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1.
Astronomy Reports - It is known from observations that the mass spectra of meteoric bodies (meteoroids) that cause the phenomena of sporadic meteors and bodies in meteor showers are close in... 相似文献
2.
AbstractThe statistics and mechanical studies of the spatial variability of rainfall do not seem to be entirely satisfactory for brief events, such as the showers generating gullying. Thus, indirect indications of vegetation may be useful. Botanical continuums do reflect some climatic changes that may be linked to the distribution of these events. As an example, the analyses of some floristic transects, located at the transition of Mediterranean and oceanic, and Mediterranean and arid climates are presented. Conducted in parallel with the dynamic interpretation of climatic fields, these analyses have, for instance, allowed to state more precisely the spatial extension of several features of rains such as their cumulated values and their regime. The floristic ‘signal’ mainly concerns the climatic variations of the cumulated value of rainfall, but, in a subtler way, vegetation may signal the variations of regime. These features are linked with the strength of showers, and, as a consequence with the erosive capacity of rainfall. Dynamic phytoclimatology is then liable to give an aid to the spatialization of external geodynamic studies. © 2000 Éditions scientifiques et médicales Elsevier SAS 相似文献
3.
《International Geology Review》2012,54(5):593-614
Basically, two main types of statistical methods – robust and outlier-based – are available for handling experimental data; we document here the application of the outlier-based method. Due to the unavailability of a suitable software system for statistically correct application of the outlier-based method, a new computer program, DODESSYS (Discordant Outlier DEtection and Separation SYStem), was written for the application of 33 discordancy test variants to experimental data, constituting contaminated or uncontaminated normal statistical samples. We illustrate the application of the discordant outlier-based scheme by five specific examples; three include univariate data for which this procedure was specifically designed and two are for bivariate data for which this methodology can be easily adopted. We thus report new statistical information on two reference materials (granite G-2 and sediment IAEA-417), bryozoan species from eastern Oman, a new improved Na/K geothermometric equation, and a more significant correlation with water depth of the abundance of meiofauna from the Gulf of Mexico. Recently, two sets of multi-dimensional discrimination diagrams for basic as well as acid rocks have been proposed from statistically correct methodology of natural logarithm-transformation of element ratios; the diagrams also require that these ratios should be normally distributed. We present numerous examples of application of these new diagrams for inferring tectonic setting of Archaean to Recent rocks, both before and after testing the datasets for discordant outliers. We recommend that outlying observations should always be evaluated for their discordancy. 相似文献
4.
5.
Dr. D. A. Velikoslavinsky 《Mineralogy and Petrology》1994,50(1-3):35-42
Summary This paper deals with the chemical composition of biotites from rapakivi granites. For this purpose, 61 chemical analyses of biotites from composite anorthosite-rapakivi plutons in the Fennoscandian and Ukranian shields were used. Figurative points of the biotite composition were plotted on diagrams designed for biotites byFoster (1960),Winchell (1949) andVelikoslavinsky (1972). Consideration of the diagrams shows that a) the biotites from rapakivi granites have exotic composition not characteristic of other biotites from other igneous rocks, b) that they belong to the annite-siderophyllite series and c) that they crystallized at low pressures and high temperatures. Model crystallization temperatures range from 760° (early intrusive phases) down to less than 435° (late intrusive phase).
With 3 figures 相似文献
Biotit von Rapakivi
Zusammenfassung Diese Arbeit hat die chemische Zusammensetzung von Biotit aus Rapakivi Graniten zum Thema. 61 Biotitanalysen von Anorthosit-Rapakivi Plutonen aus dem fennoskandischen und ukrainischen Schild werden präsentiert. Die Zusammensetzung der Biotite wird unter Verwendung von Diagrammen, die vonFoster (1960),Winchell (1949) undVelikoslavinsky (1972) entwickelt wurden, diskutiert. Diese Diagramme zeigen, daß a) Biotite aus Rapakivi eine, im Vergleich zu Biotiten aus anderen magmatischen Gesteinen, untypische Zusammensetzung haben, b) diese Biotite der AnnitSiderophyllit Serie zuzurechnen sind und c) diese Biotite bei niedrigem Druck und hoher Temperatur kristallisierten. Die Kristallisationstemperaturen reichen von 760° in den granitischen Frühphasen bis unter 435°C in den intrusiven Spätphasen.
With 3 figures 相似文献
6.
