Structural change in lead fluorapatite at high pressure

The structure of lead fluorapatite [PbFAP; Pb₁₀(PO₄)₆F₂], crystallized from the melt in a platinum capsule at 1,000°C and 1 atm, has been investigated by single-crystal X-ray diffraction. Crystal data are a = 9.7638 (6), c = 7.2866 (4) Å, space group P6₃/m, R = 0.043, R _w = 0.034. We have also studied the compressional behaviour of the c-axis channel of PbFAP up to 9 GPa at 25°C, using a diamond-anvil cell, synchrotron X-radiation, and Rietveld powder structure refinement. Pressure–volume data for the channel polyhedron of PbFAP fitted to the third-order Birch–Murnaghan equation resulted in K _T  = 33.2 ± 1.2 GPa when K _T ′ is fixed at 4. The c-axis channel of PbFAP is about twice as compressible as the unit-cell volume of PbFAP and the channel of calcium apatites. This is attributed to the anomalous narrowing of the channel of PbFAP with increase in confining pressure. Flexibility of the apatite channel is a key factor in the scavenging of toxic heavy metals by calcium apatites.     

Quantity, distribution, and impacts of coastal driftwood triggered by a typhoon

Typhoon Morakot pounded Taiwan in 2009 with record-breaking rainfall, washing an unprecedented amount of driftwood into the sea that was partially deposited at the coastal areas. According to the satellite imagery analysis, more than three million trees fell and were washed away to occupy 83.2% of the Taiwanese coastline, including 52 fishing harbors. The amount cleaned-up was only 1/7 of the total coastal driftwood. It was found that the amount of coastal driftwood is not only related to the amount of precipitation but is also related to the distance from the location of the landslide to the river mouth and to the landslide area. The amount of accumulated coastal driftwood demonstrated log-profile declines with increasing distance to the river mouth. Nearshore current and wave motion are the critical factors for driftwood deposition. Much of the driftwood washed into the sea harmed the tourism and fishing industries, endangered navigation and oceanic activities, and impacted the marine environment and ecosystem.     

Synthetic lead bromapatite: X-ray structure at ambient pressure and compressibility up to about 20 GPa

Lead bromapatite [Pb₁₀(PO₄)₆Br₂] has been synthesized via solid-state reaction at pressures up to 1.0 GPa, and its structure determined by single-crystal X-ray diffraction at ambient temperature and pressure. The large bromide anion is accommodated in the c-axis channel by lateral displacements of structural elements, particularly of Pb2 cations and PO₄ tetrahedra. The compressibility of bromapatite was also investigated up to about 20.7 GPa at ambient temperature, using a diamond-anvil cell and synchrotron X-ray radiation. The compressibility of lead bromapatite is significantly different from that of lead fluorapatite. The pressure–volume data of lead bromapatite (P < 10 GPa) fitted to the third-order Birch-Murnaghan equation yield an isothermal bulk modulus (K _T ) of 49.8(16) GPa and first pressure derivative (K_T^￠ K_{T}^{\prime } ) of 10.1(10). If K_T^￠ K_{T}^{\prime } is fixed at 4, the derived K _T is 60.8(11) GPa. The relative difference of the bulk moduli of these two lead apatites is thus about 12%, which is about two times the relative difference of the bulk moduli (~5%) of the calcium apatites fluorapatite [Ca₁₀(PO₄)₆F₂]_, chlorapatite [Ca₁₀(PO₄)₆Cl₂] and hydroxylapatite [Ca₁₀(PO₄)₆(OH)₂]. Another interesting feature apparently related to the replacement of F by Br in lead apatite is the switch in the principle axes of the strain ellipsoid: the c-axis is less compressible than the a-axis in lead bromapatite but more compressible in lead fluorapatite.     

Study of nonlinear behaviors of geological materials

Two polynomial constitutive equations of nonlinear stress–strain relationship were used to construct two nonlinear 1-D wave equations with external pressure (source term) applied. Nonlinear model 1 was a concave downward curve, and nonlinear model 2 was a concave upward curve. The time-dependent stress and strain of a 300-m length were calculated. The computation for nonlinear model 1 terminated at time t?=?5.0 s. The evaluated stress versus position at different times was mainly at position x?=?0 m. The stress versus position had a concave point at position x?=?0.3 m. Between x?=?0 m and x?=?0.3 m, the graph of stress versus position showed a small convex upward curve. Moreover, at the two sides of x?=?0.3 m, the strain position had distinctively different slopes. The distinctive difference in the slope of strain at position x?=?0.3 m can therefore be used to estimate the rupture position of a rock. The terminal evaluation time for nonlinear model 2 was at t?=?1.55 s. The stress versus position and the strain versus position to time change were within the 0–3 m interval from the pressure end. Time increase produced the phenomenon of stress and strain solitons. These stress and strain solitons moved forward and increased in peak value with time. During the compression process, the stress and strain soliton resulted in instability which rendered the rock situation more easily broken. The position of the rock rupture might have occurred away from the pressure side.     

