An ion-association model for PbO-SiO2 melts: interpretation of thermochemical,conductivity, and density data

We propose that within PbO-SiO₂ melts the component species PbO(1) and SiO₂(1) associate to form a lead orthosilicate complex [Pb₂SiO₄(1)] according to $\text{2PbO(1) + SiO}{}_2\text{(1)}\text{a}\text{?}\text{f Pb}{}_2\text{SiO}{}_4\text{(1)}$ The association constant (K) for the above reaction is near 30 at 900 and 1000°C On the premises that the enthalpy of the above reaction is the only contributor to the enthalpy of mixing and that three melt species [PbO(1), SiO₂(1), Pb₂SiO₄(1)] mix ideally (Raoult's Law obeyed), the partial molar properties and the integral mixing properties of the melt have been calculated and have been found to accurately reproduce the measured thermochemical data.The conductivity data and density data for PbO-SiO₂ melts have been calculated using the association model. The calculated data again are remarkably consistent with the experimental data.Other silicate-melt models have been reinterpreted and that of Toop and Samis is compatible with the proposed model. In fact the Toop and Samis model implicitly assumes the formation of orthosilicate species rather than condensed polymeric species.     

Retail geography and social well-being: A note on the changing distribution of pharmacies in Scotland

Dispensing Chemists represent an important component in the organization of primary medical care in the United Kingdom, as well as providing a basic retail service. Over the past 30 years, the number of dispensing chemists' shops in Scotland has fallen by 40%, a withdrawal of services which has important spatial welfare implications. In this paper the locational dynamics of pharmaceutical retailing are examined, and the pattern of change between 1950 and 1980 is described. It is shown that the major trend, at both regional and intra-urban levels, is centralization. This trend is only weakly related to patterns in population change. Rather, it is the product of a general restructuring of retailing, reinforced by changes in the structure of the health care delivery system.     

A possible dependence of tectonic strength on the age of the crust in Asia

In Asia, the intensity and style of active tectonics appears to depend on the age of the last orogenic activity. The shields have remained essentially undeformed during the India-Eurasian collision, but Paleozoic and Mesozoic orogenic belts apparently have been reactivated by this collision, with a suggestion of greater reactivation and more diffuse deformation of the younger belts. If the greater observed heat flow in regions of more recent orogenic activity reflects, at least in part, a greater heat flow from the mantle beneath the younger belts, then the temperatures in the mantle beneath the younger belts should be higher than beneath older belts and shields. Because of the strong dependence on temperature of the creep strength of minerals, particularly of olivine, the crust and mantle beneath the hotter, younger belts should be much weaker than those beneath older belts. This difference in temperature, and consequent difference in strength, may be the cause of the greater reactivation of younger belts.     

The influence of organic debris on channel morphology and bedload transport in a New Zealand forest stream

The behaviour and form of, and bedload sediment transport through, a 3.5 m wide forest stream have been monitored for nearly three years. Bedload transport is highly episodic and spatially variable, and is controlled less by water discharge than by sediment availability. Organic debris in the channel creates temporary base levels and sites at which coarse sediment may remain stored for long periods; collapse or disruption of log and debris jams makes sediment available for transport in only a small proportion of the runoff events that are actually competent to move the material. Even then, sediment travels only a short distance before being redeposited, frequently behind debris accumulations further downstream. Rates of sediment transport during a given runoff event can vary markedly over short distances along the stream, again depending on whether sediment was made available for transport by log jam collapse upstream. Organic debris is therefore a major constraint on the application of physical laws and theories to explaining sediment movement in, and the morphology of, this stream.     

Some results of induced polarisation surveys for sulphide mineralisation in parts of Rajasthan and Gujarat,India

Some results of induced polarisation surveys carried out for the exploration of sulphide mineralisation in parts of Rajasthan and Gujarat, employing both frequency-domain and time-domain techniques, are presented. A few typical examples of I.P. results along with borehole sections and results of conventional geophysical surveys have been incorporated. Frequency-domain and time-domain I.P. data have been compared for selected areas.Borehole data pertaining to several areas investigated have corroborated the I.P. results fairly well.     

