Spherulites (brown) grown on plagioclase crystals in a obsidian. Spherulites (brown) grown on plagioclase crystal in a obsidian. XPL image, 10x (Field of view = 2mm)Ĭrystallites in the groundmass. Magmatic flow (due to the alignment of small crystallites) in a obsidian. PPL image, 2x (Field of view = 7mm)Ĭoncentric cracks (Perlite) in a obsidian. (2008): Principles of Metamorphic Petrology. Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations. Magmas and magmatic rocks: an introduction to igneous petrology. A classification and glossary of terms, 2. J., Bonin, B., Bateman, P., & Lameyre, J. W., Streckeisen, A., Zanettin, B., Le Bas, M. An introduction to the rock-forming minerals (p.
(1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London. Magmatic flow in a obsidian, Big Obsidian Flow, Newberry, Oregon, USA. Obsidian lava flow, Big Obsidian Flow, Newberry caldera, Oregon, USA. Spherulitic obsidian in obsidian clast breccia, Firehole Canyon roadcut, Yellowstone, Wyoming, USA. Partially devitrified rhyolitic obsidian, Obsidian Cliff, Yellowstone, Wyoming, USA. Obsidian Cliff, Yellowstone, Wyoming, USA. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian) or an iridescent, rainbow-like sheen (rainbow obsidian).Īphyric rhyolitic obsidian. It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). Iron and magnesium typically give the obsidian a dark brown to black color. Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities.
This breakdown of obsidian is accelerated by the presence of water. The intersections of conchoidal fracture surfaces can be sharper than a razor.īecause obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic "conchoidal" fracture. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. Because of its conchoidal fracture (smooth curved surfaces and sharp edges), the sharpest stone artifacts were fashioned from obsidian. Obsidian was used by Native Americans and many other primitive peoples for weapons, implements, tools, and ornaments and by the ancient Aztec and ancient Greek civilizations for mirrors. Other types with dark bands or mottling in gray, green, or yellow are also known. Though obsidian is typically jet-black in colour, the presence of hematite (iron oxide) produces red and brown varieties, and the inclusion of tiny gas bubbles may create a golden sheen. Obsidian has a glassy lustre and is slightly harder than window glass. Obsidian is extremely rich in silica (about 65 to 80 %), is low in water, and has a chemical composition similar to rhyolite. Obsidian is a igneous rock occurring as a natural glass formed by the rapid cooling of viscous lava from volcanoes.
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