Glossary of Rock and Mineral Terms - F

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face

1) An individual flat section of a faceted gem.
2) Crystal face.

facet

A desired surface displayed in a gem. It may grow naturally but is usually hand cut. This definition includes the meaning of a specific cuts for gems.

faceting

Cutting from a rough stone into a gem, creating a facet

fall

The fastest form of mass movement, occurring when rock or sediment breaks off from a steep or vertical slope and descends at a rate of 9.8 meters per second. A fall can be extremely dangerous.

fault

A fracture dividing a rock into two sections that have visibly moved relative to each other.

fault block

A section of rock separated from other rock by one or more faults.

fault-block mountain

A mountain containing tall horsts interspersed with much lower grabens and bounded on at least one side by a high-angle normal fault.

fault-zone metamorphism

The metamorphism that acts on rocks grinding past one another along a fault and is caused by directed pressure and frictional heat.

feldspar (group)

Group of minerals that are aluminum silicates containing potassium, sodium, and/or calcium. Some minerals in this group are wrongly classified as other minerals in this group. This is the most abundant group of minerals on the earth, and the building block of many rocks. The feldspar group is in the tectosilicates subdivision of the silicates group.

feldspathoid (group)

Group of minerals that are very similar to the feldspars. The difference is that the feldspathoids lacked the amount of silica to fully make them into feldspars when they were formed. They, as feldspars, are aluminum silicates of potassium, sodium, and/or calcium. The feldspathoid group is in the tectosilicates subdivision of the silicates group.

felsic

Consisting mostly of silica (more than 65 percent), in the form of quartz and feldspar. Can also be applied in reference only to the feldspars.

ferromagnetic

Greatly attracted to magnetic fields. Ferromagnetism is the magnetic property in certain materials that causes them to be attracted to magnetic fields.

ferrous

Containing iron.

ferruginous

1) Containing iron.
2) Appearing like iron.
3) Having a dull reddish-brown color similar to rust.

fiber

Individual segment or crystal of a fibrous aggregate.

fibrous

Aggregate describing a mineral constructed of fine, usually parallel threads. Some fibrous minerals contain cloth-like flexibility, meaning they can be bent around and feel like cotton.

filiform

Aggregate composed of thin, intergrown, twisted, pipelike strands. (i.e. flos-ferri)

firn

Firmly packed snow that has survived a summer melting season. Firn has a density of about 0.4 gram per cubic centimeter. Ultimately, firn turns into glacial ice.

fishtail twin

Form of repeated twinning in which two monoclinic crystals join as on the side, one reflecting the other, forming a fishtail or flower-like model. See also swallowtail twin.

fissile

Easily split along parallel layers.

fissility

The property exhibited on fissile rocks or minerals that is responsible for the splitting along parallel layers. Fissility exhibited on minerals is either cleavage or parting.

fission tracks

Marks left in the latticework of a mineral crystal by subatomic particles released during the fission of a radioactive atom trapped inside the crystal.

fjord

A deep, steep-walled, U-shaped valley formed by erosion by a glacier and submerged with seawater.

flakey

Aggregate containing small, flat, almost bendable flakes.

flame test

Complex, scientific test which is conducted to identify a mineral. A small fragment of a mineral is placed on the end of a platinum wire and held in a flame. Different metals present in the mineral change the color of the flame (such as sodium, yellow ; copper, blue and green ; potassium, violet).

flaw

Inclusion or crack in a gemstone that usually demotes its value.

flexible / flexibility

Minerals that are capable of being bent without breaking are said to be flexible. If a flexible mineral will return to its original position after stress is released, it is said to be flexible and elastic. If it does not return to its original position, but retains the position it was given after being bent, it is said to be flexible but inelastic. If just the term flexible is used, it usually refers to flexible but inelastic. Flexible minerals are many times capable of be molded, shaped or stretched. Flexibility is a form of tenacity.

floating crystal

Crystal that developed without being attached to rock, such as being grown artificially in a lab or having been grown naturally in clay or water. All the sides of floating crystals are intact.

