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Geology glossary

Updated: May 16, 2022 00:05 GMT - Refresh


Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Black volcanic glass
Obsidian is a naturally occurring glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of cr... Read all


Olivine from basalt lava on Hawai'i (image: By <a rel="nofollow" href=" "target="_blank">incidencematrix</a> - <a rel="nofollow" href="" target="_blank">Basalt with Olivine</a>, <a href="" title="Creative Commons Attribution 2.0">CC BY 2.0</a>, <a href="" target="_blank">Link</a>)
Olivine from basalt lava on Hawai'i (image: By incidencematrix - Basalt with Olivine, CC BY 2.0, Link)
Olivine is one of the earth's most common rock-forming silicate minerals, typically contained in mafic to ultrafafic magmatic rocks with low silica content (e.g. basalt, gabbro, peridotite), which make up much of the earth's upper mantle, the oceanic crust and lava flows etc from basaltic shield volcanoes on land.
Its name refers to its often olive-green color.
Formula(Mg2+, Fe2+)2SiO4
Crystal systemorthorhombic
ColorYellowish-green to olive green
Crystal habitMassive to granular
Mohs hardness6.5–7
Specific weight3.2-4.5
Chemically, it combines two interchangeable ions of magnesium (Mg2+) or iron (Fe2+) with one silica ion (Si4+) in a densely-packed crystal lattice. The two end-members from this compositional range are forsterite (magnesium-olivine, Mg2SiO4) and and fayalite (iron-olivine, Fe2SiO4). In rare cases, Mg/Fe can also be replaced by calcium (Ca), manganese (Mn), or nickel (Ni). Read all
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