Struktur und Polymorphie des Eukryptits (Tief-LiAlSiO4). (Betrachtungen zur Polymorphie II.)

Ohne ZusammenfassungHerrn Professor Dr.Carl W. Correns zum 60. Geburtstag gewidmet.     

Stabilitätsbeziehungen zwischen Chlorit,Cordierit und Almandin bei der Metamorphose

The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe²⁺/Fe²⁺+Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture. Cordierite is formed according to the following reaction: $${\text{Chlorite + muscovite + quartz}} \rightleftharpoons {\text{cordierite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} + {\text{H}}_{\text{2}} {\text{O}}$$ . At low pressures this reaction characterizes the facies boundary between the albite-epidotehornfels facies and the hornblende-hornfels facies, at medium pressures the beginning of the cordierite-amphibolite facies. Experiments were carried out reversibly and gave the following equilibrium data: 505±10°C at 500 bars H₂O pressure, 513±10°C at 1000 bars H₂O pressure, 527±10°C at 2000 bars H₂O pressure, and 557±10°C at 4000 bars H₂O pressure. These equilibrium data are valid for the Fe-rich starting material, using thuringite as the chlorite, as well as for the Mg-rich starting mixture with ripidolite. At 6000 bars the equilibrium temperature for the Mg-rich mixture is 587±10°C. In the Fe-rich mixture almandite was formed instead of cordierite at 6000 bars. The following reaction was observed: $${\text{Thuringite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Experiments with the Fe-rich mixture, containing Fe²⁺/Fe²⁺+Mg in the ratio 8∶10, yielded three stability fields in a P,T-diagram (Fig.1):

1. Above 600°C/5.25 kb and 700°C/6.5 kb almandite+biotite+Al₂SiO₅ coexist stably, cordierite being unstable.
2. The field, in which almandite, biotite and Al₂SiO₅ are stable together with cordierite, is restricted by two curves, passing through the following points:
   1. 625°C/5.5 kb and 700°C/6.5 kb,
   2. 625°C/5.5 kb and 700°C/4.0 kb.
3. At conditions below curves 1 and 2b, cordierite, biotite, and Al₂SiO₅ are formed, but no garnet.

An appreciable MnO-content in the system lowers the pressures needed for the formation of almandite garnet, but the quantitative influence of the spessartite-component on the formation of almandite could not yet be determined. the Mg-rich system with Fe²⁺/Fe²⁺+Mg=0.4 garnet did not form at pressures up to 7 kb in the temperature range investigated. Experiments at unspecified higher pressures (in a simple squeezer-type apparatus) yielded the reaction: $${\text{Ripidolite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Further experiments are needed to determine the equilibrium data. The occurence of garnet in metamorphic rocks is discussed in the light of the experimental results.     

