Obsidian
| Obsidian | |
|---|---|
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|
| General | |
| Category | Volcanic glass |
|
Formula (repeating unit) |
70–75% SiO2, plus MgO, Fe3O4 |
| Identification | |
| Color | Deep black or blackish green |
| Fracture | Conchoidal |
| Mohs scale hardness | 5–6 |
| Luster | Vitreous |
| Specific gravity | ~2.4 |
| Optical properties | Translucent |
| References | |
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.
It is produced when felsic lava extruded from a volcano cools rapidly with minimal 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. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which were used in the past in cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.
... among the various forms of glass we may reckon Obsian glass, a substance very similar to the stone found by Obsius in Ethiopia.
The translation into English of Natural History written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called Obsian, so named from its resemblance to a stone (obsiānus lapis) found in Ethiopia by Obsius, a Roman explorer.
Obsidian is the rock formed as a result of quickly cooled lava, which is the parent material.Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis.
Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite. 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. Having a low water content when newly formed, typically less than 1% water by weight, obsidian becomes progressively hydrated when exposed to groundwater, forming perlite.
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Wikipedia