floodplain

The flat land that surrounds a stream and becomes submerged when the stream overflows its banks.

flood tide

A tide that raises the water surface of an ocean and moves the shoreline farther inland.

flow

A relatively rapid mass-movement process that involves a mixture of rock, soil, vegetation, and water moving downslope as a viscous fluid. Within a flow (such as a mudflow), each particle, regardless of its size, moves independently.

flowstone

Calcium carbonate formed by mineral-rich water that deposits the dissolved mineral on the walls of caverns and cliffs, forming a smooth and humpy growth. As long as there is water flowing down, the layer of flowstone accumulates.

flow banding

Layered grouping of different rocks formed from the flowing of lava.

fluorescence

Emission of visible light by a substance, such as a mineral, that is currently exposed to ultraviolet light and absorbs radiation from it. The light appears in the form of glowing, distinctive colors. The emission ends when the exposure to ultraviolet light ends. Minerals describes as fluorescent have the ability to fluoresce.

fluorescent lamp

Lamp that gives off ultraviolet light while blocking out white light, which causes fluorescent minerals to react.

fluorite twin / fluorite penetration twin

Form of penetration twinning where two cubic crystals form interpenetrating twins, as depicted in the figure below. Named after the mineral Fluorite, which most frequently exhibits this form of twinning.

fluvial

Said of eroded material that gets transported by rivers or streams. Synonym of alluvial.

flux

Any liquid added to another liquid to improve flow, usually to prevent the formation of oxides.

focus (plural foci)

The precise point within the Earth's crust or mantle where rocks begin to rupture or move in an earthquake.

fold

A bend that develops in an initially horizontal layer of rock, usually caused by plastic deformation. Folds occur most frequently in sedimentary rocks.

foliation

The arrangement of a set of minerals in parallel, sheet-like layers that lie perpendicular to the flattened plane of a rock. Occurs in metamorphic rocks on which directed pressure has been exerted.

forearc basin

A depression in the sea floor located between an accretionary wedge and a volcanic arc in a subduction zone, and lined with trapped sediment. See also backarc basin.

foreshock

A minor, barely detectable earthquake, generally preceding a full-scale earthquake with approximately the same focus. Major quakes may follow a cluster of foreshocks by as little as a few seconds or as much as several weeks.

foreshore

The portion of a beach that lies nearest to the sea, extending from the low-tide line to the high-tide line.

form

The setting of all the crystal faces and the structure of a mineral.

fossil

Animal or plant remains of a previous age embedded and preserved in rock. Rocks containing fossils are described as fossiliferous.

fossilize

To form into a fossil.

fossilized

Having been transformed into a fossil.

fossilization

The process of an organic substance being transformed into a fossil.

fossil fuel

A nonrenewable energy source, such as oil, gas, or coal, that derives from the organic remains of past life. Fossil fuels consist primarily of hydrocarbons.

fourling

Twinned intergrowth of four individual crystals

fractional crystallization

The process by which a magma produces crystals that then separate from the original magma, so that the chemical composition of the magma changes with each generation of crystals, producing igneous rocks of different compositions. The silica content of the magma becomes proportionately higher after each crystallization.

fracture

1. (n) A crack or break in a rock.
2. (v) To break in random places instead of cleaving.
3. The characteristic way a mineral breaks when put under stress, aside from cleavage. Said of minerals.

framework silicates

Synonym of tectosilicates.

freezing point

The temperature it takes a substance to transform from liquid to solid. Every substance has a different freezing point.

friable

Describes a mineral which easily crumbles. Minerals with an earthy fracture are friable.

fringing reef

A reef that forms against or near an island or continental coast and grows seaward, sloping sharply toward the sea floor. Fringing reefs usually range from 0.5 to 1.0 or more kilometers in width.

frost wedging

A form of mechanical weathering caused by the freezing of water that has entered a pore or crack in a rock. The water expands as it freezes, widening the cracks or pores and often loosening or dislodging rock fragments. As the ice forms, it attracts more water, increasing the effects of frost wedging.

fumarole

Volcanic vent where smoke and gases escape from.

fusion

The process of being melted by heat.