Published phase diagrams for the siliceous carbonate system CaO–MgO–SiO2–CO2–H2O are contradictory because of different estimates of the relative stability of magnesite. Experimental data on magnesite are too ambiguous to determine the validity of these estimates. Therefore, field evidence is used to select the correct phase diagram topology for siliceous carbonate and carbonate ultramafic rocks at pressures of about 2–5 kbar. The primary selection criterion is provided by the existence of the stable assemblage talc+dolomite+forsterite+tremolite+antigorite, which occurs in the Bergell contact aureole and Swiss Central Alps. Field evidence also is used to argue that the reaction magnesite+quartz=enstatite must occur at lower temperature than the reaction dolomite+quartz=diopside. T-X
CO
2 and P
CO
2-T phase diagrams consistent with these observations are calculated from experimental and thermo-dynamic data. For antigorite ophicarbonate rocks, remarkable agreement is obtained between the spatial distribution of low variance mineral assemblages and the calculated diagrams. 相似文献
7.
D. Nakamura 《Journal of Metamorphic Geology》2002,20(3):325-333
This paper describes a kinetic study on reaction textures in eclogitic rocks from the Sulu region, eastern China. Some of the eclogitic rocks display a decompressional reaction texture, whereby kyanite grains are surrounded by plagioclase coronas and are never in contact with quartz. The change in mineral parageneses with progress of the reaction was predicted by constructing chemical potential diagrams in a model system. The chemical potential diagrams indicated that the chemical potential of 2Na2O + CaO (2µNa2O + µCaO) in intergranular regions between kyanite and quartz should decrease with decreasing pressure, whereas 2µNa2O + µCaO in intergranular regions between garnet and omphacite should increase with decreasing pressure. Thus, upon decompression, an inequality in chemical potential arises in the rock. To reduce this inequality, garnet and omphacite react to produce amphibole and plagioclase and release Na2O and CaO. Then, the released Na2O and CaO components diffuse into the regions between kyanite and quartz grains and react to produce plagioclase between them. This model also indicates that the chemical potential of SiO2 should decrease around kyanite grains during the progress of the decompressional reaction, and Si‐undersaturated conditions should have formed around kyanite grains in spite of the presence of quartz in these eclogitic rocks. Thus, spinel or corundum that are not stable in the system with excess quartz can form as a metastable phase, as observed in eclogitic rocks from the study areas. Phase diagrams in the system with excess quartz should be carefully applied for analysis of such reaction textures. 相似文献
8.
Vishnevsky A. V. Belogub E. V. Charykova M. V. Krivovichev V. G. Blinov I. A. 《Geology of Ore Deposits》2018,60(7):559-567
Selenium is one of the most important minor elements in massive sulfide ores. This study focuses on selenium minerals present in the oxidation zone of the Yubeleinoe massive sulfide deposit, the South Urals, Russia: clausthalite (PbSe), tiemannite (HgSe), and naumannite (Ag2Se). These minerals are associated with goethite and siderite. Thermodynamic modeling was used to estimate the physicochemical parameters of selenide stability and the possible formation of Pb, Hg, and Ag selenites as a result of sulfide ore oxidation. The Eh–pH diagrams for the Fe–S–CO2–H2O and Fe–Se–CO2–H2O systems were calculated to estimate the physicochemical formation conditions of the Yubileinoe oxidation zone, as well as for the M–Se–Н2О and M–S–H2O (M = Hg, Pb, Ag) systems. The physicochemical parameters of clausthalite, naumannite, and tiemannite stability are consistent with these conditions. Only the formation of PbSeO3 is theoretically possible among Pb, Ag, and Hg selenites.
相似文献9.
A new real triaxial deformation apparatus for p
1p
2p
3 tests and temperatures up to 700° C has been developed. In five creep tests over 8 hours the strain-time curves at 650° C for axial and orthorhombic symmetry of stress are plotted in the three main directions and compared with the petrofabric diagrams of the deformed material. In all tests the symmetry of the petrofabrics can be correlated with the symmetry of the external loading. From the strain-time curves it can be deduced that the intermediate loading p
2 is of great influence in the stress-strain behavior.Syntectonic recrystallization is found to be common for all tests in the form of broad lensoid and irregular defined {01¯12} lamellae oriented favorably for twinning. Measurements of c-axes in newly recrystallized grains show preferential alingnement subparallel to the axis of maximum principal loading.
Der Deutschen Forschungsgemeinschaft sei für die finanzielle Unterstützung der Untersuchungen besonders gedankt. 相似文献
Der Deutschen Forschungsgemeinschaft sei für die finanzielle Unterstützung der Untersuchungen besonders gedankt. 相似文献
10.
