Appendix:Dictionary of Mining, Mineral, and Related Terms/I/2

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a. The ventilating passage in an underground mine through which fresh air is conducted via an adit, drill hole, or downcast shaft to the workings.

b. The passage by which the ventilating current enters a mine. See also: downcast. A term that is more appropriate for a shaft; intake for an adit or entry. c. Scot. Person who works underground at odd work. d. N. of Eng. Any roadway underground through which fresh air is conducted to the working face. e. The passage and/or the current of ventilating air moving toward the interior of a mine. f. The suction pipe or hose for a pump. g. In hydraulics, the point at which the water or other liquid is received into a pipe, channel, or pump. h. The headworks of a conduit; the place of diversion.

intake area

That part of the land surface where water passes downward on its way to the zone of saturation in one or more aquifers. See also: recharge.

integral pilot

A pilot-type noncoring bit having a pilot section that is an integral, nonreplaceable part of the bit.

integrated producer

A producer of metal who owns mines, smelters, and refineries, and sometimes also fabricating plants.

integrated train

A long string of cars, permanently coupled together, that shuttles continuously back and forth between one mine and one generating plant, not even stopping to load and unload, since rotary couplers permit each car to be flipped over and dumped as the train moves slowly across a trestle.

integrating meter

A meter that records the total quantity of liquid or electricity passing through it.


In petrology, the formation of larger crystals from smaller ones by recrystallization.


A circuit whose output is substantially proportional to the time integral of the input.

intense anomaly

An anomaly whose elemental values rise sharply to one or more well-defined peaks.


As applied to color, the comparative brightness (vividness) or dullness or brownishness of a color; its comparative possession or lack of brilliance; therefore, the variation of a hue on a vivid-to-dull scale. See also: tone.

intensity of magnetization

The magnetic moment per unit volume.

intensity of pressure

The pressure per unit area.

intensity of radiation

The energy per unit time entering a sphere of unit cross-sectional area centered at a given place. The unit of intensity is the erg per square centimeter second or the watt per square centimeter.

intensity scale

A standard of relative measurement of earthquake intensity. Four such systems are the Mercalli scale, the modified Mercalli scale, the Richter scale, and the Rossi-Forel scale.


Occurring between beds, or lying in a bed parallel to other beds of a different material. Syn: interstratified. CF: intercalated.


Material of any nature that lies between two or more bedded ore zones or coal seams. Term is primarily used in surface mining.


Said of layered material that exists or is introduced between layers of a different character; esp. said of relatively thin strata of one kind of material that alternates with thicker strata of some other kind, such as beds of shale intercalated in a body of sandstone. CF: interbedded.


a. The distance along a crystallographic axis to its intersection with a crystal face. This intercept is a rational number because the axial unit length of each mineral is selected to make it so. The ratio of these intersections of a face with each of the crystallographic axes constitutes a parameter, such as Miller indices, that defines the crystal face. CF: indices of a crystal face; crystallographic axis; Miller indices.

b. The part of the rod seen between the upper and lower stadia hairs of a transit or telescopic alidade; e.g., a stadia interval.

intercepting channel

A channel excavated at the top of earth cuts, or at the foot of slopes, or at other critical places to intercept surface flow; a catch drain.

intercepting drain

A drain that intercepts and diverts water before it reaches the area to be protected. Also called curtain drain.


point where the hole first encounters a specific rock or mineral body and where the hole enters a different or underlying rock formation.

b. In crystallography, the distances cut off on axes of reference by planes.

intercept time

See: delay time.


a. A radiator in which air is cooled while moving from low-pressure to high-pressure cylinders of a two-stage compressor. See also: two-stage compression.

b. In multistage compression of air, a cooling arrangement between stages. See also: aftercooler. c. A cooling device used on a turbocharged diesel engine to reduce the air volume between the turbocharger and the cylinders.


Extraction of heat from a compressed gas between two stages of compression in order to improve the efficiency of compression.

interfacial angle

The internal or dihedral angle between two faces of a crystal. It is also the "angle of dip" between faces.

interfacial energy

Tension at interfaces between the various phases of a system; may include solid, liquid, and gas interfaces, varying in their combinations and qualities.

interfacial tension

The contractile force of an interface between two phases.


