Crystal chemical and 57Fe Mössbauer study of chromite from the Nuggihalli schist belt (India)

Summary In the Nuggihalli schist belt (India), chromite bodies were affected by intense serpentinization followed by weathering. In spite of the strong oxidation of the chromite grains, some unaltered cores were preserved, and they were characterised using electron probe microanalysis, single-crystal X-ray diffraction and low temperature ⁵⁷Fe Mössbauer spectroscopy. Results of such investigations revealed that chromite cores from the Nuggihalli schist belt are effectively not oxidised, and their study revealed that chromite was not affected by greenschist-facies metamorphism. The apparently highly ordered cation distribution of the chromites, which would suggest low equilibration temperature, is just caused by the very high Cr contents, which prevent Mg–Al exchange between T and M sites. Using Fabries (1979) geothermometer a temperature of about 1180°C was retrieved (Mitra and Bidyananda, 2003), which therefore corresponds to the quenched-in igneous equilibrium.     

Microtremor survey in Talchir, India to ascertain its basin characteristics in terms of predominant frequency by Nakamura's ratio technique

The Talchir Basin, one of India's oldest basins, has been a subject of interest because of its rich coal deposits. The maximum thickness of the basin is about 1500 m. Beyond the basin is the hard metamorphic rocks of Precambrian age. The ambient noise survey data have been analyzed for the Talchir Basin using Nakamura's technique of horizontal–vertical-spectral-ratio (HVSR) to ascertain the basin structure in terms of the predominant frequency. The predominant frequency varies from 0.25 Hz to 7.8 Hz but a major portion of the basin comes under the range of 0.3 Hz–2.4 Hz while on the metamorphic rocks it is as high as 7.8 Hz. The variation in predominant frequency shows a good correlation with the sediment thickness of the basin. The results have been compared with the previous studies by other researchers and it shows consistency with the northerly dip of the basin. The present study has also been compared with the results of the synthetic seismogram that was performed for the Talchir Basin. The predominant frequency obtained from HVSR technique complements well with the frequency at which the peak response spectra ratio is observed. The present study of the predominant frequency identifies quite well the characteristics of Talchir Basin and is in good agreement with the synthetic ground motion modeling of the region.     

Homogeneous freezing of single sulfuric and nitric acid solution drops levitated in an acoustic trap

The freezing temperatures of single supercooled drops of binary and ternary sulfuric and nitric acid solutions were measured while varying the acid concentration. An acoustic levitator was used which allows to freely suspend single solution drops in air without electrical charges thereby avoiding any electrical influences which may affect the freezing process. The drops of typically 500 µm in radius were monitored by a video camera during cooling cycles down to − 85 °C to simulate the upper tropospheric and stratospheric temperature range. The present data confirm that liquid solution droplets can be supercooled far below the equilibrium melting point by approximately 35 °C. They follow the general trend of the expected freezing temperatures for homogeneous ice nucleation.     

Laboratory studies about the interaction of ammonia with ice crystals at temperatures between 0 and −20°C

In laboratory experiments the interactions of ammonia with ice crystals were studied within the temperature range between 0 and −20°C. In a first series of experiments dendritic ice crystals were grown from water vapor in presence of ammonia gas in various concentrations between 4 and 400 ppbv. In a second series of experiments pure ice crystals were exposed to a humidified ammonia–air mixture inside a horizontal flow tube. The influence of temperature, ammonia gas concentration (0.6, 1.5, and 10 ppmv), exposure time, and the presence of impurities such as sulfate on the ammonia uptake by the ice surface was investigated by determining the ammonium content in the melt water of the ice crystals by ion chromatography. During the growth of ice crystals significant amounts of ammonia (around 200 μg/l) were taken up even at small gas concentrations. In contrast, even at high gas concentrations the uptake of ammonia by non-growing ice crystals was lower by approximately one order of magnitude. The presence of sulfate on the ice surface affected an enhanced uptake of ammonia by a factor of 5–10. A model is presented which describes the uptake of ammonia by ice considering the chemical processes occurring in the ice surface layer and simultaneous diffusion of ammonia into bulk ice. Even the increased uptake of ammonia by growing ice is rather small compared to the uptake by water droplets; thus, the major process for scavenging of ammonia from the atmosphere via the ice phase might not be the direct uptake by ice crystals but the riming involving super-cooled droplets containing ammonia.     

