Chemically, Obsidian closely resembles granite and rhyolite but the processes which lead to the formation of Obsidian are different to those which shape granite. Indeed, Obsidian is not a true mineral because it lacks crystal structure. Probably a better description, and that most often used for Obsidian, is a that it is a naturally occurring form of glass. While pure Obsidian is dark in appearance, the colour varies depending on the presence of impurities. For example, iron and magnesium gives obsidian a dark green to brown shade.
The Many Variations of Obsidian Rock
Obsidian is an extreme variety of igneous rock with a glassy texture. Most popular accounts say that obsidian forms when lava cools very quickly, but that is not quite accurate. Obsidian starts with lava very high in silica more than about 70 percent , such as rhyolite. The many strong chemical bonds between silicon and oxygen make such lava very viscous, but equally important is that the temperature range between fully liquid and fully solid is very small. Another factor is that low water content may inhibit crystallization.
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Chronometric Dating in Archaeology pp Cite as. A freshly-made surface of obsidian volcanic glass of rhyolitic composition will absorb water which slowly penetrates by diffusion into the body of the artifact. Although the depth of penetration can be measured by various methods, it is generally determined by microscopic examination on thin sections of the artifact cut normal to the surface. The rate of penetration of water is dependent upon several factors, primarily the chemical composition of the glass and the temperature at which the hydration occurred. Discussions are given of techniques for measuring the hydration thickness, measurement or estimates of ambient hydration temperature, chemical composition of the obsidian, and the conversion of hydration thickness to dating the time of manufacture of the artifact.
Crystalline rock types and soils collect energy from the radioactive decay of cosmic uranium, thorium, and potassium Electrons from these substances get trapped in the mineral's crystalline structure, and continuing exposure of the rocks to these elements over time leads to predictable increases in the number of electrons caught in the matrices. But when the rock is exposed to high enough levels of heat or light, that exposure causes vibrations in the mineral lattices and the trapped electrons are freed. Luminescence dating is a collective term for dating methods that encompass thermoluminescence TL and optically stimulated luminescence OSL dating techniques.