Petrologic hypothesis testing with Pearce element ratio diagrams: derivation of diagram axes 总被引：3，自引：0，他引：3
In petrology, Pearce element ratio (PER) diagrams have been used: i) to determine whether members of a rock suite are co-genetic, ii) to identify the minerals involved in differentiation processes, and iii) to evaluate the extent to which those mineral are involved. The axis coefficients of each diagram are chosen such that sorting of minerals or combinations of minerals will generate unique and predictable trends. Unfortunately, selection of the optimal combination of axis coefficients is a difficult task, especially if the system being investigated has a large number of phases or complicated solid solution minerals. Our work has established a formal set of rules and matrix operations which facilitate the determination of PER diagram axes coefficients. This methodology can be used to determine the unit molar vector displacement caused by the addition or subtraction of a specific mineral, given a set of axis coefficients. It can also be used to create PER diagrams on which minerals have predetermined vector displacements. By designating all vector displacements to be parallel, axis coefficients for assemblage test diagrams can be determined to test the following hypothesis: the observed chemical variation is due to the addition (or removal) of a specific set of minerals. Alternatively, by designating all vector displacements to be mutually perpendicular, phase discrimination diagrams can be created which test whether the observed chemical variations require a specific phase to be involved in differentiation. Phase discrimination diagrams also provide a means to estimate the extent of that involvement. This methodology facilitates construction of powerful yet simple PER diagrams which provide an effective means of testing alternative differentiation hypotheses.
Current address: Department of Geological Sciences, Queen's University Kingston, Ontario K7L 3N6, Canada 相似文献
11.
R. F. S. Andrade O. Oliveira A. L. Cardoso L. S. Lucena F. E. A. Leite M. J. Porsani R. C. Maciel 《Computational Geosciences》2009,13(2):155-163
Self-affine properties have been observed in a large variety of rough profiles and time series from natural data sets. In
this work, seismograms used for oil prospecting, which contain information of distinct subsurface features collected by seismic
waves reflected or scattered at their interfaces, are taken into consideration. It is expected that any self-affine property,
measured by the Hurst exponent H, depends on the depth. For each seismic trace, H is evaluated locally within a moving window, which is chosen narrow enough to reveal space dependency but also wide enough
to display scale invariance. With the use of color code diagrams, it is possible to draw two-dimensional diagrams that show
the local dependence of H for the analyzed seismogram. The reliability of the method is tested by the investigation of seismograms that contain ground
roll components, as well as multiple reflections. The effect of different kinds of filter in the scaling properties is also
investigated. In this case, comparisons are drawn among the diagrams obtained from original seismograms and those subjected
to appropriate filter to eliminate spurious components.
Work supported by the Brazilian agencies CNPq, FINEP-CTPETRO and Petrobras. 相似文献
12.
《International Geology Review》2012,54(13):1522-1558
The Melrose Stock in the Dolly Varden Mountains of east-central Nevada is one of the many Mesozoic intrusion s in the Basin and Range Province. It consists of monzonites, quartz monzonites, granodiorites, and granites sharply intruding Mississippian to Triassic units. Phenocrysts of plagioclase (An38–An24) with oscillatory zoning and albitic rims, hornblende ± diopside, and biotite are common. Coexisting phases include orthoclase, quartz and accessory magnetite, apatite, titanite, ilmenite, and allanite. Mineral compositions suggest that the intrusion was emplaced at ～720 ± 40°C and 1.8–2.3 kbar. All rocks are metaluminous to slightly peraluminous, defining a calcalkalic trend in which the monzonites and syenites are shoshonitic. Rare earth element patterns indicate that all studied rock types are comagmatic. Harker plots show curvilinear trends with some kinks consistent with fractionation, and mixing/assimilation. Major-element modelling and petrographic evidence suggest three stages of fractionation/mixing: Stage 1 marked by the fractionation of diopside and plagioclase; Stage 2 by fractionation of plagioclase, hornblende ± orthoclase ± biotite, accompanied by mixing through convection; and Stage 3 by fractionation of biotite, hornblende, plagioclase, and orthoclase. Mineralogic, petrographic, and major- and trace-element data demonstrate that all rocks are I-type granitoids, suggesting a significant mantle contribution. Spider diagrams show troughs for Ti, P, and Nb, indicating magma genesis in a subduction-zone setting. Discrimination diagrams classify all rocks as late orogenic. Magma was therefore generated from mantle metasomatized by subduction, differentiated to a monzonitic magma, and emplaced in the thinned continental crust during a period of extension late in the cycle of Elko orogeny. 相似文献
13.
