The Las Cruces VMS deposit lies on the southern margins of the Iberian Pyrite Belt, Spain. The primary base metal massive sulphide is overlain by a supergene enriched zone and precious metal gossan that remains well preserved under approximately 150 metres of Tertiary marl. The mineralogy, mineral textures and associations of five boreholes containing precious metal gossan mineralisation were characterised using a combination of optical microscopy, SEM and XRD techniques. The mineralogy and geochemical profile of the gossan suggests that it was formed under near-surface weathering conditions, resulting in the development of the supergene zone and a mature gossan profile characterised by elevated levels of Au and Ag. The Au and Ag probably remobilised as chloride complexes under strongly acid, oxidising conditions, precipitating as high fineness Au and discrete Ag-bearing phases lower in the gossan profile. The original Fe-oxyhydroxkJe dominated gossan mineral assemblage has subsequently been extensively replaced by later stages of siderite, greigite, galena and high fineness Au mineralisation that reflect marked changes in the oppositional environment relative to the original gossan mineral assemblage. Fluctuating oxidising and reducing conditions, coupled with biogenic processes within the Niebla Posadas aquifer, situated directly above the present day Las Cruces gossan, provide a suitable mechanism for the formation of the extensive siderite and greigite mineralisation as well as precious metal remobilisation as a thiosulphate complex under near-neutral to alkaline conditions. Strongly negative 613C stable isotope values for the siderite are consistent with biogenic processes involving Fe3* and/or sulphate reducing bacteria as well as a significant influence from the oxidation of methane.

Clay minerals are ubiquitous and readily accessible in the natural environment and consequently have become an essential ingredient in the development of Western Society. Their structural properties are responsible for many of their uses, their layered-leaf composition enables the absorption of water and other solutes, for example. In this thesis, the focus of interest lies primarily in the chemical properties of the clay minerals, which is due to the large surface areas of varying atomistic environments comprising the mineral layers. Clay minerals offer a challenge to the electronic structure modeller as their atomistic composition is non-exact, consequently a number of constraints are automatically applied during the modelling process, the first being the choice of composition of the model. There are currently few examples of density functional theory studies using planewaves and the pseudopotential approximation, and the available experimental data is not necessarily directly applicable to theoretical data due in part, to the inexactness of the clay mineral composition. Consequently, in the studies presented in this thesis, as much time has been spent in considering the modelling methods as on the results obtained and the implication of these in the modelling environment chosen. This thesis records investigations into the decarboxylation of a fatty acid into an alkane and CO$_{2}$ with the modelling of a catalytic environment of an aluminium-bearing clay mineral; the identification of a transition state of this reaction pathway using lattice dynamics and finally, the mechanism of reduction within iron-bearing clay minerals.

Reynolds, Brook and Gourley (1957) derived a value of 5 x 10(^-4) e.s.u. for the charge separated per crystal collision when a simulated hailstone rotated in a cloud consisting of ice crystals together with supercooled water droplets of some 5μ size. From this estimate they concluded that the electrification of thunderclouds could be explained in terras of the electrification of hail by impacting ice crystals. Latham and Mason (196I B) performed similar experiments in the absence of water droplets and obtained an estimate which was 5 orders of magnitude less than Reynolds' value. They also measured the electrification of an iced probe by supercooled water droplets shattering on it and derived a value of 4 x 10(^-6) e.s.u. for the mean charge separated per droplet diameter range 40 - 100 μ. They concluded from these experiments that the charge separated by Ice crystal impacts was not sufficient to explain thunderstorm electrification. They proposed instead that the droplet shattering mechanism offered a satisfactory explanation of the magnitude of the charge separated in thunderclouds. It was the purpose of this thesis to investigate the results of these two authorities and in particular to seek an explanation for the large discrepancy between their results on ice crystal impactions. In this laboratory similar experiments to Reynolds, Brook and Gourley were performed, and it was concluded that the results could be explained qualitatively in terms of the temperature gradient theory, but quantitatively the charge separated was larger than predicted by the theory. Experiments similar to those of Latham and Mason on crystal impacts were performed. The quantity of charge separated per crystal collision and how It depended on the sign and magnitude of the measured temperature difference between the iced probe and the crystals, the presence of impurities in the probe, and the impact velocity of the ice crystals was determined. An estimate of 2.5 X 10(^-7) e.s.u. was obtained for an impact velocity of 20 m sec (^-1) and measured temperature difference of 10 C. This was 50 times greater than the value found by Latham and Mason but it was shown that the two values could be reconciled. It was shown that they could also be reconciled with the previous value of 5 x 10(^-6) e.s.u. It was further observed that the charge separated per crystal collision increased markedly as the impact velocity increased. Apparatus was built which enabled stable streams of uniformly sized uncharged water droplets in the radius range 50 - 150μ, to be produced. Smaller droplets down to 30μ radius were produced in unstable streams. Droplets were made to encounter a rotating iced robe connected to an electrometer. It was found that appreciable quantities of charge were separated only for the larger droplets. If the droplets were above about 0 C they charged the probe positively and if they were supercooled they charged it negatively. The quantity of negative charge separated decreased as the degree of supercooling increased. The maximum mean charge separated for a 150μ radius droplet was 10(^-5) e.s.u. and for a 90μ radius droplet was 4 x 10(^-7) e.s.u. It was concluded that the charge was separated by the droplets splashing. Droplets which were in the process of freezing were allowed to encounter the probe. The droplets always charged the probe negatively and mean charges of up to 2 x 10(^-4) e.s.u. per 150μ. radius droplet were separated. The charge separated appeared to be proportional to he cube of the droplet radius. Although the results were not directly comparable with the results of Latham and Mason, it was considered that a similar charge separation mechanism was operative, and that it could be explained more readily by the Workman - Reynolds effect than by the temperature gradient theory.

