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461.
Paul F. McMillan George H. Wolf Phillipe Lambert 《Physics and Chemistry of Minerals》1992,19(2):71-79
We have carried out a Raman Spectroscopic study of single crystalline quartz samples shocked to peak pressures up to 31.4GPa. Samples shocked to above 22 GPa show shifts in peak positions consistent with the quartz being under tensile stress, and new broad bands associated with the formation of high density SiO2 glass appear in the spectra. These changes are accompanied by an increase in the lattice parameters of the quartz. Formation of the diaplectic glass could be due to a metastable melting event, or spinodal lattice collapse on attainment of a mechanical stability limit of crystalline quartz, as suggested by previous studies of pressure-induced amorphization in static pressurization experiments on SiO2 and GeO2 polymorphs. 相似文献
462.
Robert D. Shannon James E. Dickinson George R. Rossman 《Physics and Chemistry of Minerals》1992,19(3):148-156
The dielectric constants and dissipation factors of LiAlSi2O6, CaAl2Si2O8 and CaMgSi2O6 in both the crystalline (α-spodumene, anorthite, and diopside) and amorphous forms were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: spodumene κ′ 11=7.30 tan δ= 0.0007 κ′22=8.463 tan δ= 0.0002 κ′33 =11.12 tan δ= 0.0007 anorthite κ′ a *=5.47 tan δ= 0.0009 κ′b *=8.76 tan δ= 0.0010 κ′c *=7.19 tan δ= 0.0013 diopside κ′11=9.69 tan δ= 0.0016 κ′22 = 7.31 tan δ= 0.0007 κ′33=7.29 tan δ= 0.00019 LiAlSi2O6 κ′=8.07 tan δ= 0.047 amorphous CaAl2Si2O8 κ′=7.50 tan δ= 0.0024 amorphous CaMgSi2O6 κ′=8.89 tan δ= 0.0021 amorphous The dielectric properties of a spodumene glass, progressively crystallized at different conditions, were also determined. As the crystallization temperature was increased from 720 to 920° C, κ′ increased from 6.22 to 6.44. The dissipation factor, tan δ, remained constant at 0.020. Similarly, as the crystallization time at 750° C increased from 0.5 hr to 6.0 hr, κ′ increased from 6.28 to 6.35. The deviations of the measured dielectric polarizabilities as determined from the Clausius-Mosotti equation from those calculated from the sum of oxide polarizabilities according to α D(mineral, glass) = σ α D(oxides) are +7.4% for α-spodumene, +1.2% for diopside, and +28.0, +19.6 and +15.9% for amorphous spodumene, anorthitie and diopside compositions, respectively. Positive deviations in α-spodumene and anorthite are consistent with lower than normal apparent cation bond valence sums and are believed to be evidence for loosely bonded “rattling” Li and Ca ions. Diopside, with Ca and Mg ions having normal bond valence sums, exhibits no abnormal deviation from additivity. Larger positive deviations in amorphous SiO2, LiAlSi2O6, CaAl2Si2O8 and CaMgSi2O6 are postulated to arise from a combination of loosely bonded cations and disordered O= ions where the oxygen dielectric polarizability increased from its normal value of 2.0 Å3 in well-behaved oxides to 2.2–3.0 Å3 in the amorphous phases. 相似文献
463.
James H. Cowan Ray S. Birdsong Edward D. Houde Jonathan S. Priest William C. Sharp George B. Mateja 《Estuaries and Coasts》1992,15(3):392-402
Experiments in 2.2 m3, in situ mesocosm enclosures indicate that black drum, Pogonias cromis, eggs and larvae potentially can survive in the lower Chesapeake Bay at ambient microzooplankton prey levels (≈200 prey 1?1) in the absence of predators. In growth experiments, larva mean growth rates to 10 d posthatch were similar (0.17 mm d?1 and 0.18 mm d?1) when fed at prey levels of 50 prey 1?1 and 200 prey 1?1. Individual growth rates, however, were more variable at 50 prey 1?1. Mortality rates also were comparable in 50 (27% d?1) and 200 (23% d?1) prey 1?1 enclosures. In a second experiment, the predation potentials of the hydromedusa Nemopsis bachei and the lobate ctenophore Mnemiopsis leidyi were estimated in relation to initial black drum egg prey density, presence of alternative <1 mm zooplankton prey, and estimated daily abundance of the jellyfish on the black drum spawning grounds. Mortality rates per medusa and ctenophore were similar (0.02–0.03 d?1), were not affected by presence of alternative prey, and were directly related to initial egg density. Results suggest that the gelatinous predators, especially the hydromedusa, could have cleared a high (≈38%) but variable fraction of the water column daily of fish eggs and yolk-sac larvae during the black drum spawning season. We hypothesize that the poor or episodic recruitment success of black drum in Chesapeake Bay results from a short spawning season that often coincides with abundance peaks of gelatinous predators and that predation on eggs and yolk-sac larvae may control recruitment. 相似文献
464.