Zusammenfassung Die granitischen Gesteine der südlichen Böhmischen Masse, der Weinsberger, Mauthausener und Eisgarner Granit, zeigen mit ihrem Chemismus eine Differentiationsreibe an. Dies wird mit Hilfe von Korrelationsdiagrammen K/Rb, Ca/Sr, Be/Sn, Ga/Sn u. a. illustriert.Die zeitliche Abfolge ist jedoch nur bedingt charakterisiert, da basischere Einschübe, wie der Freistädter Granodiorit, entsprechend ihres Gesteinschemismus herausfallen.Die vorgefundenen Unterschiede sind wahrscheinlich durch Differenzen im Ausgangsmaterial der granitoiden Schmelzen, aber auch durch die jeweils aufgeschlossene Stockwerkshöhe der Granitintrusionen zu deuten, die mit der zeitlichen Abfolge zu korrespondieren scheint.
Mit 5 Texiabbildungen
Herrn Prof. Dr.H. Leitmeier zum 80. Geburtstag gewidmet. 相似文献
Summary The granitic rocks of the Southern Bohemian Massif, the granites of Weinsberg, Mauthausen, and Eisgarn, show a series of differentiation in their chemical properties. This is illustarted with the aid of correlation diagrams of K/Rb, Ca/Sr, Be/Sn, Ga/Sn etc.However, the temporal succession of the granitic intrusions is only characterized partly, as more basic younger intrusions, for instance the granodiorite of Freistadt, do not follow the rule of the variation, of the chemism.The differences found are probabely caused by the different original material of the granitic melts, but also by the erosion depth of the intrusive bodies, which seems to correspond with the temporal succession, being in evidence.
Mit 5 Texiabbildungen
Herrn Prof. Dr.H. Leitmeier zum 80. Geburtstag gewidmet. 相似文献
14.
H. S. Edgell 《Australian Journal of Earth Sciences》2013,60(2):235-261
The palaeontology, correlation and sedimentation of the Proterozoic sequence in the Hamersley Range, Western Australia, are discussed. Fossil calcareous algal growths, such as stromatolites and onkolites are described, as well as possible medusoid impressions. These indicate shallow water accumulation of the sediments and provide evidence of the extensive existence of plant and animal life in the Late Precambrian of Western Australia. Distinctive stromatolites are shown to characterize various levels in the Proterozoic succession. A descending sequence of stromatolite assemblages is proposed denoted by:
-
Collenia frequens—Conophyton cf. inclinatum
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Collenia australasica—C. undosa
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C. cf. kona—C. brockmani
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C. sp. aff. multiflabella.
15.
Julia Semprich Nina S. C. Simon Yuri Yu. Podladchikov 《International Journal of Earth Sciences》2010,99(7):1487-1510
Constraints on density as a function of pressure, temperature, and composition are crucial to understand isostatic movements
during geodynamic processes. Here, we provide a systematic series of density diagrams extracted from thermodynamic calculations
for a variety of crustal compositions within a wide P–T range. We quantify systematic density changes in collisional settings for relevant compositional variations and attempt to
simplify the density–composition relationships. Rock densities depend strongly on pressure, temperature, and composition.
Densities at some selected pressure–temperature conditions increase linearly with increasing Al2O3 as well as MgO/FeO contents in pelitic rocks. Al- and Fe-rich pelites yield the highest densities, which is mostly due to
the formation of garnet but also depends on other minerals and changes of reactions. The effect of loading on densities is
investigated, and we show that for deep burial, a meta-pelite rich in Fe and Mg yields much larger density changes than a
dry basalt and that the burial of such a rock with a composition close to typical lower crust may result in significant negative
buoyancy. Metamorphism of hydrous lower crust due to pressurization and heating thus leads to densification of thickened lower
crust, while heating of dry crust leads to a decrease in density. Hence, water-loaded isostatic subsidence due to metamorphism
of water-saturated lower crust is substantial and increases with the thickness and depth of the reacting layer, while dry
compositions show much less or only transient densification and subsidence. The density change due to thermal expansion, an
extensively used concept in geodynamic models, predicts uplift under the same P–T conditions and is an order of magnitude smaller than the density variation calculated from petrologically consistent diagrams. 相似文献
16.