A description of the stratigraphy of the area studied is given in which the variations in lithology and sequence are illustrated. The methods of clay mineral analysis are discussed and it is concluded that X-ray analysis is most useful for the present work. The argillaceous rocks are classified into types on characteristics recognisable in the field and the results of petrographic and X-ray analyses are tabulated. Apart from kaolinite, which is post-depositional in origin, only three different clay minerals occur: illite, chlorite and mixed- layer. Different types of argillaceous rock have different clay mineral assemblages. Associated arenaceous and calcareous rocks are similarly classified on field characters and are analysed petrographically. A system of sandstone classification is proposed. The relationship between the clay mineralogy of a sedimentary rock and the origin, transport, deposition and post-depositional history of the minerals is discussed. Recent shallow water marine and deltaic sedimentation are reviewed and the various facies recognised in the Gulf of Mexico and the Mississippi Delta are enumerated. It is postulated that the Yoredale sediments were deposited in a shallow water marine environment from a series of small shoal-water deltas. The different sediment types and their mineralogies are related to different conditions in the various facies of the environment. Repetition of the sedimentation in the form of cycles is discussed briefly. An additional chapter deals with the igneous rocks intruding the sediments and the metamorphic effects of the Whin Sill upon the country rocks.

The mineralogy of the sulphosalts is not well understood, principally because of the very wide variations in chemical composition of the natural examples and their common admixture with other sulphide mineral species. The synthesis of pure sulphosalt compositions is considered to be a valuable means of solving the problems involved. Changes in characteristic physical properties can then be measured as elements are substituted systematically into the structure. A typical sulphosalt, tetrahedrite, general formula (CuAg)(_10)(ZnFeCu)(_2)(SbAs)(_4)S(_13) was selected for study by synthesis in sealed evacuated silica glass capsules. The phase relations of the Cu-Sb-S: system were studied. Phases obtained were: tetrahedrite, Cu(_12)Sb(_4)S(_13); non-stoichiometric tetrahedrite, Cu(_13.04)Sb(_3.86)S(_13), which has high and low temperature cubic polymorphs; stibioluzonite, Cu(_3)SbS(_4); chalcostibite, CuSbS(_2); and Gu(_3)SbS(_3). Substitution of As for Sb in tetrahedrite (the tetrahedrite-tennantite series) was found to result in a decrease in cubic cell edge, increase in microhardness, decrease in reflectivity and slight changes in quantitative colour measurements. Non-stoichiometry of the series also affected these properties. Substitution of Zn and Fe for Cu in tetrahedrite resulted in an increase in cubic cell edge, increase in microhardness and increase in reflectivity. Substitution of Ag for Cu resulted in an increase in unit cell edge, decrease in microhardness and decrease in reflectivity. Only Ag substitution proved to be temperature dependant, the low temperature breakdown products of Ag-tetrahedrite being (CuAg)(_3)SbS(_4) and (AgCu)(_3)SbS(_3). Chemical bonding in the phases synthesised is discussed in detail. Substitution and the resultant effect on physical properties are discussed in relation to the atomic properties of the substituting element.