Fractionation of sulfur isotopes during laboratory synthesis of pyrite at low temperatures

The reaction products and the accompanying sulfur isotope fractionations during the reaction of H₂S with goethite in aqueous media at 22–24°C for periods from 0.5 hr to 65 days were studied. Fine-grained pyrite formed within two days and was isotopically 0.8‰ lighter than the H₂S source. After 65 days reaction time the pyrite had nearly the same isotopic value as the H₂S. Aqueous precipitation of pyrite from H₂S and goethite at room temperature involved three major steps, namely: (1) the rapid oxidation of H₂S and reduction of Fe³⁺ during which elemental S is formed; (2) the formation of acid-volatile sulfides and the disappearance of elemental S; and (3) the formation of pyrite at the expense of acid-volatile sulfides.     

An investigation of the morning dissipation of the valley wind by means of a one-dimensional model

A simple numerical model is constructed to investigate the time-history of the dissipation of the valley wind system. The local wind system is considered as a thermally induced circulation. The Coriolis force, effect of slope of the valley floor and advection of heat and momentum are not considered in this study. The non-dimensional forms of the heat-diffusion and momentum equations along the direction of the valley axis are numerically integrated with a set of specified initial conditions, boundary conditions and assumed heat sources (and sinks). Three cases are presented in this report. The numerical solutions are in good agreement with the observed features of the dissipation of the valley wind provided that a relative heat source at ridge-line level and a relative heat sink at the ground are hypothesized.The major part of the research presented in this paper was done while the author was at the Department of Meteorology and Oceanography, New York University.     

Raman spectroscopy of shocked enstatite-rich meteorites

We present Raman patterns of enstatite in different classes of enstatite-rich chondrites and achondrites of various shock levels as previously reported from petrographic observations and X-ray diffraction analyses. Thin sections or mineral separates of four enstatite chondrites (LaPaz Icefield [LAP] 02225, MacAlpine Hills [MAC] 02837, Pecora Escarpment [PCA] 91020, and Itqiy), three aubrites (Larkman Nunatak [LAR] 04316, Khor Temiki, and Allan Hills [ALH] 84008), and a ureilite (Sayh al Uhaymir [SaU] 559) were examined by laser Raman spectroscopy. We find that the frequencies of fundamental Raman peaks of enstatites from the chondrites and aubrites deviate by ≤2 cm⁻¹ from the values for unshocked enstatite. This small difference implies a negligible effect of shock metamorphism on peak positions. Significant differences (<6 cm⁻¹) for peak positions are found for the pyroxenes of SaU 559 and may be attributed to minor substitution of Fe and Ca for Mg. Linear regressions of peak widths of enstatite chondrites against their established shock stages show a strong positive correlation for each mode (r² > 0.94). From this linear relationship, the 343 and 1014 cm⁻¹ peaks of the aubrites coincide with S4 determined from petrography. For Itqiy, we find S4–5, while the shock levels of SaU 559 exceed the petrologic scheme (S1–6), suggesting that the ureilite might have sustained multiple shock events or have been deformed in a high-pressure environment. Alternatively, for Itqiy (peak 343 cm⁻¹) and SaU 559 (all peaks) enstatites, minor substitutions of Fe and Ca for Mg may have further broadened their peak widths.     

An examination of τc-Pd earthquake early warning method using a strong-motion building array

The use of characteristic period τ_c and peak displacement amplitude Pd of the initial P wave in earthquake early warning (EEW) was proposed by Wu and Kanamori 1, 2, 3 and 4. Here we apply this approach to strong-motion records from a building sensor array installed in Taitung County, Taiwan. This building was damaged during the 2006 M_w=6.1 Taitung earthquake with a peak ground velocity (PGV) of up to 38.4 cm/s at an epicentral distance of 14.5 km. According to our analysis, the peak displacement amplitude Pd is a better indicator for the destructiveness of an earthquake than τ_c because τ_c is more sensitive to the signal-to-noise ratio (SNR) than Pd. In accordance with previous studies, only the structurally damaging Taitung earthquake generated a Pd value larger than 0.5 cm (a threshold for identifying damaging events). Using Pd as an indicator for destructive earthquakes does not lead to missing or false alarms for EEW purposes.     

Analysis of landslide dam geometries

The geometry of a landslide dam is an important component of evaluating dam stability. However, the geometry of a natural dam commonly cannot be obtained immediately with field investigations due to their remote locations. A rapid evaluation model is presented to estimate the geometries of natural dams based on the slope of the stream, volume of landslides, and the properties of the deposit. The proposed model uses high resolution satellite images to determine the geometry of the landside dam. These satellite images are the basic information to a preliminary stability analysis of a natural dam. This study applies the proposed method to two case studies in Taiwan. One is the earthquake-induced Lung-Chung landslide dam in Taitung, and the second is the rainfall-induced Shih-Wun landslide dam in Pingtung.