain extension angle and Si tetrahedral volume in kosmochlor decrease linearly with pressure, whereas above 31 GPa the kinking ceases and the rate of Si tetrahedral compression increases by greater than a factor of two. No evidence of phase transitions was observed over the studied pressure ranges.  相似文献   
6.
The nickel arsenide (B81) and related crystal structures are among the most important crystallographic arrangements assumed by Fe and Ni compounds with light elements such as Si, O, S, and P, expected to be present in planetary cores. Despite the simple structure, some of these materials like troilite (FeS) exhibit complex phase diagrams and rich polymorphism, involving significant changes in interatomic bonding and physical properties. NiP (oP16) represents one of the two principal structure distortions found in the nickel arsenide family and is characterized by P–P bonding interactions that lead to the formation of P2 dimers. In the current study, the single-crystal synchrotron X-ray diffraction technique, aided by first principles density functional theory (DFT) calculations, has been applied to examine the compression behavior of NiP up to 30 GPa. Two new reversible displacive phase transitions leading to orthorhombic high-pressure phases with Pearson symbols oP40 and oC24 were found to occur at approximately 8.5 and 25.0 GPa, respectively. The oP40 phase has the primitive Pnma space group with unit cell a = 4.7729(5) Å, b = 16.6619(12) Å, and c = 5.8071(8) Å at 16.3(1) GPa and is a superstructure of the ambient oP16 phase with multiplicity of 2.5. The oC24 phase has the acentric Cmc21 space group with unit cell a = 9.695(6) Å, b = 5.7101(9) Å, and c = 4.7438(6) Å at 28.5(1) GPa and is a superstructure of the oP16 phase with multiplicity of 1.5. DFT calculations fully support the observed sequence of phase transitions. The two new phases constitute logical next stages of P sublattice polymerization, in which the dilution of the P3 units, introduced in the first high-pressure phase, decreases, leading to compositions of Ni20(P3)4(P2)4 and Ni12(P3)4, and provide important clues to understanding of phase relations and transformation pathways in the NiAs family.  相似文献   
7.
At ambient conditions, witherite is the stable form of BaCO3 and has the aragonite structure with space group Pmcn. Above ~10 GPa, BaCO3 adopts a post-aragonite structure with space group Pmmn. High-pressure and high-temperature synchrotron X-ray diffraction experiments were used to study the stability and equation of state of post-aragonite BaCO3, which remained stable to the highest experimental P–T conditions of 150 GPa and 2,000 K. We obtained a bulk modulus K 0 = 88(2) GPa with $K'$  = 4.8(3) and V 0 = 128.1(5) Å3 using a third-order Birch-Murnaghan fit to the 300 K experimental data. We also carried out density functional theory (DFT) calculations of enthalpy (H) of two structures of BaCO3 relative to the enthalpy of the post-aragonite phase. In agreement with previous studies and the current experiments, the calculations show aragonite to post-aragonite phase transitions at ~8 GPa. We also tested a potential high-pressure post–post-aragonite structure (space group C222 1 ) featuring four-fold coordination of oxygen around carbon. In agreement with previous DFT studies, ΔH between the C222 1 structure and post-aragonite (Pmmn) decreases with pressure, but the Pmmn structure remains energetically favorable to pressures greater than 200 GPa. We conclude that post–post-aragonite phase transformations of carbonates do not follow systematic trends observed for post-aragonite transitions governed solely by the ionic radii of their metal cations.  相似文献   
8.
A comparative study of damage accumulation in magnesium aluminate spinel (MgAl2O4) has been conducted using ionoluminescence (IL), cathodoluminescence (CL) and Rutherford Backscattering Spectrometry/channeling (RBS/C) techniques. MgAl2O4 single crystal and polycrystalline samples were irradiated with 320 keV Ar+ ions at fluencies ranging from 1 × 1012 to 2 × 1016 cm?2 in order to create various levels of radiation damage. RBS/C measurements provided quantitative data about damage concentration in the samples. These values were then compared to the luminescence measurements. The results obtained by IL and RBS/C methods demonstrate a two-step character of damage buildup process. The CL data analysis points to the three-step damage accumulation mechanism involving the first defect transformation at fluencies of about 1013 cm?2 and second at about 1015 cm?2. The rate of changes resulting from the formation of nonluminescent recombination centers is clearly nonlinear and cannot be described in terms of continuous accumulation of point defects. Both, IL and CL techniques, appear as new, complementary tools bringing new possibilities in the damage accumulation studies in single- and polycrystalline materials.  相似文献   
9.
Understanding the partitioning of aqueous boron species into marine carbonates is critical for constraining the boron isotope system for use as a marine pH proxy. Previous studies have assumed that boron was incorporated into carbonate through the preferential uptake of tetrahedral borate B(OH)4−. In this study we revisit this assumption through a detailed solid state 11B magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic study of boron speciation in biogenic and hydrothermal carbonates. Our new results contrast with those of the only previous NMR study of carbonates insofar as we observe both trigonal and tetrahedral coordinated boron in almost equal abundances in our biogenic calcite and aragonite samples. In addition, we observe no strict dependency of boron coordination on carbonate crystal structure. These NMR observations coupled with our earlier re-evaluation of the magnitude of boron isotope fractionation between aqueous species suggest that controls on boron isotope composition in marine carbonates, and hence the pH proxy, are more complex that previously suggested.  相似文献   
10.
Three isotypic crystals, SiO2 (α-cristobalite), ε-Zn(OH)2 (wülfingite), and Be(OH)2 (β-behoite), with topologically identical frameworks of corner-connected tetrahedra, undergo displacive compression-driven phase transitions at similar pressures (1.5–2.0 GPa), but each transition is characterized by a different mechanism resulting in different structural modifications. In this study, we report the crystal structure of the high-pressure γ-phase of beryllium hydroxide and compare it with the high-pressure structures of the other two minerals. In Be(OH)2, the transition from the ambient β-behoite phase with the orthorhombic space group P212121 and ambient unit cell parameters a = 4.5403(4) Å, b = 4.6253(5) Å, c = 7.0599(7) Å, to the high-pressure orthorhombic γ-polymorph with space group Fdd2 and unit cell parameters (at 5.3(1) GPa) a = 5.738(2) Å, b = 6.260(3) Å, c = 7.200(4) Å takes place between 1.7 and 3.6 GPa. This transition is essentially second order, is accompanied by a negligible volume discontinuity, and exhibits both displacive and reversible character. The mechanism of the phase transition results in a change to the hydrogen bond connectivities and rotation of the BeO4 tetrahedra.  相似文献   
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