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991.
992.
Brian L. Phillips John G. Thompson Yuehui Xiao R. James Kirkpatrick 《Physics and Chemistry of Minerals》1993,20(5):341-352
Nuclear magnetic resonance spectroscopic data are presented for the cristobalite polymorphs of AlPO4 and SiO2 from RT to 770 K, through their respective α-β transitions. The nuclear magnetic resonance (NMR) data include chemical shifts for 31P, 27Al, and 29Si, 27Al quadrupole coupling parameters, and 31P and 27Al spin-lattice relaxation rates. Also presented are electron diffraction patterns of β-cristobalite AlPO4 that show diffuse scattering similar to that reported previously for SiO2. For the α-phases of both AlPO4 and SiO2, the chemical shifts decrease approximately linearly with increasing temperature from RT to Tc and discontinuously by -2 to -3 ppm from α to β. This result is consistent with a small, continuous increase in the mean T-O-T angle (〈θ〉) of the α-phases with increasing T and an increase of 〈θ〉 by about 4° across the α-β transition for both cristobalite and its AlPO4 analogue. Based on the 29Si chemical shifts, the mean Si-O-Si angle for β-cristobalite is 152.7±1° near Tc. For AlPO4-cristobalite, the 27Al nuclear quadrupole coupling constant (CQ) decreases approximately linearly from 1.2 MHz at RT to 0.94 MHz near Tc (493±10 K). At the α-β transition the 27Al CQ approaches zero, in agreement with the cubic average structure observed by diffraction. The satellite transitions retain a small frequency distribution above the α-β transition from electric field gradients attributed to defects. The short-range cubic symmetry of the Al-site and non-linear Al-O-P angle support a dynamically disordered model of the β-cristobalite structure. Complete averaging of the 27Al quadrupole coupling in the β-phase indicates that the lifetime of any short-range ordered domains must be shorter than about 1 μs. 相似文献
993.
994.
995.
Juliana Boerio-Goates Johanne I. Artman Brian F. Woodfield 《Physics and Chemistry of Minerals》1990,17(2):173-178
Heat capacity measurements have been made on a synthetic sample of langbeinite, K2Mg2(SO4)3, from 13 to 342 K in an adiabatic calorimeter. Three phase transitions, at 51.0, 54.9 and 63.8 K, have been observed in this material. Our study is the first to report the existence of such phase transitions in K2Mg2(S04)3 and disputes predictions that none would take place below 77 K. Two models which have been proposed to explain the transition in potassium langbeinites are discussed in light of these results. 相似文献
996.
Effects of forest land management on erosion and revegetation after the eruption of Mount St. Helens
The 1980 eruption of Mount St. Helens covered soils with a tephra blanket and killed the forest tree cover in a 550 km2 area. After the eruption, rates of sheetwash and rill erosion, and plant cover were measured on tephra-covered hillslopes which had been subject to three land-management practices: grass seeding; scarification, and salvage logging. On rapidly-eroding hillslopes subject to grass seeding, limited plant covers were established only after erosion had declined sharply. Logging of trees downed by the eruption and scarification of previously logged surfaces slowed erosion, although the effect was small because erosion rates had already slowed substantially by the time these two practices were implemented. The factors controlling erosion, revegetation, and their relative timing at Mount St. Helens are similar to those following explosive volcanic eruptions elsewhere, suggesting that grass seeding is not likely to be effective at slowing erosion following most tephra eruptions, and that early mechanical disturbance could be an effective erosion-control measure. The results also indicate that even without deliberate conservation measures, processes which mechanically disturb a surface layer of low hydraulic conductivity (such as frost-action or trampling) can radically reduce runoff and erosion before revegetation has an important effect. 相似文献
997.
The rate, with respect to area and time, at which grains are dislodged from a sand bed for given wind conditions is an important factor in determining the grain transport rate and the intensity of grain activity in each of the transport modes. The literature of the subject contains little direct information about particle dislodgement. The paper describes a series of experiments in which dyed sand grains, spread on the surface of quartz dune sand in a wind-tunnel, were photographed at five second intervals while the sand was exposed to wind. The data on rate of loss of coloured grains was used, for two of three chosen size fractions, to deduce the dislodgement rate for each size fraction. The variation of this dislodgement rate with shear velocity is shown graphically for values of u* between 24 cm s?1 and 50 cm s?1. Because of the artificial method of distribution of the coloured sand grains, the results should be applied with caution to natural conditions. The interpretation of the observations of dyed grain loss involved the numerical simulation of the process which comprises removal of coloured grains, slightly offset by replenishment as upwind coloured grains settle briefly in the observed zone. An estimation of grain excursion length has to be incorporated in the simulation. This estimation was made by trial, but general corroboration was found from earlier work. Comparisons are made between dislodgement rates obtained thus and rates estimated by Anderson (1986) and by Jensen and Sorensen (1986). Reasonable agreement with the latter is found in the u* range 30 cm s?1. 相似文献
998.
Data for the major element oxides and seventeen trace elements (Ba, Co, Cu, Ga, La, Mo, Nb, Ni, Pb, Rb, Sb, Sn, Sr, U, Y, Zn and Zr) have been determined by X-ray fluorescence spectrometry for the CANMET ore standards MA-1, TLG-1, BH-1, CT-1, OKA-1, PR-1, HV-1, MP-2 and TAN-1. 相似文献
999.
1000.
Nikolaos P. Nikolaidis Gary A. Robbins Michelle Scherer Brian McAninch Gordon Binkhorst Jay Asikainen Steven L. Suib 《Ground Water Monitoring & Remediation》1994,14(3):150-159
The vertical distribution and partitioning (between the solid and aqueous phase) of chromium in a glaciofluvial aquifer in northeastern Connecticut were assessed. Most of the chromium (99 percent of its mass) is bound to the soil. Retardation is primarily the result of binding to organic matter and adsorption to iron oxide coatings. However, other attenuation mechanisms also appear to be significant. If the degree of chromium binding observed here is representative of other chromium contaminated sites, pump-and-treat remediation will not remove the vast amount of chromium from the subsurface. However, most of the chromium may be immobile, and removal may not be required following the initial pumping to remove the mobile fraction. Further knowledge of the mechanisms that bind chromium to the soil, their reversibility, and their kinetics is essential to developing effective remediation strategies. 相似文献