Cation disorder in garnets along the Mg3Al2Si3O12-Mg4Si4O12 join: an infrared,Raman and NMR study     

Paul McMillan  Masaki Akaogi  Eiji Ohtani  Quentin Williams  Ronald Nieman  Robert Sato 《Physics and Chemistry of Minerals》1989,16(5):428-435

We have obtained infrared and Raman spectra for garnets synthesized at high (static) pressures and temperatures along the join Mg₃Al₂Si₃O₁₂ (pyrope) — Mg₄Si₄O₁₂ (magnesium majorite). The vibrational spectra of Mg-majorite show a large number of additional weak peaks compared with the spectra of cubic pyrope garnet, consistent with tetragonal symmetry for the MgSiO₃ garnet phase. The Raman bands for this phase show no evidence for line broadening, suggesting that Mg and Si are ordered on octahedral sites in the garnet. The bands for the intermediate garnet compositions are significantly broadened compared with the end-members pyrope and Mg-majorite, indicating cation disorder in the intermediate phases. Solid state ²⁷Al NMR spectroscopy for pyrope and two intermediate compositions show that Al is present only on octahedral sites, so the cation disorder is most likely confined to Mg-Al-Si mixing on the octahedral sites. We have also obtained a Raman spectrum for a natural, shock-produced (Fe,Mg) majorite garnet. The sharp Raman peaks suggest little or no cation disorder in this sample.  相似文献   

Toward an improved global network for determination of tropospheric ozone climatology and trends     

Ronald G. Prinn 《Journal of Atmospheric Chemistry》1988,6(3):281-298

An examination of typical tropospheric ozone variability on daily, monthly, annual and interannual timescales and instrumental precision indicates that the current ozonesonde network is insufficient to detect a trend in tropospheric ozone of 1% per year at the 2 level even at stations with records a decade in length. From a trend prediction analysis we conclude that in order to detect a 1% per year trend in a decade or less it will be necessary to decrease the time between observations from its present value of 3–7 days to 1 day or less. The spatial distribution of the current ozonesonde stations is also inadequate for determining the global climatology of ozone. We present a quantitative theory taking into account photochemistry, surface deposition, and wind climatology to define the effectively sampled region for an observing station which, used in conjunction with the instrumental precision and the above prediction analysis, forms the basis for defining a suitable global network for determining regional and global ozone climatology and trends. At least a doubling of the present number of stations is necessary, and the oceans, most of Asia, Africa, and South America are areas where more stations are most needed. Differential absorption lidar ozone instruments have the potential for far more frequent measurements of ozone vertical profiles and hence potentially more accurate climatology and trend determinations than feasible with ozonesondes but may produce a (fair weather) biased data set above the cloud base. A strategy for cloudy regions in which either each station utilizes both lidars and sondes or each station is in fact a doublet comprised of a near-sea-level lidar and a proximal-mountain-top lidar could serve to minimize this bias.  相似文献   

Surface inhomogeneity effects on convective diffusion     

Gary A. Briggs 《Boundary-Layer Meteorology》1988,45(1-2):117-135

It is suggested that convective scaling, with appropriate extensions, provides the most useful framework for estimating the effects of urban-scale surface inhomogeneities on diffusion in convective conditions. Strong contrasts in surface heat flux exist between cropland, forests, urban areas, and water or marshland surfaces. It is argued that a typical fetch for convective turbulence to readjust to changed heat (or buoyancy) input from the surface below is 2(U/w ^*)h, where U is the mean wind speed in the mixing layer, w ^* is the convective scaling velocity, and h is the mixing depth. In contrast, the fetch required for wind speed to readjust to new underlying surface roughness is of the order (U/u ^*)²h/2, where u ^* is the friction velocity.The ratio w ^*/U is the best index of diffusion rates in moderately to very unstable conditions. General urban effects on heat flux, h, and U are discussed separately, then their combined effects on w ^*/U are estimated. While this ratio can double over a large city during light winds, its increase is much less for small cities, or during moderate winds. Finally, some examples of heat flux in- homogeneities causing stationary convective features are presented. Steady downdrafts associated with these features are of the order of 0.4w ^*, and could significantly increase surface concentrations from elevated sources.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.This paper is based on a presentation made at the AMS Specialty Conference on Air Quality Modeling of the Urban Boundary Layer, in Baltimore, late 1983.  相似文献   

The Sensitivity of Four Monitoring Well Sampling Systems to Low Concentrations of Three Volatile Organics     

Ronald Schalla  David A. Myers  Mary Ann Simmons  John M. Thomas  Antony P. Toste 《Ground Water Monitoring & Remediation》1988,8(3):90-96