Prof. Dr. E. Schroll 《Mineralogy and Petrology》1966,11(3-4):317-347
Zusammenfassung Die Analyse der seltenen Elemente in granitoiden Gesteinen ist ein wertvolles. Hilfsmittel zur Erfassung und Charakterisierung petrogenetischer Vorgänge, wenn auch vielfach noch nach dem heutigen Wissensstand die Vorausseizungen zur Lösung spezieller Probleme fehlen.Der Cheismus basaltischer, granitischer und sedimentärer Gesteinstypen wird verglichen und in Diagrammen dargestellt. Diskussion der geochemischen Klassifikation der seltenen Elemente und der räumlichen und zeitlichen Verteilungsgesetzmäßigkeiten. Eine zeitliche Abhängigkeit der seltenen Elemente scheint nur auf Grund der räumlichen Verteilung vorhanden zu sein, wobei die Aufschlußtiefe der kontinentalen Stockwerke von größerer Bedeutung ist.Die Auswertung von Literaturangaben und Analysen aus dem alpinen und moldanubischen. Raum zeigt streng geselzmäßige Konzentrationsverteilungen an, die an Hand von Korrelationsdiagrammen, wie z.B. K/Rb, Ca/Sr, Cr/Sc, Cr/Ni, Ni/Co, Mn/V, Y/Sc, Nb/Ta, diskutiert werden.Gehalte an femaphilen Elementen erlauben eine Ausscheidung von Paragesteinen sowie tonalitischer und granodioritischer Gesteinstypen, wobei die Stoffaufnabme aus dem basischen Milien charakteristisch ist.
Mit 7 Textabbildungen
Herrn Prof. Dr.H. Leitmeler zum 80. Geburtstag gewidmet. 相似文献
Summary The analysis of the rarer elements in granitoidic rocks is a valuable aid for realizing and characterizing petrogenetic processes, although according to the present knowledge the premisses are lacking to solve special problemsThe chemism of basaltic, granitic and sedimentary types of rocks is compared and shown in diagrams. The geochemical classification of the rarer elements and the spatial and temporal laws of distribution are discussed. A temporal dependance of the rarer elements seems to be present only according to the spatial distribution, where as the depth of erosion of the continental floars is more imporiant.The evaluation of data of literature and of analyses of samples from the Alpine and Moldanubic region shows strict laws of the distribution in trace element concentrations, which are described using concentration diagrams, for instance K/Rb, Ca/Sr, Cr/Sc, Cr/Ni, Ni/Co, Mn/V, Y/Sc, Nb/Ta etc.Concentrations of femaphile elements allow the separation of para-rocks or tonalitic or granodioritic types, where as the addition of basic substance is remarkable.
Mit 7 Textabbildungen
Herrn Prof. Dr.H. Leitmeler zum 80. Geburtstag gewidmet. 相似文献
17.
Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H2O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H2O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H2O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X
A as well as P and T. The predicted mineral associations for the low- and high-X
A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X
A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO2 and H2O) for low- and high-X
A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X
A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X
A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X
A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H2O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased 相似文献
18.
《International Geology Review》2012,54(3):325-347
The four tectonic discrimination diagrams of Pearce et al. [Journal of Petrology, v. 25, p. 956–983] for granitic rocks were first evaluated using the literature cited by these authors as well as from our new database. The first diagram (Y?Nb) cannot discriminate volcanic-arc and collision settings. Both Y?Nb and Yb?Ta diagrams have an overlapping field for within-plate and ocean-ridge granitoids. The remaining two diagrams (Y?+?Nb?Rb and Yb?+?Ta?Rb) use a mobile element (Rb) in their y-axis. Although these diagrams successfully discriminate volcanic-arc and within-plate granites, they perform less well for collision tectonics. Besides, felsic or acid rocks are scarce in ocean-ridge settings, which limits the usefulness of these diagrams for this geological environment. Therefore, using an extensive database, we proposed a set of five new discriminant-function-based multi-dimensional diagrams for acid magmas from four tectonic settings (island arc, continental arc, continental rift, and collision). The very similar tectonic settings of island and continental arcs are discriminated for the first time. These diagrams are based on correct statistical treatment of compositional data, because they use natural logarithm transformation of major-element ratios and linear discriminant analysis (LDA). The use of discordant outlier-free samples prior to LDA improved the success rates by about 3–5%. Success rates of these diagrams as inferred from a testing set were between 76% and 88% for island arc, 60% and 92% for continental arc, and 72% and 84% for both continental rift and collision settings. Finally, application of these new diagrams to case studies not compiled in our initial database used for constructing these diagrams provided the following results: a collision setting for the Himalayas at about 30 Ma; an island arc setting for Quaternary acid rocks from geothermal boreholes in El Salvador; an island- or continental-arc setting for northern Italy at 35–52 Ma; a continental-arc setting for the Italy–Austria border at about 30 Ma; either a rift or a collision setting for northern Nigeria at about 164 Ma; a collision setting for central Nigeria at about 144 Ma and for the Cretaceous Masirah ophiolites of Oman; and an island arc setting for the Cretaceous Semail ophiolites of Oman. In spite of the relative mobility of major elements, these applications suggest utility of the new discrimination diagrams for all four tectonic settings. 相似文献
19.
