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21.
22.
Roccamonfina, part of the Roman Potassic Volcanic Province, is an example of a composite volcano with a complex history of caldera development. The main caldera truncates a cone constructed predominantly of this caldera may have been associated with one of the ignimbritic eruptions of the Brown Leucitic Tuff (BLT) around 385 000 yr BP. The Campagnola Tuff, the youngest ignimbrite of the BLT, however, drapes the caldera margin and must postdate at least the initial stages of collapse. During the subsequent history of the caldera there were several major explosive eruptions. The largest of these was that of the Galluccio Tuff at about 300 000 yr BP. It is likely that there was further collapse within the main caldera associated with these eruptions. It is of note that despite these subsequent major explosive eruptions later collapse occurred within the confines of the main caldera. Between eruptions caldera lakes developed producing numerous lacustrine beds within the caldera fill. Extensive phases of phreatomagmatic activity generated thick sequences of pyroclastic surge and fall deposits. Activity within the main caldera ended with the growth of a large complex of basaltic trachyandestite lava domes around 150 000 yr BP. Early in the history of Roccamonfina sector collapse on the northern flank of the volcano formed the northern caldera. One of the youngest major events on Roccamonfina occurred at the head of this northern caldera with explosive activity producing the Conca Ignimbrite and associated caldera. There is no evidence that there was any linkage in the plumbing systems that fed eruptions in the main and northern calderas. 相似文献
23.
Summary Anandite has an approximate formula of Ba(Fe3+, Fe2+)3[Si2(Fe3+, Fe2+, Si)2O10–x(OH)x] (S, Cl) (OH), withx=0–1, and belongs to the 2 O brittle mica group. It is orthorhombic; space groupPnmn;a=5.468(9) Å,b=9.489(18)Å,c=19.963(11) Å;Z=4.The structure was determined from 3dim. Weissenberg-data, starting with an approximate structure in the pseudo space groupCcmm. Least squares refinement resulted inR=0.061 for 409 photometric intensities, andR=0.131 for all 853 observedhkl-reflexions.The iron of the tetrahedral layer is concentrated in one of the two crystallographically different kinds of tetrahedra. The basal oxygen rings of the tetrahedral layer form approximate hexagons and have not the ditrigonal configuration of the common micas. This peculiarity is considered to be a consequence of the size and charge of the barium ion. The role of OH in the common micas is played partly by S2– and Cl– in anandite.
With 4 Figures 相似文献
Die Kristallstruktur des 2 O Sprödglimmers Anandit
Zusammenfassung Anandit hat die ungefähre Formel Ba(Fe3+, Fe2+)3[Si2(Fe3+, Fe2+, Si)2O10–x(OH)x] (S, Cl) (OH) mitx=0–1 und gehört zur 2O Sprödglimmergruppe. Er ist rhombisch; RaumgruppePnmn; a=5,468(9) Å,b=9,489(18) Å,c=19,963(11) Å;Z=4.Die Struktur wurde aus Weissenberg-Daten bestimmt, wobei mit einer approximativen Struktur in der PseudoraumpruppeCcmm begonnen wurde. Die Verfeinerung nach der Methode der kleinsten Quadrate führte für 409 photometrierte Reflexe aufR=0,061 und für alle 853 beobachtetenhkl-Reflexe aufR=0,131.Der Eisengehalt der Tetraederschicht ist in einer der beiden kristallographisch verschiedenen Tetraederarten konzentriert. Die basalen Sauerstoffringe der Tetraederschicht bilden annäherungsweise Sechsecke und haben nicht die ditrigonale Konfiguration der gewöhnlichen Glimmer. In Anandit spielen S2– und Cl– teilweise die Rolle der Hydroxylgruppen in den gewöhnlichen Glimmern.
With 4 Figures 相似文献
24.
Summary The refined lattice parameters of beryllonite are:a=8.178 (3) Å,b=7.818 (2) Å,c=14.114 (6) Å, =90.00° (2); space groupP21/n,Z=12. Integrated Weissenberg photographs were taken by using CuK radiation and multiple film packs. The anisotropic refinement of the crystal structure by means of least-square methods gave a finalR value of 0.063 for the 1388 observed reflections. In the crystal structure PO4 and BeO4 tetrahedra, linked by shared oxygen atoms in a three-dimensional network, form pseudo-ditrigonal rings perpendicular to theb axis. The independent Na atoms lying in the channels formed by the rings are coordinated as an irregular nine-cornered polyhedron and as distorted octahedra.
