Frontiers in natural and un-natural glasses: An interdisciplinary dialogue and review

Abstract

The purpose of this review is to compare and contrast silicate natural glasses with so-called un-natural glasses, bringing together literature and common issues in geology and technical glass science and engineering. The review is organized into three proposed ‘frontiers’ of glass science of interdisciplinary interest: 1) extreme conditions for glass formation, 2) crystallization in complex glasses, and 3) long-term glass alteration. In each frontier, examples of natural (geologic) as well as un-natural (accidental anthropogenic) glasses are presented, and their characteristics are compared and contrasted. Since crystallization is an important consideration, not only fully amorphous glasses but also partially crystallized glasses are considered. Natural glasses discussed include primarily impactites, obsidian, and basaltic glass, though glasses formed by other means are also considered. Un-natural glasses discussed include those created in atomic blasts, vitrified Iron Age hillforts, and slags from metallurgical processes. Conditions for extreme glass formation in different types of glasses formed by shock metamorphism are compared to modern time-resolved shock and high-pressure experiments for observing polyamorphism and densification changes. Silicate magmas producing obsidian and basaltic glasses are presented and the differences in crystallization pathways are discussed as they relate to changes in flow properties and eruptions. Vitrified hillfort materials and combustion natural glasses are then compared with respect to their felsic-mafic liquid phase separation and crystal assemblage. Finally, an appeal is made for the use of natural and un-natural glasses for understanding long-term glass corrosion as a function of composition. Glass corrosion models are considered as applied to both types of glasses, particularly relating to the neo-formed crystals, biologically-induced glass corrosion, and nuclear waste management. It is hoped that this unorthodox presentation will foster dialogue between earth scientists and technical glass scientists and further research into these and other frontiers of natural glass science. Keywords: Natural glass, Obsidian, Tektites, High-pressure studies, Bio-corrosion, Crystallization, Libyan Desert glass, Shock metamorphism, Volcanic glass, Glass corrosion