The meeting of two wave systems resulting in increased amplitude (constructive interference) if they are in phase, i.e., crest to crest, and decreased amplitude (destructive interference) if they are out of phase, i.e., crest to trough. In polarized-light microscopy (PLM), phase differences are generated when white light passes through an anisotropic (i.e., doubly refracting) crystal or crystal fragment, these differences being determined primarily by birefringence, light wavelength, and crystal thickness. Waves of light in different parts of the visible spectrum interfere both constructively and destructively when resolved in the microscope analyzer to give an interference color. Since anisotropic minerals have a range of birefringence, interference colors are useful as an aid to their identification. See also: optic sign; index of refraction.

interference color

One of the spectral colors produced by the strengthening or the weakening of certain wavelengths of a composite beam of light in consequence of interference. This is an important characteristic in determining minerals in thin section or in fragments under the polarizing microscope.

interference figure

a. An optical figure composed of a series of spectrally colored rings combined with a blank cross (if uniaxial) or a series of spectrally colored curves or rings with two black parabolic curves called isogyres (if biaxial). The figure is observed when a properly oriented thin section or fragment of a mineral is examined in convergent light through the polarizing microscope. The interference figure, which is caused by the birefringence of the mineral and by the orientation of the mineral so that it presents an optic axis in the field of the microscope, is one of the most valuable optical aids in identifying minerals. Also called the direction image.

b. An optical pattern produced by conoscopic illumination of anisotropic crystals which appears on a spherical focal surface located above the objective lens of a polarized-light microscope. CF: melatope.

interference methanometer

A combustible-gas detector based on the velocity of light. A beam of light is split into two parts that pass respectively through chambers containing pure air and the test air at velocities characteristic of the gases. When methane is present, the light beams are out of step and this movement becomes a measure of the methane concentration. See also: refractometer.


The area between two rivers flowing in the same general direction. Syn: interstream area.


Between formations, such as an interformational unconformity. CF: intraformational.

intergranular corrosion

Corrosion that occurs preferentially at grain boundaries of a metal or alloy.


a. Of coal and mineral matter, naturally associated and separable only by crushing or grinding.

b. In crystallography, a descriptive term for mineral species that have crystallized simultaneously and therefore become intertwined or interlocked.


The state of interlocking of grains of two different minerals as a result of their simultaneous crystallization. CF: graphic granite.

interior angle

Horizontal angle between adjacent sides of a polygon, measured within the polygon.

interior coalfields

U.S. Includes Eastern Interior Field, Illinois, Indiana, and Kentucky; Western Interior Field, Great Plains States from Iowa to Arkansas; Southwestern Field, Texas; and Northern Field, Michigan.

interior span

A continuous beam or slab, both supports of which are continuous with adjacent spans.


a. Intercalated in very thin layers.

b. A term used in remote sensing to describe a technique of storing digital image data (pixel interleaved, line interleaved, or band interleaved.)


The clutch in steel sheet piles.

interlocking controls

A system of electrical controls for a system of conveyors that maintains a controlled relationship between the units of the system. Sometimes applied to sequence-starting controls.

interlocking wedge-type capping

This type of wire-rope capping is simple to apply. The sleeves are first threaded on the rope, a white metal bob is then formed on the end of the rope by untwisting the wires and cutting out the hemp core, if present, and white metal is run into a mold around the wires. The bob is allowed to cool. Two tapered interlocking steel wedges are then fitted on to the rope clear of the bob so that wedges can move forward toward the bob and grip the rope as the load is applied to the capping. The rope is cleaned of lubricant where the wedges will grip, and the groove in the wedges must be of such a size that a gap is left between the wedges so that they can grip the rope firmly. The edges of the wedges should come opposite a valley between the strands of the rope. The outer socket is now placed over the wedges and the sleeves are lightly tapped into position to hold the parts together.


a. A secondary or auxiliary horizontal passage driven between levels in a mine, which may extend from a raise or stope and, depending upon its orientation, may be either an intermediate drift or a crosscut. Syn: sublevel.

b. Said of an igneous rock that is transitional between basic and silicic (or between mafic and felsic), generally having a silica content of 54% to 65%; e.g., syenite and diorite. "Intermediate" is one subdivision of a widely used system for classifying igneous rocks on the basis of their silica content; the other subdivisions are acidic, basic, and ultrabasic. Syn: mediosilicic.

intermediate constituent

A constituent of alloys that is formed when atoms of two metals combine in certain proportions to form crystals with a different structure from that of either of the metals. The proportions of the two kinds of atoms may be indicated by formula, e.g., CuZn; hence, these constituents are also known as intermetallic compounds.

intermediate cooler

In a blast furnace, a water-cooled casting, usually of copper, that is installed inside the cinder cooler.