No description available.

This PhD was made possible by funding from the Natural Environment Research Council and the British Geological Survey. Additional funds came from the Society of Economic Geologists, Robert’s Skills Fund, Whitaker Fund, the Mineralogical Society and the Geochemical Society.

Previous work concerning the structure of chrysotile is first reviewed, to provide a basis for discussion of the natures of two of the less common varieties of chrysotile - Povlen-type chrysotile and para-chrysotile. The results of x-ray diffraction, electron diffraction and electron-optical experiments on Povlen-type chrysotile are presented. On the basis of these observations a lath-like model for this material is rejected. In the course of discussion of the diffraction effects arising from Povlen-type chrysotile, the earlier theoretical treatment of the diffuse reflections of chrysotile is extended. In particular, the effects on the profiles of these reflections of a variety of distributions of fibril diameters within a fibre bundle have been investigated. The character of the reflections to be expected from chrysotile, based upon a high-order helical structure are described in the course of the evaluation of a possible helical model for Povlen-type chrysotile. This model is, however, rejected in favour of a polygonal model, which accounts satisfactorily for the observed diffraction effects. An explanation is offered for the "lumpiness" of the diffuse reflections apparent in some x-ray fibre photographs of chrysotile. After a review of the few previous observations of para-chrysotile the results of electron-optical and electron diffraction studies are presented. An attempt is made to interpret these results in terms of a possible structural model for para-chrysotile. As in the case of Povlen-type chrysotile a lath-like model is rejected. Computations of the profiles of some of the diffuse reflections of para-chrysotile (based upon a cylindrical structure), which have not previously been published, are reported. In the light of the results of these calculations, which were carried out both for fibres consisting of bundles of fibrils exhibiting various distributions of diameters (applicable to x-ray diffraction) and for the case of single fibrils of various diameters (applicable to electron diffraction), a cylindrical model is considered to be inadequate to explain the present diffraction results. It is suggested that a polygonal model, similar to that proposed for Povlen-type chrysotile, might be more appropriate. Computer programs have been written to carry out the computation of the reflection profiles discussed in these two chapters, and are given in an Appendix. In the succeeding chapters observations concerning several commonly occurring accessory minerals of serpentinites are reported. X-ray diffraction and electron diffraction studies of fibrous brucite are described. The intergrowth of brucite with chrysotile, particularly with para-chrysotile, is discussed in the light of the present observations in the electron microscope. The nature of 'nemalite' is discussed and a model, based upon intergrowth with chrysotile, is proposed to account for the published accounts of this material. The structural and chemical characteristics of some of the hydroxycarbonate minerals are reviewed. The stability of brucite in the weathering environment and some chemical and crystallographic aspects of its alteration to various hydroxycarbonate-type minerals are considered. Observations on the textural development of magnetite in serpentinites are reported. Some chemical data are presented, together with the results of an x-ray investigation into the structural orientation of fibrous magnetite intergrown with other minerals in cross-cutting veins. Finally, an attempt is made to relate some of the observations reported in the present study to recent discussion of the processes involved in serpentinisation.

A new spectroscopic study of the orthorhombic amphiboles has been undertaken, using Mandouml;ssbauer and infrared spectroscopic techniques to determine site occupancies. The crystal structure of amphiboles is reviewed, and a new approach to the structures of chain silicates is outlined. Hydroxyl spectra of the specimens are of better quality than in the previous study of the orthoamphiboles (Law, 1969/70) and this allows a hyperfine structure to be seen in spectra of anthophyllites. Spectra are generally more difficult to interpret qualitatively than are spectra of related minerals, because of their complexity or because of low absorption intensities which characterise gedrite spectra. The development of a computer model for investigating the relationship between site occupancies and peak intensities allows, in principle, separate site populations for M1 and M3 sites to be determined from hydroxyl spectra. The Mandouml;ssbauer spectra, particularly of aluminium-rich gedrites, are too complex to be resolved completely, and this limits the accuracy of the site population information which can be obtained. Site occupancies are presented for the orthoamphiboles, however. A distinction is observed between high- and low-aluminium gedrites, and this empirical distinction is useful in discussing the properties of these amphiboles.

No description available.