Noble gases in the earth's mantle

The abundances and isotopic compositions of noble gases in two samples from ultramafic xenoliths in alkali basalt, a young kaersutitic amphibole separated from a peridotite xenolith from Dish Hill, California and an ancient whole-rock lherzolite xenolith from Baja California, are reported and compared with the results of analyses on other mantle samples. In addition to previously recognized excesses of ³He and ¹²⁹Xe, our results indicate that ambient gases in the mantle show a general enrichment of the lighter-mass nonradiogenic isotopes of Ar, Kr and Xe, and Ar with ⁴⁰Ar/³⁶Ar = 3 · 10².     

A fast method for the solution of a system of simultaneous linear equations—A method adapted to a particular problem

Mathematical Geosciences - In mineral deposit evaluation, the estimation of block values can involve the solution of as many as 100,000 systems of linear equations. To minimize computing costs it...     

Effects of thermal discharges on the microstructural growth ofMercenaria mercenaria

The effects of thermal discharges from the Oyster Creek Nuclear Generating Station at Barnegat Bay, New Jersey, are recorded in the microstructural growth ofMercenaria mercenaria, a common coastal marine pelecypod. The analysis of the shell microstructure shows that this bivalve acts as an effective monitor of the environmental conditions existing in the marine waters adjacent to the power station. Many physiological and environmental events such as spawning, winter (freeze) shocks, summer (heat) shocks, thermal shocks, tidal cycles, and major storms are clearly recorded in the shell microstructure. The exact time of occurrence of these events can be determined by counting daily growth increments backwards from the outer shell margins of freshly killed individuals. Microstructural growth patterns reflected in Barnegat Bay specimens indicate that these pelecypods were affected mainly by temperature extremes, temperature variations, tides, type of substratum, and age. Growth patterns in specimens from areas surrounding Oyster Creek (affected by thermal effluent) are significantly different from those from other bay localities (unaffected by thermal effluent).Mercenaria mercenaria within approximately a 1.6km radius of Oyster Creek show a lower summer growth rate (10 percent to 25 percent lower) and a greater number of growth breaks (2 to 6 more per clam) than those away from the creek. The lower summer growth rates occur in bivalves subjected to the effluent because the added heat during the summer months causes water temperatures to exceed a critical threshold for optimum growth in the species. The greater number of growth breaks takes place, in turn, because many of the breaks (thermal shock breaks) are generated by rapidly fluctuating temperatures associated with abrupt shutdowns, massive load reductions and rapid renewal of operations following shutdowns or load reduction periods at the nuclear power station. In addition, the effluent may be upsetting natural spawning events in the clams when abrupt changes in power station operations overlap with spawning periods. In this respect, spawning may be precluded by sharp temperature changes which result in physiological shocks to the animal.     

Phytoplankton diversity and chlorophyll-A in a polluted estuary

The quantity of phytoplankton in Newark Bay, New Jersey as indicated by chlorophyll-a content of the water, is low in the winter and early spring, and fluctuates greatly during the spring and summer. Chlorophyll-a concentrations are generally less than 20 μg/l until April. Between April and August, three phytoplankton blooms were indicated by chlorophyll-a concentrations as high as 81.4 μg/l. Net phytoplankton diversity values indicated generally eutrophic conditions; however, there was no significant correlation between diversity and chlorophyll-a concentrations. A role of nannoplankton in blooms is indicated.     

The oxidation state of europium as an indicator of oxygen fugacity

The distribution of Eu between plagioclase feldspar and magmatic liquid has been determined experimentally for basaltic and andesitic systems as a function of temperature and oxygen fugacity at one atmosphere total pressure. Using the approach of Philpotts the ratios $\text{Eu}{}^{\mathrm{2+}}\text{Eu}{}^{\mathrm{3+}}$ in plagioclase and coexisting magmatic liquid have been calculated. These ratios appear to be simply related to oxygen fugacity for the bulk compositions studied here. Using published trace element distribution data for natural rocks oxygen fugacities may be calculated from these experimental results. For terrestrial basalts calculated oxygen fugacities average 10^?7 with little dispersion from this value. Andesites average 10^?8.1 with considerable dispersion, while dacites and rhyodacites average 10^?9.1, also with considerable dispersion. Oxygen fugacities for lunar ferrobasalts cluster tightly around 10^?12.7. Data on achondritic meteorites are limited, but calculations indicate oxygen fugacities of two-to-five orders of magnitude lower than lunar ferrobasalts.