intermediate cut

See: middle cut.

intermediate-duty fireclay brick

A fireclay refractory having a pyrometric cone equivalent not lower than No. 29, or having a refractoriness of not more than 3.0% deformation as measured by the load test at 2,460 degrees F (1,349 degrees C) (ASTM requirements).

intermediate electrode

See: bipolar electrode.

intermediate gate

A gate between the central gate and the end gates, particularly in double-double unit layouts.

intermediate haulage

a. The transportation of mined coal or ore from the face haulage to that point where it is accessible to the main line. It is accomplished by conveyors, belts, or locomotives and mine cars.

b. Mine haulage used to collect loads and deliver empties from and to the sections. These are taken to and from central sidetracks served by the main line motor. Locomotives and track are frequently lighter than those on the main line. See also: haulage.

intermediate haulage conveyor

Generally 500 to 3,000 ft (152 to 915 m) in length. It is used to transport material between the gathering conveyor and the main haulage conveyor.

intermediate layer

See: sima.

intermediate loading station

See: loading station.

intermediate microcline

See: mesomicrocline.

intermediate packs

Packs built between gates with wastes on each side and usually supported by packwalls.

intermediate principal plane

The plane normal to the direction of the intermediate principal stress.

intermediate principal stress

The principal stress whose value is neither the largest nor the smallest (with regard to sign) of the three.

intermediate section

The part of a mining belt conveyor that consists of the framing and the belt idlers supported by the framing, both of which guide and support the belts between the head end and the tail end. There are two general types of intermediate sections: rigid side framed and wire-rope side framed.

intermediate shaft

A shaft that is driven by one shaft and drives another.

intermediate transfer point

The point along a conveyor, which may already be carrying a load, at which coal is delivered from another panel conveyor.

intermediate vein zone deposit

A deposit thought to have been formed at a depth ranging from 4,000 to 12,000 ft (1.2 to 3.7 km) below the surface and at a temperature between 175 degrees C and 300 degrees C. Such a deposit may take the form of a fissure vein, a series of parallel fissures called a sheeted zone, a replacement of the wall rock of fissures, or a large disseminated deposit. Much of the gold, silver, copper, lead, and zinc of the Western United States comes from these deposits.

intermetallic compound

An intermediate phase in an alloy system, having a narrow range of homogeneity and relatively simple stoichiometric proportions, in which the nature of the atomic binding can vary from metallic to ionic. See also: intermediate constituent.

intermittent cutter

Coal-cutting machine of the pick machine and breast machine type. They are called intermittent cutters because they must be frequently reset, whereas with continuous cutters, a continuous cut can be made the full width of the face without stopping the machine.

intermittent filters

These usually consist of a number of filtering leaves that are simply rectangular frames carrying filter cloth on the outer surface. A number of these leaves are mounted in a suitable tank, and the clear liquid passes through the filter cloth and out through pipes leading from the interior of each filter leaf. The solid material forms a cake on the outside of the leaf.


Lying between mountains.

internal-discharge bucket elevator

A type of bucket elevator having continuous buckets abutting, hinged, or overlapping; designed for loading and discharging along the inner boundary of the closed path of the buckets. Syn: internal elevator. See also: bucket elevator.

internal drainage

Drainage that does not reach the ocean by surface streams, such as drainage toward the lowermost or central part of an interior basin. It is common in arid and semiarid regions, as in western Utah.

internal elevator

See: internal-discharge bucket elevator.

internal ribbon conveyor

A trunnion-supported, revolving cylinder, the inner surface of which is fitted with continuous or interrupted ribbon flighting.

internal stress

Residual stress existing between different parts of metal products, as a result of the differential effects of heating, cooling, or working operations, or of constitutional changes in the solid metal.

internal waste

a. Barren rock between two or more bands (veins) or reef which are mined simultaneously.

b. See: horse.

international ampere

The electric current that, when passed through a solution of silver nitrate in water, will deposit silver at the rate of 0.001118000 g/s. The unit of current in common use.

International Ellipsoid of 1930

Equation relating variation of gravity with latitude, adopted by an international commission as best expressing the normal gravity field of the Earth to the approximation of an ellipsoid of revolution.

international metric carat

See: carat.

interpenetration twin

Two or more individual crystals twinned into such a position that they penetrate one another. See also: penetration twin. CF: contact twin; juxtaposition twin.


In physical chemistry, the transition layer, zone of change, zone of shear, or zeta layer, through which the characteristic qualities of each contacting phase diffuse outward with diminishing strength toward the adjoining phase. Not an interface, since the division is not sharp.


