It is translucent in thin cleavage fragments and along sharp margins. It does not scratch glass, thus corresponding to the hardness of 5 or 6 in the Mohs scale. The obsidian exhibits a pronounced vitreous lustre. It does not contain any macroscopically discernible inclusions of gases, liquids or solids, such as minerals or rock fragments. Petrographic characteristics unaltered compact obsidian with black colour, pronounced conchoidal fracture and sharp edges. Japan, Kenya, Mexico, New Zealand, Peru, Russia, United States, and many other Significant deposits of obsidian are found in Argentina,Ĭanada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, It older than a few million years is rare because the glassy rock is rapidly destroyed or altered by weathering, heat, or other processes. It is confined to areas of geologically recent volcanic activity. Obsidian is found in many locations worldwide. Hence, obsidian cannot be classified under any crystal system.This glassy rock is metastable-over time, it becomes fine-grained, mineral crystals. As a result, the lava gets cooled and consolidated instantly without any crystallization. As silicate-rich magma extrudes from inside the earth’s crust, it falls over the water bodies or other surfaces that are in contact with water. It is commonly found in areas where the cooling of lava was rapid-such as at the margins of the lava flows and in places where the lava entered a lake or sea. Obsidian is produced from volcanoes when felsic lava cools and freezes without sufficient time for crystal growth. Rarely founded is a composition similar to basalt and gabbro.These name of galasy are tachylyte Obsidian Formation Its from various localities in the World differ partly in terms of their trace element contents. Weathering is accelerated in the presence of water. It is unstable in atmospheric conditions and it temporally crystallizes to very finely grained crystal aggregate. Al2O3 content is lower than common rhyolite and granite.As a rule, granite and rhyolite are peraluminous, with corundum in their norms, whereas the Lipari obsidian is metaluminous, devoid of the normative corundum. It is dominant SiO2, MgO and Fe2O3 is most commont component. Chemical Composition of ObsidianĬhemical composition of it is close to these rock rhyolite and granite. Another common black type is snowflake obsidian, having inclusions of small white crystals. Sheen obsidian has golden effects, while rainbow obsidian possesses characteristic rainbow luster. Volcanic glasses also differ in terms of their structure and texture.Īir bubbles that are imparted in the course of magma flow, are aligned in a regular pattern in some types. Not have their own classification, but they can be characterized using the TASĭiagram in terms of their chemical composition. Volcanic rocks containing more than 80 vol. Reason, the obsidian is sometimes regarded as a mineraloid. Structural ordering similar to crystals only in very short distances. Major minerals: Amorphous volcanic glass that exhibits the 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 obsidian (lapis obsidianus), discovered in Ethiopia by Obsidius It is ascribed to Theophrastus who used it for the first time in 320 BC. Name origin: The name obsidian is very old. It is fragile, tough and it disintegrates to very sharp slivers.
Diffusion in the very viscous acid and felsic lavas with highly polymerized SiO2 tetrahedrons is slow, thus inhibiting the nucleation of crystals and supporting the origin of the glassy structure. Obsidian most frequently originates by a rapid coooling of a felsic viscous lava. Those are combination of black and brown colours. Hand-specimen rock is common combination of two colours. Colour is controlled factor by trace element content and the character of inclusion. Also It occur red, brown-green, green, yellow and rare transparent colourless. It usually colours dark, black, grey-black to grey.
Result of this formed, glass occupies more than 80 % and that have seeming conchoidal fracture and vitreous luster.
It is occurring as a natural glass formed by the rapid cooling of viscous lava from volcanoes.