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Obsidian

Obsidian
Obsidian found in the Oregon
General
Category	Volcanic rock
Empirical formula	70-75% SiO _2, MgO addition, Fe ₃ O ₄
Identification
Color	gray, dark green, red, black
Fracture	conchoidal
Mohs	7 to 7, 5
Brightness	glassy
Units of SI & STP unless otherwise stated.

The obsidian is a volcanic rock glass and rich in silica.
Gray, dark green, red or black, it comes from a washing acid (type rhyolite). Bulk vitrification is made possible by the high degree of polymerization of the lava (see External Links). This phenomenon has nothing to do with the chilled margins of a few millimeters to several centimeters observed basic lavas (dikes, pillows lavas) which vitrification is due to rapid cooling of lava (contact with rocks or with cold water).

Its name comes from Obsius, a character in ancient Rome who first pointed out the presence of this rock in Ethiopia.

Physical properties

Obsidian is transparent to translucent and has a texture and a glassy luster. Its fracture is significantly conchoidal , hardness Mohs high: 7 to 7.5 (it scratches the glass).

Obsidian "snowflake" with pebbles of recrystallizations

Recrystallisation

The glass of the obsidian can recrystallize to give the spherulites of feldspar. This gives obsidian "snowflake" with pebbles of recrystallization.

Field, age and varieties

Obsidian is formed from lava flows thick and very rich in silica.

Unusual rock, obsidian is found in abundance:

the Lipari Islands , one of the Aeolian Islands ( Italy ), in Sardinia ( Monte Arci )
near Landmannalaugar ( Iceland )
as well as Mexico , Peru , United States , Japan and Armenia ...

Obsidian is also very present around the volcanoes of Easter Island.

The obsidian is usually dated Pliocene (2 to 4 million years) is no earlier than the Triassic : indeed, this rock is devitrifies time.

The obsidian blue glass imitations are manufactured, the color does not exist naturally in the rock.

Using prehistoric

Arrowhead obsidian

Obsidian was used for the manufacture of edged sword for weapons and tools during the prehistory , particularly in pre-Columbian America. The pebbles used in the manufacture of jewelry and hardness of this mineral and its relative ease of size can make the blades.

There are also many traces of the use of obsidian in southern Europe in the Neolithic , where a form of trade and transportation of the stone was in place for deposits of volcanoes in Italy today. Obsidian tools have been found during excavation archaeological, particularly in the south of France. With the advent of the Neolithic in the Western Mediterranean 8000 years ago, exchange networks are being established between the different agro-pastoral communities. A recent study of prehistoric obsidian can lift a veil on how some of these were organized long-distance trade between the islands and shores of the Mediterranean: trade was partly in the hands of skilled craftsmen who moved by sea and distributing the proceeds of their craft communities visited. Archaeological sites attest to the shaping of cores around the deposits (mainly Lipari and Monte Arci in Sardinia ) and the detachment of blades and blades in the villages "consumers" ^Notes