Glossary of Rock and Mineral Terms - R

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radiating

Aggregate composed of tiny, slender crystals compacted together radiating from a central point. The radiation can be flat or three dimensional. If three dimensional, this aggregate commonly occurs with circular, ball-like masses, and is known as spherulitic

radiation

The emission of alpha, beta, or gamma rays.

radical

Compounds that act as a single atom when combining with other elements to form minerals. Radicals contain one or more unpaired electrons.

radioactive

1) Term given to describe substances that contain atoms whose nuclei are are unstable, which causes the substance to slowly decay and emit radiation. Radioactive minerals usually take an extremely long time to decay if kept in proper settings.
2) (verb) - Emitting alpha, beta, and gamma rays.

radioactivity

The effect of radioactive material, emitting alpha, beta, and gamma rays. Also used to describe in present tense the nature of radioactive material.

rare earth (elements)

1) Group of elements, very similar in chemical properties, once thought to be extremely rare. They take up numbers 57 through 71 on the periodical table. Also known as the lanthanides.1) The rare earth elements.
 2) Oxides of the rare earth elements.

rain shadow effect

The result of the process by which moist air on the windward side of a mountain rises and cools, causing precipitation and leaving the leeward side of the mountain dry.

recrystallization

The diagenetic process by which unstable minerals in buried sediment are transformed into stable ones.

reflection

The event occurring when light hits a material and bounces off of it. There are different degrees of reflection, the strongest being light rays bouncing off a smooth, flat, polished surface, such as a mirror, where they bounce off and form a reversed image on the surface. Polished Hematite is a great example describing a reflective mineral, where in some cases it produces a mirror-like reflection.

refract

Bend.

refraction

The bending of white light upon entering a new dimension, such as from air to water or from air to a crystal, and splitting the white light into the colors of the spectrum.

refractive index

The amount of refraction that takes place in a particular substance, which is a direct connection to the speed of light in that substance. The higher the refractive index, the greater the amount of dispersion, which increase the brilliance of a material. The refractive indices of gems are measured with a refractometer. Gems can be identified by measuring their refractive indices(plural)

refractometer

Instrument that measures the refraction of light when it enters through a different dimension, such as from air to water or from air into a crystal.

regional metamorphism

Metamorphism that affects rocks over vast geographic areas stretching for thousands of square kilometers.

relative dating

The fixing of a geologic structure or event in a chronological sequence relative to other geologic structures or events. See also numerical dating.

relative density

Synonym of specific gravity

reniform

Aggregate describing smooth, rounded, kidney-like agglomerations. Rounded agglomerations of reniform aggregates are larger than botryoidal agglomerations but smaller than mammilary agglomerations.

repeated twinning

Form of twinning where two or more crystals form a repeated pattern. Examples: cassiterite twin, rutile twin, plagioclase twin, and fishtail twin.

replace/replacement

Having some or all the atoms being exchanged for atoms of a similar element. The process of one mineral taking the place of another mineral or material, with one or two atoms per molecule in the structure being exchanged with a different one with similar characteristics, thus creating a new mineral that retains the shape of the first mineral. (See Pseudomorph for more information.) Also refers to one element of a mineral to partially or fully substitute itself to another element.

reserve

A known resource that can be exploited for profit with available technology under existing political and economic conditions.

reservoir rock

A permeable rock containing oil or gas.

resinous luster

Luster describing yellow, dark orange, or brown minerals with slightly high refractive indices -- honey like, but not necessarily the same color.

resistance

The unyielding of material to destructive acids and tough environments

resource

A mineral or fuel deposit, known or not yet discovered, that may be or become available for human exploitation.

reticulated

Aggregate composed of long crystals in a netting-like form, where all the crystals crisscross each other.

reverse fault

A dip-slip fault marked by a hanging wall that has moved upward relative to the footwall. Reverse faults are often caused by the convergence of lithospheric plates.

rhombic dodecahedron

See dodecahedron

rhombohedral clevage

Type of cleavage exhibited on minerals that crystallize in the hexagonal system and as rhombohedrons, in which small rhombohedrons cleave off of the existing rhombohedron. Example is Calcite.

rhombohedron

Six sided polyhedron with each side shaped as a rhombus.

rhomb-scalenohedron

Mixture of a rhombohedron and a scalenohedron.

rhyolite

Any of a group of felsic igneous rocks that are the extrusive equivalents of granite.

Richter scale

A logarithmic scale that measures the amount of energy released during an earthquake on the basis of the amplitude of the highest peak recorded on a seismogram. Each unit increase in the Richter scale represents a 10-fold increase in the amplitude recorded on the seismogram and a 30-fold increase in energy released by the earthquake. Theoretically the Richter scale has no upper limit, but the yield point of the Earth's rocks imposes an effective limit between 9.0 and 9.5.

rift

Crack in a rock. On a large scale it refers to the separation of plates on fault areas, forming a depression in the earth.

rifting

The tearing apart of a plate to form a depression in the Earth's crust and often eventually separating the plate into two or more smaller plates.

ring silicates

Same as cyclosilicates - Group of silicate minerals that have their tetrahedrons linked into rings. Each silicon atom is bound by two oxygen atoms that are part of another tetrahedron. Each ring consists of three, four, or six linked tetrahedrons.

rip current

A strong, rapid, and brief current that flows out to sea, moving perpendicular to the shoreline.

ripple marks

A pattern of wavy lines formed along the top of a bed by wind, water currents, or waves.

riprap

A pile of large, angular boulders built seaward of the shoreline to prevent erosion by waves or currents. See also seawall.

rock

A naturally formed aggregate of an indefinite mixture of naturally occurring substances, mainly minerals. Its composition may vary in containment of minerals and organic substances, and are never exact.

rock crystal

Transparent, colorless, crystal of Quartz.

rock cycle

A series of events through which a rock changes, over time, between igneous, sedimentary, and metamorphic forms.

rock-forming mineral

One of the 20 or so minerals contained in the rock that composes the Earth's crust and mantle.

rock shop

Commercial store where minerals and rough gemstones can be purchased.

rock show

Periodical show in a large, usually indoor arena where mineral and gem dealers come to sell their stock at wholesale prices.

rosette

Mineral with concentric aggregates resembling rose flowers.

rough

Without any crystal faces. In regard to gemstones it refers to unfaceted material.

rubidium-strontium dating

A form of radiometric dating that relies on the 47-billion-year half-life of radioactive isotopes of rubidium, which decay into isotopes of strontium, to determine the age of rocks in which strontium is present. Rubidium-strontium dating is used for rocks that are at least 10 million years old, deep-Earth plutonic rocks, and Moon rocks.

rust

Powdery or crusty, brownish-yellow coating of iron oxides commonly see on iron materials after they come in contact with water.

rusting

Developing a coat of rust on the surface, or containing a rust coating from a previous rusting.

rutilated

Containing the needle like mineral Rutile as impurities.

rutile twin

Two or more crystals that twin in a repeated pattern ("repeated twinning") as depicted in the figure below. Named after the mineral Rutile, which most frequently exhibits this form of twinning. If there are six repeated crystals (i.e. another three crystals are added to the figure below in the same pattern), the agglomerate forms a circular structure, known as a sixling.