Fundamental Minerals

Sialic Minerals

Feldspar

Feldspathoid

Quartz

Femic Minerals

Accessory Minerals

Very Rare Minerals

Secondary Minerals

Texture

Alteration Products

Glass and related Texture

Volcanic Rocks

Komatiites

Lamprophyres

Obsidian

Obsidian is a igneous rock occurring as a natural glass formed by the rapid cooling of viscous lava from volcanoes. Obsidian is extremely rich in silica (about 65 to 80 %), is low in water, and has a chemical composition similar to rhyolite. Obsidian has a glassy lustre and is slightly harder than window glass. 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. Other types with dark bands or mottling in gray, green, or yellow are also known. 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. Because of its conchoidal fracture (smooth curved surfaces and sharp edges), the sharpest stone artifacts were fashioned from obsidian.
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. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic "conchoidal" fracture. This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass. The intersections of conchoidal fracture surfaces can be sharper than a razor.

Because 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 breakdown of obsidian is accelerated by the presence of water. Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color. 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). 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. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian) or an iridescent, rainbow-like sheen (rainbow obsidian).

obsidian2020(3).jpg

Aphyric rhyolitic obsidian. Obsidian Cliff, Yellowstone, Wyoming, USA. From James St. John

Partially devitrified rhyolitic obsidian, Obsidian Cliff, Yellowstone, Wyoming, USA. From James St. John

Spherulitic obsidian in obsidian clast breccia, Firehole Canyon roadcut, Yellowstone, Wyoming, USA. From James St. John

Obsidian lava flow, Big Obsidian Flow, Newberry caldera, Oregon, USA. From USGS

Magmatic flow in a obsidian, Big Obsidian Flow, Newberry, Oregon, USA. From All trails

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Magmatic flow (due to the alignment of small crystallites) in a obsidian. PPL image, 2x (Field of view = 7mm)	Concentric cracks (Perlite) in a obsidian. PPL image, 10x (Field of view = 2mm)	Concentric cracks (Perlite) in a obsidian. PPL image, 10x (Field of view = 2mm)
Concentric cracks (Perlite) in a obsidian. PPL image, 10x (Field of view = 2mm)	Concentric cracks (Perlite) in a obsidian. PPL image, 10x (Field of view = 2mm)	Concentric cracks (Perlite) in a obsidian. PPL image, 10x (Field of view = 2mm)
Magmatic flow (due to the alignment of small crystallites) in a obsidian. PPL image, 2x (Field of view = 7mm)	Magmatic flow (due to the alignment of small crystallites) in a obsidian. PPL image, 2x (Field of view = 7mm)	Magmatic flow (due to the alignment of small crystallites) in a obsidian. XPL image, 2x (Field of view = 7mm)
Magmatic flow (due to the alignment of small crystallites) in a obsidian. PPL image, 10x (Field of view = 2mm)	Magmatic flow (due to the alignment of small crystallites) in a obsidian. PPL image, 10x (Field of view = 2mm)	Magmatic flow (due to the alignment of small crystallites) in a obsidian. XPL image, 10x (Field of view = 2mm)
Crystallites in the groundmass. PPL image, 20x (Field of view = 2mm)	Crystallites in the groundmass. PPL image, 20x (Field of view = 2mm)	Crystallites in the groundmass. PPL image, 20x (Field of view = 2mm)
Spherulites (brown) grown on plagioclase crystals in a obsidian. PPL image, 2x (Field of view = 7mm)	Spherulites (brown) grown on plagioclase crystals in a obsidian. PPL image, 2x (Field of view = 7mm)	Spherulites (brown) grown on plagioclase crystal in a obsidian. PPL image, 10x (Field of view = 2mm)
Spherulites (brown) grown on plagioclase crystals in a obsidian. PPL image, 10x (Field of view = 2mm)	Spherulites (brown) grown on plagioclase crystals in a obsidian. PPL image, 10x (Field of view = 2mm)	Spherulites (brown) grown on plagioclase crystals in a obsidian. PPL image, 10x (Field of view = 2mm)
Spherulites (brown) grown on plagioclase crystal in a obsidian. PPL image, 10x (Field of view = 2mm)	Obsidian with brecciated structure (the various fragments consist of obsidian). PPL image, 2x (Field of view = 7mm)	Obsidian with brecciated structure (the various fragments consist of obsidian). PPL image, 2x (Field of view = 7mm)
Obsidian with brecciated structure (the various fragments consist of obsidian). PPL image, 2x (Field of view = 7mm)	Obsidian with brecciated structure (the various fragments consist of obsidian). PPL image, 10x (Field of view = 2mm)	Obsidian with brecciated structure (the various fragments consist of obsidian). PPL image, 10x (Field of view = 2mm)
Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)	Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)	Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)
Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)	Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)	Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)
Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)	Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)	Feldspar crystals within a glassy, fluidal groundmass. PPL image, 1x (Field of view = 9mm)
Feldspar crystals within a glassy, fluidal groundmass. PPL image, 2x (Field of view = 7mm)	Feldspar crystal within a glassy, fluidal groundmass. PPL image, 2x (Field of view = 7mm)	Feldspar crystal within a glassy, fluidal groundmass. XPL image, 2x (Field of view = 7mm)
Feldspar and pyroxene crystals within a glassy, fluidal groundmass. PPL image, 2x (Field of view = 7mm)	Feldspar and pyroxene crystals within a glassy, fluidal groundmass. PPL image, 2x (Field of view = 7mm)	Pyroxene crystal within a glassy, fluidal groundmass. PPL image, 2x (Field of view = 7mm)
Feldspar crystal within a glassy, fluidal groundmass. PPL image, 2x (Field of view = 7mm)	Feldspar crystal within a glassy, fluidal groundmass. PPL image, 2x (Field of view = 7mm)	Feldspar and pyroxene crystals within a glassy, fluidal groundmass. PPL image, 2x (Field of view = 7mm)
Feldspar crystal within a glassy, fluidal groundmass. PPL image, 10x (Field of view = 2mm)	Feldspar crystal within a glassy, fluidal groundmass. PPL image, 10x (Field of view = 2mm)	Feldspar crystal within a glassy, fluidal groundmass. PPL image, 10x (Field of view = 2mm)