Planewave pseudopotential calculations of supercell total energies were used as bases for first-principles calculations of
the CaCO3–MgCO3 and CdCO3–MgCO3 phase diagrams. Calculated phase diagrams are in qualitative to semiquantitative agreement with experiment. Two unobserved
phases, Cd3Mg (CO3)4 and CdMg3(CO3)4, are predicted. No new phases are predicted in the CaCO3–MgCO3 system, but a low-lying metastable Ca3Mg(CO3)4 state, analogous to the Cd3Mg(CO3)4 phase is predicted. All of the predicted lowest-lying metastable states, except for huntite CaMg3(CO3)4, have dolomite-related structures, i.e. they are layer structures in which A
m
B
n
cation layers lie perpendicular to the rhombohedral [111] vector.
Received: 6 May 2002 / Accepted: 23 October 2002
Acknowledgements This work was partially supported by NSF contract DMR-0080766 and NIST. 相似文献
20.
Systematic mineralogical and chemical study of the younger Taupovolcanic ash showers, collected from the Terraces Quarry, Taupo,has enabled the showers to be grouped into ten eruptive sequences(numbers refer to the stratigraphical position of the deposits,after Baumgart (1954)): 3; 4; 5–8; 9–13; 14–15;16; 17–18; 19–22; 23–25; 26. The showers arecomposed of rhyolitic pumice and finer glass fragments; accessoryfragments dominantly of rhyolite, with minor dacite, ignimbrite,and andesite; crystals of plagioclase, hypersthene, and magnetite.The proportions of these components provide a useful guide todistinguishing the showers for correlation purposes. Refractiveindices of the glasses show wide variations, even in one pumicefragment, and increase markedly with increased weathering. Correlationof natural glasses by this method must, therefore, be done withextreme care. Vesiculation of pumice from most of the sequences has been studiedquantitatively by measurements of density and porosity (by modalanalysis). These give a measure of the extent of vesiculation,and in the sequences studied it is shown that there is a progressivedecrease in vesiculation with time in each sequence, here attributedto progressive volatile loss. In the large Taupo (3) and Waimihia(15) deposits, there was a rapid initial rise of intensity ofvesiculation. The last phase of several sequences appears tohave been relatively hotter than the earlier magma, as shownby more calcic plagioclase and more magnesian-rich pyroxenes. As a result of differences in nucleation of the various pumices,three general textures can be recognized: (i) finely cellularwith thick intercellular walls; (ii) finely cellular with thinintercellular walls; (iii) coarsely cellular pumice, usuallywith widely varying vesicle size. Glasses from seven members have been chemically analysed, andshow a close similarity. However, they fall into two distinctgroups, comprising the older showers (19, 24, 25) and the youngershowers (3, 5, 8, 15). The former have higher normative quartz,orthoclase, and anorthite. The compositions are rhyolitic. Plagioclase (An36-An48), hypersthene (Mg48-Mg64), and magnetiteform the characteristic mineralogical assemblage. The plagioclasehas well-developed normal oscillatory zoning, which is believedto be due to movement of crystals within the magma into zonesof varying vapour pressure. Many crystals have sharp discontinuitiesof some zones, which are attributed to sharp drops in vapourpressure, possibly due to eruption of preceding members of thesequence. Three chemical analyses of plagioclase are presented. From vesiculation and chemical results, it is believed thatsequences 3, 5–8, 14–15, and 17–18 crystallizedunder a very high vapour pressure, and were probably eruptedas a result of this. Sequences 19–22 and 23–25 crystallizedunder much lower vapour pressure. From the Ab-Or-Q-H2O diagram,a vapour pressure of 2,000–3,000 kg/cm2 is suggested forthe former sequences, equivalent to about 4 miles lithostaticload. Comparison is made between the mineralogy of the Tauporhyolitic glasses and recently published work on some Britishand Icelandic Tertiary rhyolitic glasses. 相似文献