Estimation of a statistical value from its mathematical or graphical position intermediate in a series of determined points.

interpolation of contours

The process of drawing contour lines by inferring their plan position and trend from spot levels or from other contours, assuming the intervening ground to have uniform slope. Where the spot levels are sparse, the process requires knowledge of the land or lie of the seams. See also: contour plan.


A device, usually automatic, for rapidly and frequently breaking and making an electric circuit, as in an induction coil.


a. To cut across or meet, as a borehole cuts through a stratum of rock or encounters a vein.

b. In mining, to cut across or meet a vein or lode with a passageway; also, the point at which a vein or lode cuts across an earlier formation.


a. The point at which a deliberate deflection of the trend of a borehole is made.

b. The point at which a drill hole enters a specific orebody, fault, or rock material. c. Meeting of two orebodies or veins, or the point at which a vein or orebody meets a fault, dike, or rock stratum. d. The point at which two underground workings connect. e. A method in surveying by which the horizontal position of an unoccupied point is determined by drawing lines to that point from two or more points of known position. Syn: resection.

intersection angle

The angle of deflection, as measured at the intersection point, between the straights of a railway or highway curve.

intersection point

That point at which two straights or tangents to a railway or road curve would meet if produced. See also: tangent distance.

intersection shoot

An ore shoot located at the intersection of one vein or vein system with another. It is a common type of ore deposit.


Said of the texture of a porphyritic igneous rock in which the groundmass, composed of a glassy or partly crystalline material other than augite, occupies the interstices between unoriented feldspar laths, the groundmass forming a relatively small proportion of the rock. CF: hyalophitic; hyalocrystalline.

interspersed carbide

Small-size (1/8 inch or 3 mm and larger), irregular-shaped fragments of tungsten carbide slugs mixed with a suitable matrix metal; applied to cutting faces of bits or other cutting tools as a weldment. Also called clustered carbide.


a. An opening or space, as in a rock or soil. Syn: void; pore. Adj. interstitial.

b. Small void in the body of a metal.


Said of a mineral deposit in which the minerals fill the pores of the host rock. CF: Frenkel defect; impregnated.

interstitial water

Subsurface water in the voids of a rock. Syn: pore water. CF: connate water.


The state or condition of occurring between strata of a different character.


a. Interbedded; strata deposited between or alternatingly with other strata.

b. Of coal and mineral matter, associated in random horizontal layers, usually with a natural cleavage.

interstream area

See: interfluve.


Lying between beds of trap.


a. The distance between two points or depths in a borehole; core intersection.

b. The vertical distance between strata or units of reference. c. The contour interval is the vertical distance between two successive contour lines on a topographic, structural, or other contour map. Syn: contour interval.

interwoven conveyor belt

A construction of conveyor belt similar to the solid woven type of belt and having the plies interwoven to the extent that it is impossible to separate them.

in-the-seam mining

The conventional system of mining in which the development headings are driven in the coal seam. CF: horizon mining. See also: unproductive development.

into the house

Newc. The upstroke of a pump engine.

into the solid

Said of a shot that goes into the coal beyond the point to which the coal can be broken by the blast.


Within or across the crystals or grains of a metal. Syn: transgranular.


The interior curve of an arch, as of a tunnel lining.


The fractured ground within the fracture zone. CF: extradosal.


a. Formed within a geologic formation, more or less contemporaneously with the enclosing sediments. The term is esp. used in regard to syndepositional folding or slumping; e.g., intraformational deformation or intraformational breccia.

b. Existing within a formation, with no necessary connotation of time of origin. CF: interformational.

intraformational breccia

A rock formed by brecciation of partly consolidated material, followed by practically contemporaneous sedimentation. It is similar in nature and origin to an intraformational conglomerate, but contains fragments showing greater angularity.

intraformational conglomerate

a. A conglomerate in which the clasts are essentially contemporaneous with the matrix, developed by the breaking up and rounding of fragments of a newly formed or partly consolidated sediment (usually shale or limestone) and their nearly immediate incorporation in new sedimentary deposits; e.g., an edgewise conglomerate.

b. A conglomerate occurring in the midst of a geologic formation, such as one formed during a brief interruption in the orderly deposition of strata. It may contain clasts external to the formation. The term is used in this sense esp. in England.

intraformational contortion

Intricate and complicated folding, resulting from the subaqueous slumping or sliding of unconsolidated sediments under the influence of gravity.