With 2 Figures 相似文献
Verfeinerung der Kristallstruktur des Beryllonits,NaBePO 4
Zusammenfassung Verfeinerte Gitterkonstanten des Beryllonits lauten:a=8,178 (3) Å,b=7,818 (2) Å,c=14,114 (6) Å, =90,00 (2)°; Raumgruppe:P21/n,Z=12. Integrierte Weissenbergaufnahmen wurden mit CuK-Strahlung und multiplen Filmpaketen aufgenommen. Die anisotrope Verfeinerung der Kristallstruktur nach der Methode der kleinsten Quadrate ergab für 1388 beob. Reflexe einen abschließendenR-Wert von 0,063. In der Kristallstruktur bilden PO4-und BeO4-Tetraeder, die über gemeinsame Sauerstoffe zu einem dreidimensionalen Gerüst verknüpft sind, pseudo-ditrigonale Ringe senkrecht zurb-Achse. Die kristallographisch unabhängigen Arten von Na-Atomen, welche in den aus Ringen gebildeten Kanälen liegen, sind in der Form eines unregelmäßigen neuneckigen Polyeders bzw. in der Form verzerrter Oktaeder koordiniert.
With 2 Figures 相似文献
25.
ABSTRACTWater indicators and indices are useful tools to assess river basin performance, that is, to measure whether the basin operates satisfactorily under a wide range of possible future demands and hydrological conditions. Spanish regulations assess the performance of water demands by using reliability indicators (RIs), established by law in 2008. This article raises the possibility of updating RIs by comparing them with sustainability indicators (SIs). SIs are widely used for the assessment of river basin performance and several policy scenarios. We applied a water allocation model to the Guadiana River basin in Spain to compare indicators under three scenarios. The study was framed within the science of socio-hydrology, combining the physical environment of a water system with its influence on social aspects. SIs gave better results than RIs when comparing future scenarios. We also propose the introduction of a vulnerability indicator into Spanish regulations. 相似文献
26.
Per?WestmanEmail author Johanna?Borgendahl Thomas?S.?Bianchi Nianhong?Chen 《Estuaries and Coasts》2003,26(3):680-689
This study corroborates the hypothesis that nitrogen-fixing cyanobacteria have probably occurred as an important component
of the phytoplankton community in the Baltic Sea at least since brackish water conditions were initiated 8,50014C yr BP. Pigment analyses indicate that extensive occurrences started prior to a sharp increase in nutrient levels dated to
7,10014C yr BP. The cyanobacteria could have functioned as a natural trigger for eutrophication in the Baltic Sea by importing nitrogen.
This is also verified by a contemporaneous drop in the δ15N values from 4‰ to around 2‰. We further conclude that the spreading of cyanobacteria was probably caused by a decrease in
nitrogen∶phosphorus (N∶P) in the water mass that resulted from the intrusion of oceanic water with high P levels. The fractionation
of P in sediments indicated that iron-bound P was efficiently sequestered under anoxic conditions that occurred as a consequence
of the establishment of a stable stratification caused by the marine intrusion. This pool only showed minor variations around
3 μmol g−1 at the freshwater-brackish water transition. All P pools except the CaCO3 fraction showed a distinct increase around 9,30014C yr BP prior to the transition. We interpreted this increase as a change in preservation of organic matter or in the source
of the sediment. Slightly after 4,00014C yr BP there was a dramatic drop in all P pools without any corresponding decreases in total N and carbon. Total P decreased
from around 75 to 25–30 μmol g−1. The most dramatic drop occurred in the organic bound and the detrital apatite fractions, which decreased by a factor of
3–4. We explain this as a preferential regeneration of P, especially organic P, compared to other nutrients due to more prevalent
anoxic conditions. 相似文献
27.
28.
J. I. Davies P. Alton S. Bianchi & M. Trewhella 《Monthly notices of the Royal Astronomical Society》1998,300(4):1006-1014
We have constructed a numerical model of a galaxy that consists of a stellar, gas and dust disc imbedded within a dark halo. We have used this model to assess the radiation, gravitational and viscous forces on dust grains and to trace their motion through the interstellar medium over a period of 109 yr. We conclude that the disc opacity is a crucial factor in understanding the motion of the grains. Large grains (≈0.1 μm) with low disc opacity will lead to dust expulsion from the stellar disc, while high opacity leads to dust retention. Reasonable disc opacities lead to the recycling of the larger grains from the outer to the inner regions of the galaxy. The larger grains travel at higher velocities than small grains (0.01−0.001 μm), and so the smaller grains remain relatively close to their formation sites. Dust can 'leak' out over the entire surface of the disc because of the imbalance of radiation and gravitational forces. The dust is dynamically coupled to the gas and so although the gas lags behind the dust it is carried along with it. This explains the close correlation between the far-infrared emission from dust and the gas column density. We use a simple analytical model to show how the dust mass of a galaxy may evolve with time and how a significant fraction (90 per cent) of the total dust mass produced may have been expelled into the intergalactic medium. 相似文献
29.
30.