Anthophyllit und Hornblende in einigen metamorphen Reaktionen

Reactions which occur at the lower boundary of the hornblende-hornfels facies and in the so-called pyroxene-hornfels facies were experimentally investigated for an ultrabasic rock at 500, 1000 and 2000 bars H₂O pressure.The starting material used was a mixture of natural chlorite, talc, tremolite and quartz such that its composition, except for surplus quartz, corresponded to that of an ultrabasic rock. The atomic ratio Fe²⁺+Fe²⁺/Mg+Fe³⁺+Fe³⁺ in the system was 0.16.The lower boundary of the hornblende-hornfels facies was defined by the formation of the orthorhombic amphibole anthophyllite and hornblende according to the following idealized reaction: chlorite+talc+tremolite+quartz hornblende+anthophyllite+H₂O In effect, this reaction consists of the two bivariant reactions: chlorite+tremolite+quartz hornblende+anthophyllite+H₂O talc+chlorite anthophyllite+quartz+H₂OThe equilibrium temperatures obtained for the two reactions in the given system are practically the same and are as follows: 535±10°C at 500 bars H₂O pressure 550±20°C at 1000 bars H₂O pressure 560±10°C at 2000 bars H₂O pressure 580±10°C at 4000 bars H₂O pressureAt 2000 bars and higher temperatures within the hornblende-hornfels facies, anorthite is formed in addition to hornblende and anthophyllite, probably according to the following reaction: hornblende₁+quartz hornblende₂+anthophyllite+anorthite+H₂O; because of the formation of anorthite it is to be expected that the hornblende in this case is poorer in aluminium than the hornblende at 500 and 1000 bars. Winkler (1967) suggests renaming the pyroxene-hornfels facies as K-feldspar-cordierite-hornfels facies which, in turn, is subdivided into a lower-temperature orthoamphibole subfacies without orthopyroxene and a higher-temperature orthopyroxene subfacies without orthoamphibole. The orthopyroxene subfacies itself may in its lower temperature part still carry hornblende which finally disappears in the higher temperature part.The appearance of orthopyroxene characterizes the transition from the orthoamphibole to the orthopyroxene subfacies of the K-feldspar-cordierite hornfels facies. The following reaction takes place at pressures lower than 2000 bars: hornblende₁+anthophyllite hornblende₂+enstatite+anorthite+H₂OSince at 2000 bars an Al-poor hornblende already exists in the hornblende-hornfels facies, it is very likely that here only anthophyllite breaks down to give enstatite+quartz+H₂O.The equilibrium temperatures for these reactions which give rise to enstatite are: 650±10°C at 250 bars H₂O pressure 690±10°C at 500 bars H₂O pressure 715±10°C at 1000 bars H₂O pressure 770±10°C at 2000 bars H₂O pressureOnly after an increase in temperature to about 710°C at 500 bars and about 770°C at 1000 bars does hornblende in the system investigated here break down completely according to the reaction: hornblende = enstatite+anorthite+diopside+H₂OExcept at very small H₂O-pressures (see Fig. 3), there exists, therefore, a region within the orthopyroxene subfacies where hornblende, enstatite and anorthite coexist. As a result we have, as mentioned above, a lower-temperature and a higher-temperature part of the orthopyroxene subfacies, and it is only in the latter part that the parageneses correspond to the pyroxene-hornfels facies as stated by Eskola (1939).Summing up, the starting material consisting of chlorite, talc, tremolite plus quartz remains unchanged in the albite-epidote-hornfels facies; this gives rise in the hornblende-hornfels facies to the paragenesis hornblende+anthophyllite, or — at higher pressures — to hornblende+anthophyllite+anorthite. For the particular composition of the starting material, however, no reactions take place at the transition of the hornblende-hornfels facies to the orthoamphibole subfacies of the K-feldspar-cordierite-hornfels facies as this transition is typified by the breakdown of muscovite in the presence of quartz. However, at the end of the orthoamphibole subfacies the breakdown of anthophyllite, by which orthopyroxene is formed, heralds the onset of the orthopyroxene subfacies. In this subfacies — at greater than about 300 bars — hornblende is still present and coexists with enstatite and anorthite, but with rising temperature hornblende breaks down to give way to the paragenesis enstatite+anorthite+diopside. The experimentally determined parageneses confirm known petrographic occurrences.

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Für die Förderung dieser Arbeit danken wir der Deutschen Forschungsgemeinschaft vielmals. Der Dank von Choudhuri gilt dem Akademischen Auslandsamt der Universität Göttingen für ein Stipendium, das ihm den Abschluß seiner Studien an der Universität Göttingen ermöglichte.     

The dynamics of H2O in minerals

The application of infrared, proton nuclear magnetic resonance, and dielectric spectroscopy and incoherent neutron scattering for the elucidation of the dynamics of H₂O incorporated into minerals is reviewed. The examples given include beryl, cordierite, gypsum, bassanite, layer silicates and zeolites. It is demonstrated that for such structures static models may be inappropriate, and dynamic models have to be used to describe the role and behavior of the H₂O molecules.     