a. Said of a phenocryst, of an earlier generation than its groundmass, that formed at depth, prior to extrusion of a magma as lava.

b. Said of the period of crystallization occurring deep within the Earth just prior to the extrusion of a magma as lava. c. Located, formed, or originating deep within the Earth.

intrinsically safe apparatus

Apparatus that is so constructed that, when installed and operated under the conditions specified by the certifying authority, any electrical sparking that may occur in normal working, either in the apparatus or in the circuit associated therewith, is incapable of causing an ignition of the prescribed flammable gas or vapor.

intrinsically safe circuit

A circuit in which any electrical sparking that may occur in normal working under the conditions specified by the certifying authority, and with the prescribed components, is incapable of causing an ignition of the prescribed flammable gas or vapor.

intrinsically safe machine

A machine that is safe in itself, without having to be placed inside a flameproof enclosure. It implies that the machine cannot produce any spark that is capable of igniting mixtures of combustible gases and air in mines.

intrinsic safety

In a circuit, safety such that any sparking that may occur in that circuit in normal working, or in reasonable fault conditions, is incapable of causing an explosion of the prescribed inflammable gas.

introductory column

In casing a borehole, the highest and first column that is inserted.


a. In geology, a mass of igneous rock that, while molten, was forced into or between other rocks.

b. A mass of sedimentary rock occurring in a coal seam.

intrusion breccia

See: contact breccia.

intrusion displacement

Faulting coincident with the intrusion of an igneous rock.

intrusion grouting

A method of placing concrete by intruding the mortar component in position; it is then converted to concrete by intruding the mortar component into its voids. One of the chief advantages of the method is that it permits the placing of concrete underwater.


Of or pertaining to intrusion--both the processes and the rock so formed. n. An intrusive rock. CF: extrusive.

intrusive vein

An igneous intrusion resembling a sheet, apparently formed from a magma rich in volatiles.


The property of some silicate minerals (e.g., stilbite, vermiculite, scapolite) or rocks (e.g., perlite) to expand permanently when heated to form an irregular or vesicular structure. CF: exfoliation.


An inrush of water, on a large scale, that floods the entire mine or a large section of the workings. See also: tapping old workings.


An alloy of nickel and iron, containing about 36% nickel, and having an extremely low coefficient of thermal expansion. It is used in the construction of surveying instruments, such as pendulums, level rods, first-order leveling instruments, and tapes.


a. Mex. A mining trespass.

b. See: transgression.

inverse distance squared

A method for interpolating spatial sample data and determining values between data points. A value interpolated for any spatial point is determined by applying a weighting factor based on distance between the spatial point and surrounding sample data. Selection of sample points to include in the calculation may be determined by minimum and/or maximum distance, azimuth orientation, and the minimum and/or maximum number of the nearest sample data points. Abbrev. IDS.

inverse initiation

The placing of the detonator at the back of the shothole. This is the usual practice when using delay detonators to minimize the danger of cutoff holes. See also: direct initiation. Also known as indirect priming. Syn: indirect initiation.

Inverse Square Law

Law that governs the distance-dependence of physical effects, such as intensity of light, magnetism, and gravitational force. The effect at a point due to an emitting source varies as the square of the distance between them.


a. Construction of a geophysical model from a set of measurements; e.g., using numerous gravity measurements to infer subsurface density distributions, or using slip vectors and spreading rates to define global plate motions. Inversion models are inherently ambiguous.

b. A change of crystalline phase brought about by a change in temperature or pressure; e.g., the inversion between alpha quartz and beta quartz at 573 degrees C. c. See: center of symmetry.

inversion point

a. A point representing the temperature at which one polymorphic form of a substance, in equilibrium with its vapor, reversibly changes into another under invariant conditions.

b. The temperature at which one polymorphic form of a substance inverts reversibly to another under univariant conditions and a specific pressure. c. More loosely, the lowest temperature at which a monotropic phase inverts at an appreciable rate into a stable phase, or at which a given phase dissociates at an appreciable rate, under given conditions. d. A single point at which different phases are capable of existing together at equilibrium. Syn: transition point; transition temperature.


The floor or bottom of the internal cross section of a closed conduit, such as an aqueduct, tunnel, or drain. The term originally referred to the inverted arch used to form the bottom of a masonry-lined sewer or tunnel.


