Untersuchungen mit der Elektronen-Mikrosonde an zonargebauten Mineralen der Pyromorphitgruppe

Zusammenfassung Natürliche Glieder der Pyromorphitgruppe wurden mit einer Elektronen-Mikrosonde untersucht. Bei allen untersuchten Proben tritt gegenseitige isomorphe Ersetzung des Phosphors, Arsens oder Vanadins auf. Die fast reinen Endglieder zeigen dabei einen ziemlich gleichmäßigen, geringfügigen Einbau der Nebenkomponente unabhängig vom optischen Zonarbau, während für die Mischglieder starke zonare Schwankungen in der chemischen Zusammensetzung in Abhängigkeit von den optisch erkennbaren Zonen typisch sind.In einem Falle wird zonar Blei isomorph durch Kalzium ersetzt. Der Chlorgehalt scheint von keiner der übrigen Komponenten abhängig zu sein und unterliegt nur z.T. zonaren Schwankungen. Zonarbau durch Farbänderungen und Einbau von Fremdpartikeln, fast nur bei den Mischgliedern zu finden, bedingt zumeist entsprechende zonare chemische Veränderungen der Phosphor-, Arsen- oder Vanadingehalte. Zonarbau infolge unterschiedlicher optischer Orientierung kann sowohl ohne chemische Zonen vorkommen (bei den Endgliedern), als auch mit gleichlaufenden Veränderungen der Komponenten bei den Mischgliedern.Zonarbau als Folge unterschiedlicher optischer Orientierung, Felderteilung und Zweiachsigkeit haben nach den vorliegenden Untersuchungen nicht isomorphen Ersatz als Ursache; Veränderungen der physikalischen Bedingungen während oder Sekundärvorgänge nach Beendigung des Wachstums der Kristalle könnten eine Rolle gespielt haben. In Frage kommt auch eine Abweichung von der hexagonalen Symmetrie; eine kürzlich von U. Keppler (Karlsruhe) durchgeführte Einkristall-Untersuchung einer Mimetesit-Zone ergab für diese eine monokline Symmetrie (im Druck).Das Auftreten von diffusen Reflexen oder Doppellinien in den Röntgendiagrammen der Mischglieder kann zwanglos durch die gegenläufigen Schwankungen der Phosphor-, Arsen- oder Vanadin-Gehalte gedeutet werden.

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Electron microprobe investigation of members of the pyromorphite group with zoned structure

Natural members of the pyromorphite group, pyromorphite Pb₅(PO₄)₃Cl, mimetite Pb₅(AsO₄)₃Cl, kampylite Pb₅(PO₄, AsO₄)₃Cl, and vanadinite Pb₅(VO₄)₃Cl were investigated by the electron microprobe method. Mutual isomorphic replacement of P, V and As was observed even in the purest available end members occurring in nature. The frequently appearing zoned structure, observed in thin sections, may or may not coincide with corresponding chemical changes with respect to the V, As or P contents, mainly for the intermediate members, while for the rather pure end members nearly no connection was found between optical zones and chemical composition (also as to the division of basal sections into six triangular areas).

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Den Herren Prof. Dr. F. Laves, Zürich, und Prof. Dr. H. Wondratschek, Karlsruhe, danken wir für die Ermöglichung dieser Arbeit und für das fördernde Interesse. Der Deutschen Forschungsgemeinschaft danken wir für die finanzielle Unterstützung dieser Arbeit durch eine Beihilfe.     

The structural and textural features of felsic volcanics of the Sarbai Formation (Southern Urals)

The results of a microscope study of samples of felsic volcanic rocks from the upper horizons of the Sarbai Formation (O₂–S₁ sb) in the Southern Urals that compose the formation are presented. A detailed study of the samples and an evaluation of the results of a silicate analysis of these rocks showed their volcanogenic genesis. The rocks have been classified as volcanic glass that underwent several stages of transformation. Three newly-formed structural and textural types of mineralization were defined: spherulitic (felsophyric), axiolitic, and felsitic.     

A FAST FOURIER TRANSFORM METHOD FOR RAPID COMPUTATION OF GRAVITY AND MAGNETIC ANOMALIES DUE TO ARBITRARY BODIES*

The spectrum of a magnetic or a gravity anomaly due to a body of a given shape with either homogeneous magnetization or uniform density distribution can be expressed as a product of the Fourier transforms of the source geometry and the Green's function. The transform of the source geometry for any irregularly-shaped body can be accurately determined by representing the body as closely as possible by a number of prismatic bodies. The Green's function is not dependent upon the source geometry. So the analytical expression for its transform remains the same for all causative bodies. It is, therefore, not difficult to obtain the spectrum of an anomaly by multiplying the transform of the source geometry by that of the Green's function. Then the inverse of this spectrum, which yields the anomaly in the space domain, is calculated by using the Fast Fourier Transform algorithm. Many examples show the reliability and accuracy of the method for calculating potential field anomalies.     

Comparison of the multimode surface wave response in structures with and without a low-velocity channel (Part II: dip-slip sources)

Summary A comparison is made between the multimode Rayleigh wave response to dip-slip fault-plane motion in earth models with and without a low-velocity channel in the upper mantle. In general, higher-mode amplitude spectra are useful for discriminating between the two types of structures.     

A discussion of the Cepheid amplitude behaviour in the instability strip

An analysis is presented which seeks to reveal the possible causes for discrepancy in the results on the dependence of Cepheid amplitudes on star positions in the instability strip noted by different investigators.A comparison of the data used in two of these investigations—that of Yakimova (1970) and that of Sandage and Tammann (1971)—shows that it is most unlikely to be able to explain completely the conflicting results obtained in these studies. But if we analyse the data of Sandage and Tammann following the approach of Yakimova — namely, by considering the Cepheids in only one stellar system and taking into account the stellar colours in maximum light — we obtain Yakimova's result: the light amplitude is the larger, the nearer to the low temperature edge of the instability strip a Cepheid is. Therefore, it appears that differences between the various approaches contribute greatly to the different results of Sandage and Tammann and Yakimova (and the other investigators).     

Geochronology and petrology of recent volcanics in the eastern Aegean Sea (West Anatolia and Lesvos Island)

During the lower and middle Miocene the western Anatolia and the eastern Aegean Sea were dominated by a calcalkaline volcanism associated with minor acid and basic volcanics. The basic subcrustal volcanics consist mainly of alkali basalts and hawaiites (9.7–11.9 m.y.), nepheline hawaiites and nepheline trachyandesites (Kula area from 1.1 m.y. to the recent times). The rhyolitic volcanics (12.5 m.y.) derived by a partial melting process in the upper crust (⁸⁷Sr/⁸⁶Sr=0.7121). The calcalkaline suite (16.2–21.5 m.y., mean value⁸⁷Sr/⁸⁶Sr=0.708) shows a trend from latite-andesites to dacites and rhyodacites; a latite andesite system related to a sinking slab of lithosphere and constituted by a mixing of oceanic crust (tholeiite), oceanic sediments and/or tectonic fragments of sialic crust is envisaged.