Glossary of Rock and Mineral Terms - D

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daughter isotope

An isotope that forms from the radioactive decay of a parent isotope. A daughter isotope may or may not be of the same element as its parent. If the daughter isotope is radioactive, it will eventually become the parent isotope of a new daughter isotope. The last daughter isotope to form from this process will be stable and nonradioactive.

debris flow

1. The rapid, downward mass movement of particles coarser than sand, often including boulders one meter or more in diameter, at a rate ranging from 2 to 40 kilometers per hour. Debris flows occur along fairly steep slopes. 2. The material that descends in such a flow. deflation The process by which wind erodes bedrock by picking up and transporting loose rock particles.

decomposition

The molecular breakdown of certain minerals which cause a mineral to disintegrate.

decrepitation

An explosive shattering of minerals, usually through tube tests or blowpipe tests.

degradation

The process by which a stream's gradient becomes less steep, due to the erosion of sediment from the stream bed. Such erosion generally follows a sharp reduction in the amount of sediment entering the stream.

dehydration

The removal of water from a substance. Many minerals naturally lose water in their structure at normal conditions, and the mineral transforms into another mineral.

delta

An alluvial fan having its apex at the mouth of a stream.

dendritic

Aggregate composed of skeletal or tree-like formations. May be a single entity, or a formation that forms from mineral-rich solutions that deposit the mineral in rock and form a tree or plant structure embedded in rock. There sometimes is a distinction noted between the two aggregates; in some guides the former aggregate (single entity) is known as skeletal, and the latter as dendritic (embedded in rock). Other guides, don't distinguish the two, and term them both as dendritic. A dendrite is a mineral with a dendritic shape.

dendrochronology

A method of numerical dating that uses the number of tree rings found in a cross section of a tree trunk or branch to determine the age of the tree.

deposit

An accumulation of certain minerals within a rock formation.

desert

A region with an average annual rainfall of 10 inches or less and sparse vegetation, typically having thin, dry, and crumbly soil. A desert has an aridity index greater than 4.0.

desertification

The process through which a desert takes over a formerly non-desert area. When a region begins to undergo desertification, the new conditions typically include a significantly lowered water table, a reduced supply of surface water, increased salinity in natural waters and soils, progressive destruction of native vegetation, and an accelerated rate of erosion.

desert pavement

A closely packed layer of rock fragments concentrated in a layer along the Earth's surface by the deflation of finer particles.

desert varnish

A thin, shiny red-brown or black layer, principally composed of iron manganese oxides, that coats the surfaces of many exposed desert rocks.

detrital sediment

Sediment that is composed of transported solid fragments of preexisting igneous, sedimentary, or metamorphic rocks.

detritus

Minerals or rock fragments that eroded and end up in a different region from natural causes, such as downstream currents.

diamagnetic

Repelled by magnetic fields. Diamagnetism is the property which causes a mineral to be repelled from magnetic fields.

diaphanous

Able to be seen through, being either transparent or translucent. 

Diaphaneity

The quality of of a substance to be seen through. In regard to minerals, it is variable with transparency.

divitrification

Changing over from a natural glass to a mineral with a crystalline structure. To devitrify is the process of a natural glass to lose its glassy nature and crystallize.

diagenesis

The set of processes that cause physical and chemical changes in sediment after it has been deposited and buried under another layer of sediment. Diagenesis may culminate in lithification.

dichroic / dichroism

Literally means "two colors". A mineral that exhibits one color when viewed from one angle but a different color when viewed from a different angle is said to display dichroism.

diffraction

The bending of light when it enters from one medium into another. For example, light bends as it travels from air into another substance, such as water. Diffraction also occurs when light enters from the air into a mineral, and the amount of diffraction varies among minerals.

dike

A discordant pluton that is substantially wider than it is thick. Dikes are often steeply inclined or nearly vertical. See also sill. dilatancy The expansion of a rock's volume caused by stress and deformation.

diorite

Any of a group of dark, phaneritic, intrusive rocks that are the plutonic equivalents of andesite.

dip

The angle formed by the inclined plane of a geological structure and the horizontal plane of the Earth's surface.

dip-slip fault

A fault in which two sections of rock have moved apart vertically, parallel to the dip of the fault plane.

dipyramid

In form with a plane dividing a crystal into two pyramids base to base. Synonym of bipyramid.

directed pressure

Force exerted on a rock along one plane, flattening the rock in that plane and lengthening it in the perpendicular plane.

