Glossary of Rock and Mineral Terms - O

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oblique slip

Fault motion that involves both dip-slip and strike-slip movement of fault blocks.

occurrence

The area where a particular mineral is found.

ocean trench

A deep, linear, relatively narrow depression in the sea floor, formed by the subduction of oceanic plates.

ochre

Amorphous, yellow to red substance composed of iron compounds such as Hematite and Limonite.

octacube

Three dimensional polyhedron that is a combination of a cube and octahedron.

octahedral cleavage

Type of cleavage exhibited on minerals of the isometric system that crystallize as octahedrons. The method of cleavage is that flat, triangular "wedges" peel off of the existing octahedron. Example is Fluorite.

octahedron

Eight sided polyhedron; all sides are equidimensional and bisect at at the same angle. Minerals shaped as octahedrons belong to the isometric system.

octet

A scientific law stating that all atoms, except those of hydrogen and helium, require eight electrons in the outermost energy level to maintain chemical stability.

oil of vitriol

Synonym of sulfuric acid.

oil sand

A mixture of unconsolidated sand and clay that contains a semi-solid bitumen.

oil shale

A brown or black clastic source rock containing kerogen.

oily luster

Synonym of greasy luster: Luster of a mineral that appears coated with grease. Some minerals are coated with chemicals to induce a greasy luster.

oolitic

Aggregate composed of very small, spherical particles.

opalescence

Effect seen in a few minerals, chiefly opal (hence its name) which cause it to exhibit a glimmer of different colors when rotated or seen in different angles. Opalescent describes mineral exhibiting this effect.

opalization

The common habit of Opal that it replaces material, such as wood, shells, and other minerals.

opalize

To be chemically altered to Opal.

opaque

Not able to transmit light, which in effect disables it from letting an object be seen through it.

optical properties

Physical properties of a mineral or gem that have to do with optics, such as dispersion, absorption spectra, refractive index, asterism, and dichroism, just to name a few.

optics

Branch of physics that deals with light and the electromagnetic radiation; certain areas are dispersion, absorption spectra, reflection, and refraction.

ophiolite suite

The group of sediments, sedimentary rocks, and mafic and ultramafic igneous rocks that make up the oceanic lithosphere.

ore

A mineral deposit that can be mined for a profit. Material which has a valuable constituent, usually a precious metal, that gives it value and makes it profitable for extraction. A fine example is Calaverite, which is extracted because of the gold it contains. "Ore" also refers to the precious material that was extracted from the ore.

organic

Composed of carbon compounds; being from the source of living organisms.

orogenesis

Mountain formation, as caused by volcanism, subduction, plate divergence, folding, or the movement of fault blocks. Also called orogeny.

orthorhombic crystal system

Any mineral that falls under the following specifications belongs to the orthorhombic crystal system: Three axes, all are unequal in length. All three axes are at 90° to each other.

orthosilicates

Synonym of nesosilicates: Group of silicate minerals that contain only single, non-combining groups of tetrahedrons.

oscillatory motion

The circular movement of water up and down, with little or no change in position, as a wave passes.

outcrop

Bedrock revealed at the surface of the earth.

outwash

A load of sediment, consisting of sand and gravel, that is deposited by meltwater in front of a glacier.

oxbow lake

A crescent-shaped body of standing water formed from a single loop that was cut off from a meandering stream, typically by a flood that allowed the stream to flow through its floodplain and bypass the loop.

oxidation

The process of combining with oxygen ions. A mineral that is exposed to air may undergo oxidation as a form of chemical weathering.

oxidation  minerals

Minerals that form after being altered from being exposed in the oxidation zone.

oxidation zone or oxidized zone

Area of a deposit where the rock is exposed to air and therefore is affected by wind, rain, pressure, and air, which chemically affects the minerals embedded in the rock and alters them to secondary minerals.

oxides

Group of minerals that are compounds of one or more metallic elements combined with oxygen, water, or hydroxyl (OH). The oxide group contains the greatest variations of physical properties. Some are hard, some soft. Some have a metallic luster, others are clear and transparent. The Oxide group is divided into the Simple Oxides, Hydroxides, Multiple Oxides

oxidize

Property exhibited in certain minerals that cause them to tarnish, or discolor upon contact with air. Also term used to describe the chemical alteration of one mineral into another mineral through oxidation.

oxidized

Having gone through a chemical change as a result of exposure to oxygen.