Four state-of-the-art ground water sampling systems were analyzed to determine their reliability in providing representative samples of the volatile chlorinated hydrocarbons trichloroethylene (TCE), perchloroethylene (PCE), and 1,1,1-trichloroethane (TCA) from a simulated monitoring well. The sampling systems studied represent four commonly used devices, including a stainless steel and Teflon® piston pump, a Teflon bailer, a Teflon bladder pump, and a PVC air-lift pump.
Controlled laboratory sampling experiments were conducted in a tank and well test chamber designed to approximate field conditions. A well purging and sampling procedure was used in the test apparatus to determine the accuracy and precision of each device for detecting low concentrations of the compounds in ground water. The compounds selected are some of the most ubiquitous hazardous contaminants found in shallow aquifers near hazardous waste sites throughout the United States.
No significant statistical difference was found among the four sampling systems in detecting the compounds.  相似文献   

The relationship of precipitation seasonality to the flora and stable isotope chemistry of soils in the Vizcaíno desert, Baja California, México     

Ronald Amundson  Ernesto Franco-Vizcaíno  R.C. Graham  Michael DeNiro 《Journal of Arid Environments》1994,28(4)

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Variability of dissolved organic carbon in sediments of a seagrass bed and an unvegetated area within an estuary in Southern Texas     

Eric T. Koepfler  Ronald Benner  Paul A. Montagna 《Estuaries and Coasts》1993,16(3):391-404

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1^?1 to 107 mg C 1^?1 of pore water, or 9–71 μg C cm^?3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm^?3) and high bacterial production rates (mean = 1.92 μg C cm^?3 h^?1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm^?3) and bacterial production rates were low (mean = 0.43 μg C cm^?3 h^?1) at the bare station. PWDOC turnover times (T_t), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, T_t values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.  相似文献   

The equivalence of predictions from universal kriging and intrinsic random-function kriging     

Ronald Christensen 《Mathematical Geology》1990,22(6):655-664

A proof is provided that the predictions obtained from kriging based on intrinsic random functions of orderk are identical to those obtained from anappropriate universal kriging model. This is a theoretical result based on known variability measures. It does not imply that people performing traditional universal kriging will get the same predictions as those using intrinsic random functions, because traditionally these methods differ in how variability is modeled. For intrinsic random functions, the same proof shows that predictions do not depend on the specific choice of the generalized covariance function. It is argued that the choice between these methods is really one of modeling and estimating the variability in the data.  相似文献   

Comprehensive Rock Engineering, Principles, Practice & Projects Volume 3 Rock Testing and Site Characterization.     

Ronald B. Meade 《Engineering Geology》1994,37(3-4):314-315

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A mechanism for preferential H2O leakage from fluid inclusions in quartz,based on TEM observations     

Ronald J. Bakker  J. Ben H. Jansen 《Contributions to Mineralogy and Petrology》1994,116(1-2):7-20

Preferential leakage of H₂O from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H₂O-CO₂-rich inclusions in natural quartz. Knowledge of the physical and chemical characteristics of the leakage mechanism, which may be very complex, increases the value of natural fluid inclusions to metamorphic geology. It is proposed that crystal defects play a major role in nondecrepitative preferential H₂O leakage through quartz, and remain effective during metamorphism. Inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects (like Dauphiné twin boundaries). These defects allow preferential loss of H₂O from H₂O-CO₂-rich inclusions at supercritical conditions. The transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures. The nucleation of these bubbles seems to be a recovery process in strained crystals. Solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement, caused by the growth of small bubbles at the expense of the larger original fluid inclusion. Natural fluid inclusions from Naxos (Greece) are always associated with numerous interlinked dislocations. These dislocations may have been produced by plastic derormation or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these dislocations indicates that a similar leakage mechanism for H₂O must have occurred in these rocks.  相似文献   

The status and prospects of risk assessment     

Ian Burton  Ronald Pushchak 《Geoforum》1984,15(3):463-475

After discussing the changing nature of perceived risk problems, the status of risk assessment is described in relation to its origins and in particular to its roots in the environmental impact statement process. The nature of risk, its component elements and the manner in which existing concepts of risk have been reflected in risk assessment methods are described. The paper considers two emerging schools of thought in current risk assessment studies: one that calls for more accurate measures of risk and increasingly comprehensive event prediction models to determine risk acceptability and another which argues that the acceptance of risk is less dependent on the accuracy of risk analyses than it is on the nature of the decision-making process and in particular on whether compensation is provided for those bearing a disproportionate share of risk.  相似文献