disappearing stream

A surface stream that drains rapidly and completely into a sinkhole.

disilicates

Synonym of phyllosilicates.

dispersion

The splitting of white light into the colors of the spectrum. When white light enters a crystal, it splits up; some rays are reflected and some are absorbed, forming the mineral's color. Gemstones are cut in a fashion that the split-up light in the crystal fans out its colors and reflects from an exact face. This causes "fire" in a gem, which is best seen on colorless gems.

displaced terrane

A fault-bounded body of rock - sometimes thousands of square kilometers in area - that originated elsewhere geographically and has then moved, perhaps long distances, by plate motion.

dissolution

A form of chemical weathering in which water molecules, sometimes in combination with acid or another compound in the environment, attract and remove oppositely charged ions or ion groups from a mineral or rock.

dissolved load

A body of sediment carried by a stream in the form of ions that have dissolved in the water.

distributary

One of a network of small streams carrying water and sediment from a trunk stream into an ocean.

divergence

The process by which two lithospheric plates separated by rifting move farther apart, with soft mantle rock rising between them and forming new oceanic lithosphere. See also convergence.

dodecahedron

Twelve sided polyhedron; all sides are equidimensional and either rhombic or pentagonal. If the dodecahedron is composed of rhombs, it is known as a rhombic dodecahedron, or simply as a dodecahedron. If it is composed of pentagons, it is known as a pentagonal dodecahedron or pyritohedron. Minerals shaped as dodecahedrons belong to the isometric system. Minerals with this property are described as dodecahedral.

dolar

Flat, spherical disc of radiating crystals. Also a variety of Pyrite / Marcasite.

dolostone

A sedimentary rock composed primarily of dolomite, a mineral made up of calcium, magnesium, carbon, and oxygen. Dolostone is thought to form when magnesium ions replace some of the calcium ions in limestone, to which dolostone is similar in both appearance and chemical structure.

dome

A round or oval bulge on the Earth's surface, containing the oldest section of rock in its raised, central part. See also basin. drainage basin The area from which water flows into a stream. Also called a watershed.

double refraction

Phenomenon exhibited on all non-opaque minerals except for amorphous ones and ones that crystallize in the isometric system. A light ray enters the crystal and splits up into two separate rays, making anything observed through the crystal appear as double. The double refraction on most minerals is so weak that it cannot be observed without special instruments. However, in some minerals, such as the Iceland Spar variety of Calcite it is strongly seen. The double refraction is different in every mineral, and thus can be used to identify gems. Double refraction is measured with a refractometer

doublet

One of the many gem fakes in which a thin, flat section of a real gem is pasted atop a thick base of glass or rock crystal.

doubly terminated

Exhibiting a pinched crystal figure on both bases.

drainage divide

An area of raised, dry land separating two adjacent drainage basins.

drainage pattern

The arrangement in which a stream erodes the channels of its network of tributaries.

drumlin

A long, spoon-shaped hill that develops when pressure from an overriding glacier reshapes a moraine. Drumlins range in height from 5 to 50 meters and in length from 400 to 2000 meters. They slope down in the direction of the ice flow.

druse

Cavity in a mineral or rock filled with protruding crystals. The hole is either completely filled with crystals or just partially.

drusy

Aggregate composed of prismatic crystals protruding from a cavity or wall.

dry lake

Saline lake that evaporated or was drained. Dry lakes leave many evaporite minerals, including salts, borates, and nitrates.

dry lake deposit

Deposit containing an accumulation of evaporite minerals from the evaporation or drainage of a saline lake. As the water gets exhausted, the minerals it is rich in remain, increasing in content, and eventually all that is left is accumulation of the mineral that was once present in the water.

ductile

Capable of being stretched into a thin wire. A form of tenacity. Ductility is the capability of being able to stretched into a thin wire.

dull luster

The luster of minerals with very poor optical properties.

dump

Area where left over material is placed after being extracted from a mine.

dune

A usually asymmetrical mound or ridge of sand that has been transported and deposited by wind. Dunes form in both arid and humid climates.

dusting

Very thin coating of one mineral on another mineral; a very thin sprinkling.

dynamothermal metamorphism

A form of regional metamorphism that acts on rocks caught between two converging plates and is initially caused by directed pressure from the plates, which causes some of the rocks to rise and others to sink, sometimes by tens of kilometers. The rocks that fall then experience further dynamothermal metamorphism, this time caused by heat from the Earth's interior and lithostatic pressure from overlying rocks.