Impact of Farakka barrage on the hydrology and fishery of Hoogly estuary

Construction of the Farakka barrage on the Ganga River in April 1975 to augment water supply to the Calcutta port has brought about a significant increase in freshwater discharge in its distributary, the Hoogly estuary. This has naturally resulted in major changes in the ecology of this estuary, causing modifications in the structure of its fishery resources, fishing pattern, and fish production. This paper presents observations on salinity, plankton, bottom biota, fishery resource, and fish production of different zones of the Hooghly estuary during the period 1982–1992. Comparison with similar studies made before and immediately after commissioning of the Farakka barrage (1975–1977) has revealed that the increased freshwater discharge has resulted in considerable decrease in salinity throughout the estuary. The freshwater zone now extends toward the mouth of the estuary. The true estuarine zone has moved seaward and the marine zone has been restricted to the area near the mouth of the estuary. This has effected major changes in plankton dynamics, sharp decline in the fishery of marine and neritic species in the upper estuary, caused a significant increase in catch ofTenualosa ilisha and an over twofold increase in the average annual fish landings from the estuary as a whole. New zonations have been proposed based on the presently, observed salinity values, which are the most significant factor in determining the fishery of any estuary. An interdisciplinary study of the ecology of the new zones is needed to establish their correct biological characteristics.     

Chromitites of komatiitic affinity from the Archaean Nuggihalli greenstone belt in South India

Summary Mineral chemistry and petrological data of chromites from chromitite bands in the N–S trending schist belt of Nuggihalli (southern Karnataka, India), belonging to the Dharwar craton of South India, are presented in this paper. Crystal chemical data indicate a komatiitic affinity of the chromitite. P–T calculations of the chromite-hosting peridotites yielded a pressure range of 13 to 28 kbar and temperatures ranging from 775 to 1080 °C; the oxygen fugacity (log fO₂) varies from +0.5 to +1.6 above the QFM buffer. The P, T and fO₂ data indicate that Nuggihalli chromitites crystallized in an environment akin to the upper mantle. The studied samples also show partial resetting; the lower temperatures ranging from 515 to 680 °C are ascribed to subsequent metamorphism of the area.     

On the scavenging of gaseous nitrogen compounds by large and small rain drops: II. Wind tunnel and theoretical studies of the simultaneous uptake of NH3, SO2 and CO2 by water drops

An experimental investigation of the simultaneous absorption of NH₃ and SO₂ from the ambient atmosphere by freely falling water drops has been carried out in the Mainz vertical wind tunnel. The experimental results were found to be in good agreement with the results derived from computations with the Kronig-Brink convective diffusion model and also with a model which assumes a drop to be well mixed at all times. Encouraged by this agreement, these computation schemes for the uptake of gas by single drops where incorporated in a pollution washout model with realistic SO₂, NH₃ and CO₂ gas profiles. This model allows an entire raindrop size distribution to fall through a gas layer. The results of this plume-model show that the SO₂ uptake is strongly dependent on the NH₃ concentration in the atmosphere and on the rainrate. We also find that the small drops contribute more towards the washout of these gases. In the case of simultaneous presence of NH₃ and SO₂, desorption of these gases is negligible.     

On the scavenging of gaseous nitrogen compounds by large and small rain drops I. A wind tunnel and theoretical study of the uptake and desorption of NH3 in the presence of CO2

An experimental study has been carried out in the Mainz vertical wind tunnel to determine the rate at which NH₃ in the presence of CO₂ is absorbed by freely suspended water drops. The experimental uptake rates were found to be in good agreement with the rates predicted by the Kronig-Brink convective diffusion model and, for gas concentrations in the ppbv range also by the model in which it is assumed that the absorbed gas is well mixed inside the drop (henceforth called well mixed model). The same conclusion was shown to apply also to the desorption of NH₃ from a drop previously exposed to NH₃. The latter result is in contrast to the desorption of SO₂ which must be described by a model which accounts for the diffusion of the species inside the drop. Comparison of our experimental results with theory show further that the uptake of NH₃ in presence of CO₂ is significantly overestimated if the slow reaction CO_2(aq)+H₂OHCO ₃ ^– +H⁺ is neglected in the theoretical computation.