Vitreous state in nature—Origin and properties

Vitreous materials are quite routinely found in natural settings. Most of them are aluminosilicates, which often occur in large deposits. Considering the geological formations in which naturally occurring vitreous aluminosilicates are found, they have generally remained stable for more than 1 Ma on the earth's surface, even in different geological and climatic environments. These non-crystalline solids played a very important role in the development of ancient human civilizations, long before the introduction of metallic tools. Today, however, the properties of natural glasses are of interest to mankind for completely different reasons. For example, industrial glasses are used today for encapsulating toxic wastes, especially radioactive waste, which remains active for centuries or more, in order to prevent the unwanted transfer of harmful materials to the environment. The chemical compositions of industrially produced glasses are in large part different from the compositions of natural glasses. Little is quantitatively known about the stability of industrial glasses over very long periods of time (>10,000 years). However, the physical and chemical stability of natural aluminosilicate glasses is known to extend over very long periods of time.The advancement of technological design to prevent or at least minimize the melt down of toxic waste during the encapsulation process is currently a major challenge, using glasses of natural chemical composition. Brecciated glass, which is found frequently in natural settings, provides a special clue to the possibility of producing vitreous solids by sintering glass fragments without melting the cullets. It is essential to prevent melting of the cullets because the melt has the potential of chemically reacting with the toxic waste.This paper summarizes the geological, chemical, and physical facts concerning naturally produced glasses, and seeks to establish a recognized database for further research in the domain of understanding the glass-forming processes that occur in nature. Furthermore, the authors hope to stimulate research into the utilization of natural resources that to solve the problem of storing of toxic waste safely.Major and trace element data have been collected over the past 100 years. These data constitute a sufficient basis for the chemical characterization of natural glasses. More information about the major elements is not required, in order to understand the chemical properties of these materials. On the other hand, large gaps in compositional data exist where other related components are concerned: e.g., in the case of “water-species”, with its different forms of bonding in silicates or oxygen (oxygen fugacity), CO₂-, sulphur - or hydrocarbons (methane)-, hydrogen-, chlorine-and fluorine-species. All these components have a significant impact on the properties of glasses, even when present only in minor quantities. Glass textures and crystal morphologies reflect the processes of nucleation and crystal growth in a glass-forming matrix during the cooling and reheating cycles which are currently not thoroughly understood. In nature, the processes that led to the formation of vitreous materials are very different from those used in the production of industrial glasses. The different genetic conditions under which glass formation occurs permit differentiation between magmatic and metamorphic vitreous solids. Sedimentary and biogenetic processes also contribute to the formation of non-crystalline solids.     

Late Cretaceous reactivation of major crustal shear zones in northern Namibia: constraints from apatite fission track analysis

Namibia's passive continental margin records a long history of tectonic activity since the Proterozoic. The orogenic belt produced during the collision of the Congo and Kalahari Cratons in the Early Proterozoic led to a zone of crustal weakness, which became the preferred location for tectonism during the Phanerozoic. The Pan-African Damara mobile belt forms this intraplate boundary in Namibia and its tectonostratigraphic zones are defined by ductile shear zones, where the most prominent is described as the Omaruru Lineament–Waterberg Thrust (OML–WT). The prominance of the continental margin escarpment is diminished in the area of the Central and Northern Zone of the Damara belt where the shear zones are located. This area has been targeted with a set of 66 outcrop samples over a 550-km-long, 60-km-broad coast-parallel transect from the top of the escarpment in the south across the Damara sector to the Kamanjab Inlier in the north. Apatite fission track age and length data from all samples reveal a regionally consistent cooling event. Thermal histories derived by forward modelling bracket this phase of accelerated cooling in the Late Cretaceous. Maximum palaeotemperatures immediately prior to the onset of cooling range from ca. 120 to ca. 60 °C with the maximum occurring directly south of the Omaruru Lineament. Because different palaeotemperatures indicate different burial depth at a given time, the amount of denudation can be estimated and used to constrain vertical displacements of the continental crust. We interpret this cooling pattern as the geomorphic response to reactivation of basement structures caused by a change in spreading geometry in the South Atlantic and South West Indian Oceans.     

The eastern continuation of the Cadomian orogen: U–Pb zircon evidence from Saxo-Thuringian granitoids in south-western Poland and the northern Czech Republic

We report U–Pb single zircon ages from three pre-Variscan granitoids in the NE part of the Bohemian Massif. The Platerówka granodiorite from the Lausitz-Izera Unit, the Polish Sudetes, has been dated at 533±9 Ma. The Bitouchov granite form the SW part of the South Krkonoe Unit, the Czech Sudetes, gave an age of 540+11/–10 Ma, and the Wdroe granodiorite in the Fore-Sudetic Block yielded 548±9 Ma. All these latest Vendian/Early Cambrian granitoids represent the post-tectonic expression of a late Proterozoic Cadomian orogenic cycle and demonstrate the eastward extent of the Cadomian basement into the Variscan orogen. Granodiorites of similar age have so far been reported from Brittany and especially from the Saxo-Thuringian Terrane to the NE and SW of the Elbe Fault Zone. We conclude that the Saxo-Thuringian Terrane extends across the Elbe and Sudetic Marginal Fault Zones into the Fore-Sudetic Block.     

Boundary-layer diffusion modelling: The Gaussian plume approach versus the spectral solution

The equation of turbulent diffusion is solved for a vertical area source within the planetary boundary layer. The traditional Gaussian-plume approach is compared with the spectral solution of the diffusion equation used together with the barotropic boundary-layer model of Lettau and Dabberdt (1970). The results of the numerical computations are presented and the differences between the solutions are discussed.     

Sr-gorceixite,a weathering product in rich iron ores from the Córrego do Feijão Mine,Minas Gerais,Brazil

A sulfate-bearing Sr-Ba phosphate (Sr-gorceixite) formed by supergene alteration in rich iron ores from Minas Gerais, Brazil, has been studied by means of microprobe techniques. Analytical data and the structural formula are presented.