Obsidian: Dark volcanic glass of felsic composition.

Octahedral coordination: The packing of six ions around an ion of opposite charge to form an octahedron.

Oil field: An underground accumulation of oil and gas concentrated beneath an impermeable trap, preventing its escape upward.

Oil shale: A dark-colored shale containing organic material that can be crushed and heated to liberate gaseous hydrocarbons.

Oolite: A sedimentary carbonate particle composed of spherical grains precipitated from warm ocean water on carbonate platforms. Also a rock composed of such particles.

Opaque mineral: A mineral which transmits no light through a thin section under a microscope. Usually a native metal, sulfide, or metallic oxide mineral.

Ophiolite suite: An assemblage of mafic and ultra-mafic igneous rocks with deep-sea sediments supposedly associated with divergence zones and the sea-floor environment.

Orbit: The elliptical or hyperbolic path traced by a planet or meteorite or satellite in the presence of a more massive body.

Ore: A natural deposit in which a valuable metallic element occurs in high enough concentration to make mining economically feasible.

Ore mineral: The mineral of an ore that contains the useful element.

Orogenic belt: A linear region, often a former geo-syncline, that has been subjected to folding, and other deformation in a mountain-building episode.

Orogeny: The tectonic process in which large areas are folded, thrust-faulted, metamorphosed, and subjected to plutonism. The cycle ends with uplift and the formation of mountains.

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Outgassing: The release of juvenile gases to the atmosphere and oceans by volcanism.

Outwash: A glaciofluvial sediment that is deposited by meltwater streams emanating from a glacier. Overturned fold: A fold in which a limb has tilted past vertical so that the older strata are uppermost. Oxbow lake: A long, broad, crescent-shaped lake formed when a stream abandons a meander and takes a new course.

Oxidation: A chemical reaction in which electrons are lost from an atom and its charge becomes more positive.

Oxidized element: An element occurring in the more positively charged of two common ionic forms.