Eliminierung von Abwasserinhaltsstoffen durch Kombination biochemischer und grenzflächenchemischer Mechanismen

Recent developments and future trends of waste water treatment by combined biochemical and adsorptive elimination steps are presented. Various mechanisms are discussed to explain the enhancement of conventional activated sludge systems by addition of different solid adsorbents. Special emphasis is given to the improvement of elimination capacity and operation stability caused by powdered or granular activated carbons. The results of laboratory experiments as well as large-scale applications suggest that combinations of biological systems with by-product carbonaceous materials are most likely to gain future importance in waste water treatment facilities.     

Effects of long soil surface residence times on apparent cosmogenic nuclide denudation rates and burial ages in the Cradle of Humankind,South Africa

In situ cosmogenic nuclides are an important tool for quantifying landscape evolution and dating fossil-bearing deposits in the Cradle of Humankind (CoH), South Africa. This technique mainly employs cosmogenic 10-Beryllium (¹⁰Be) in river sediments to estimate denudation rates and the ratio of 26-Aluminium (²⁶Al) to ¹⁰Be (²⁶Al/¹⁰Be), to constrain ages of sediment burial. Here, we use ¹⁰Be and ²⁶Al concentrations in bedrock and soil above the Rising Star Cave (the discovery site of Homo naledi) to constrain the denudation rate and the exposure history of soil on the surface. Apparent ¹⁰Be-derived denudation rates obtained from pebble- to cobble-sized clasts and coarse-sand in soil (on average 3.59 ± 0.27 m/Ma and 3.05 ± 0.25 m/Ma, respectively) are 2-3 times lower than the bedrock denudation rates (on average 9.46 ± 0.68 m/Ma). In addition, soil samples yield an average ²⁶Al/¹⁰Be ratio (5.12 ± 0.27) that is significantly lower than the surface production ratio of 6.75, which suggests complex exposure histories. These results are consistent with prolonged surface residence of up to 1.5 Ma in vertically mixed soils that are up to 3 m thick. We conclude that the ¹⁰Be concentrations accumulated in soils during the long near-surface residence times can potentially cause underestimation of single-nuclide (¹⁰Be) catchment-wide denudation rates in the CoH. Further, burial ages of cave sediment samples that consist of an amalgamation of sand-size quartz grains could be overestimated if a pre-burial ²⁶Al/¹⁰Be ratio calculated from the surface production is assumed. © 2019 John Wiley & Sons, Ltd.     

The age and growth of hottentot seabream Pachymetopon blochii before and after the South African linefish state of emergency in 2000

The hottentot seabream Pachymetopon blochii is a small-sized (maximum 2.67 kg) sparid endemic to southern Africa. It is an important target in South Africa's Western Cape traditional linefishery, particularly in the absence of more valuable pelagic species (such as Thyrsites atun and Seriola lalandi). In 2000, South Africa's linefishery was declared to be in a state of emergency, and commercial fishing effort was consequently reduced by 70%. A subsequent increase in stock biomass and intraspecific competition, coupled with environmental changes, were hypothesised to have thereafter altered the growth rate of hottentot, from 2000 to 2010. This study aimed to revise outdated age–growth models for the hottentot by using modern techniques (sectioned otoliths), and to compare age–growth relationships before and after the declared linefish state of emergency. The maximum age observed was 19 years, with no difference in the growth rate between sexes (p = 0.39–0.43) or time-periods (p = 0.96). Although the growth rate did not change, there is evidence that the age structure of the stock changed between time-periods as a result of changes in fishing pressure between 2000 and 2010. The enhanced recent growth model for hottentot, described as L_t = 418.063 (1 – e^{?0.104(t – [?4.709])}) (pooled sexes; n = 206), indicates a considerably slower growth rate for this species than was proposed previously using whole otoliths and has major implications for effective stock management.     

Zur geomorphologischen und geologischen Entwicklungsgeschichte der Ostabdachung der Zentralalpen in der Miozänzeit

Ohne Zusammenfassung