Geology glossary
obsidian
- Volcanology
Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Detail of obsidian with spherolites (small bubbles where the glass has crystallized around the bubble)Black volcanic glassObsidian 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... -> See whole entry olivine
- Minerals
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.Name olivine Category Formula (Mg2+, Fe2+)2SiO4 Crystal system orthorhombic Color Yellowish-green to olive green Streak None Luster Vitreous Crystal habit Massive to granular Mohs hardness 6.5–7 Specific weight 3.2-4.5 Uses Other 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). -> See whole entry
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