George Pararas-Carayannis 《Natural Hazards》1992,5(2):115-123
A tsunami source mechanism, resulting from the collapse of the cone of the Santorin Volcano, accounted for the catastrophic sea waves observed in the Aegean Archipelago and the Eastern Mediterranean in the Bronze Age. The tsunami and destruction resulting from the explosion and collapse of the volcano of Santorin are discussed in this study. On the basis of recent geological evidence, it is concluded that the final caldera collapse resulted from a large earthquake, along an about NE-SW trending normal fault, along which the Santorin Volcanic field has developed. Such a source mechanism can account for the size and destructiveness of this Bronze Age Tsunami with its interesting archaeological and historical implications. 相似文献
465.
A computer simulation model (FINICS) was used to project the interactive effects of CO2-induced climate change on a drought-deciduous shrubland. FINICS simulates the competitive relations of five dominant shrub species of Californian coastal sage scrub, based on their aboveground growth and reproductive behavior. The model was used to simulate the separate and combined effects of altered precipitation, temperature, ambient ozone levels, and fuel loads and fire intensity, on species composition. Both growth chamber and field data were used to parameterize the model. Projections show that changes attributed to climate variation alone were markedly accentuated when the indirect effects of climate change on fire intensity and ozone pollution were considered. Model results emphasized that change in community composition will result from shifting competitive abilities of individual species under the changed environmental conditions. While neither all of the secondary effects of climate change nor all possible species were included in the model, the projections suggest that inclusion of secondary interactions and species competition will be important in predicting vegetation change realistically. 相似文献
466.
George D. Guthrie Jr. David R. Veblen 《Contributions to Mineralogy and Petrology》1991,108(3):298-304
Partially turbid alkali feldspars from hydrothermally altered Tertiary granites on the Isle of Skye (the Red Hills granites)
were studied using light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ray spectroscopy.
Limpid cores and turbid rims of individual crystals were compared to determine the causes of the turbidity. The limpid cores
were cryptoperthitic, with lamellar widths of 0.1–0.3 μm. In contrast, the turbid rims contained K-rich and Narich areas coarsened
to >0.5 μm. Turbid regions contained abundant inclusions, whereas limpid regions did not. Two generations of turbidity were
recognized. Feldspars from the Beinn an Dubhaich granite, a granite with near-normal values for 18O/16O possessed limpid cores surrounded by turbid rims that cast a reddish-brown hue in transmitted light. When viewed in darkfield
light microscopy, the regions with the reddish-brown turbidity were blue. This is consistent with the hypothesis that the
cloudy appearance of these turbid regions arises from the scattering of light by micrometerto submicrometer-sized inhomogeneities
in refractive index caused by fluid-filled cavities. Feldspars from the Loch Ainort granite, a granite with low values for
18O/16O possessed limpid and reddish-brown-turbid cores surrounded by turbid rims that cast a blackish hue in transmitted light.
Ion thinning of the turbid areas produced an abundance of small holes (≤1–2 μm) apparently the remains of fluid inclusions.
Transmission electron microscopy revealed that some holes from regions of reddish-brown turbidity contained non-feldspar material,
including halite and metal-rich phases of various compositions. In contrast, blackish turbid regions contained cavities filled
with alteration products, such as kaolinite. Hence, the feldspars from granites on the Isle of Skye apparently record interactions
with at least two fluids: a saline fluid (possibly a late-stage magmatic fluid) and a meteoric fluid. 相似文献
467.
Physical properties (Schmidt hammer in situ rock strength; degree of weathering; orientation, spacing, width and continuity of joints, bedding planes and faults; groundwater flow) of bedrock-incised rills and gullies were ecamined at selected localities in southern Africa. The resulting data was compared to froms of erosion observed in the field to test for possible influences of rock physical properties on rill and gully development. Bedrock-incised rill erosion occurs at low RMS (Rock Mass Strength) values (40–50). Rills generally do not form where RMS values are high (> 50). Though V-shaped bedrock-incised gullies form in lithologies with a wide range of RMS values (40–69), these values exert strong control on longitudinal morphometry. Future studies should concentrate on establishing a database on properties of erosion forms to aid identification of lithological conditions (e. g. deep regolith) which are potentially susceptible to accelerated erosion. 相似文献
468.
A fossil dune complex on the east coast of southern Africa is presently undergoing extensive accelerated erosion, with concomitant dune reactivation and degradation, unconfined erosion (sheetwash and wind deflation) and extensive development of V-shaped, ravine-type gully forms on hillslopes composed of thick sequences of Quaternary (Berea Formation?) dune sands. The sands comprise a thick, reddened, ferralitic sand carapace overlying white, poorly cemented quartzitic sands. Soil erosion has resulted from degradation of the red dune sand cover. Despite the cohesionless nature of the host materials, gully forms developed on dune sands are typologically similar to those developed in regolith or soft bedrock. This demonstrates that gully forms are influenced by the degree of homogeneity, rather than the absolute strength, of the host material subject to erosion. Factors which have contributed to erosion are discussed. 相似文献
469.
470.