See: overturned.

inverted fold

See: overturned.

inverted heading and bench

See: heading-overhand bench.

inverted pendulum

An instrument in which the acceleration of gravity is determined by measuring the swinging period of a mass that is supported on top of a spring.

inverted plunge

The plunge of folds, or sets of folds, whose inclination has been carried past the vertical, so that the plunge is now less than 90 degrees in a direction opposite from the original attitude. It is a rather common feature in excessively folded or refolded terranes.

inverted relief

A topographic configuration that is the inverse of the geologic structure, as where mountains occupy the sites of synclines and valleys occupy the sites of anticlines.

inverted siphon

a. A pipeline crossing over a depression or under a highway, railroad, canal, etc. The term is common but inappropriate, as no siphonic action is involved. The term "sag pipe" is suggested as a substitute.

b. A pipe or tube in the shape of a siphon, but inverted, as for carrying water across the depression of a ravine to a lower level. c. See: drowned level.

invert level

The datum level of the lowest part of an invert.

invert strut

Flat strut that sometimes is used instead of an arch on the bottom of a tunnel cross section.

invisible light

Wavelengths in the electromagnetic spectrum too short or too long to be detected by the human eye; e.g., ultraviolet and infrared light. CF: visible light.


a. The refractory lining of the stack of a blast furnace. See also: stack.

b. The interior walls or lining of a shaft furnace.

inwall brick

Fireclay brick for use in lining the inwall section of a blast furnace.


A monoclinic mineral, Ca (sub 2) B (sub 6) O (sub 6) (OH) (sub 10) .8H (sub 2) O ; forms large, soft, colorless, tabular crystals associated with colemanite and other boron minerals in Inyo County, CA; dehydrates to meyerhoffite.


A hexagonal mineral, AgI ; soft, sectile, waxy; normally a secondary mineral; in the oxidized zone of silver deposits from primary ores containing argentite, tennantite or tetrahedrite, and native silver; a source of silver; in arid regions where volcanic rocks are common--Southwestern United States, Mexico, Chile, and Broken Hill, New South Wales, Australia. Formerly called iodyrite or iodite. CF: chlorargyrite; bromargyrite.


A name to replace iodobromite whose composition is not definite. Used to designate minerals of the cerargyrite group containing chlorine, bromine, and iodine, Ag(Cl,Br,I).

iodide metal

Hafnium produced by the van Arkel and de Boer process.

iodide process

The process developed by van Arkel and de Boer; used for refining zirconium and hafnium by the decomposition of iodide on a hot wire. Syn: van Arkel and de Boer process. Also called crystal bar process.

iodide process for producing titanium

Process involving the reaction of impure titanium metal with iodine to form the volatile tetraiodide, which is then decomposed on a hot wire at temperatures between 2,000 degrees F and 2,730 degrees F (1,093 degrees C and 1,500 degrees C) to form high-purity titanium and iodine.


Volumetric analysis involving either titration with a standardized solution of iodine, or the release by a substance under examination of iodine in soluble form, so that its concentration can be determined by titration, using starch as an indicator. The method is used with substances that can oxidize potassium iodide to release free iodine, or conversely with substances which combine with free iodine. The basis of reaction is I (sub 2) +2Na (sub 2) S (sub 2) O (sub 3) --> 2NaI+Na (sub 2) S (sub 4) O (sub 6) . Also called iodometry.


A nonmetallic, bluish-black, lustrous solid element, volatilizing at ordinary temperatures into a blue-violet gas with an irritating odor. Symbol, I. Occurs sparingly in sea water, in saltpeter and nitrate-bearing earth (known as caliche), in brines, and in brackish waters from oil and salt wells. Its compounds are used in medicines, photography, and organic chemistry.

iodine pentoxide method

Accurate determinations of very low concentrations of carbon monoxide are required of samples from pits troubled with spontaneous combustion and those in which diesel locomotives are operating. This method involves the passage of a known volume of the sample first through a train of reagents for purifying and drying and then through a heated tube of iodine pentoxide; if carbon monoxide is present, proportional amounts of iodine and carbon dioxide are formed, either of which may be determined.

iodine pentoxide test

An iodine pentoxide (Hoolamite) test consists of a glass tube filled with a reagent mixed with fuming sulfuric acid and carried by crushed pumice through which a known volume of the air to be sampled is discharged from a rubber bulb after preliminary drying. The bulb is squeezed 10 times; the carbon monoxide present changes the color of the reagent from grey to green, and the concentration is obtained by reference to a color chart supplied with the instrument. The instrument reads down to about 0.07% carbon monoxide.


See: iodembolite.


See: iodargyrite.


See: cordierite.


An atom or a group of atoms combined in a radical or molecule that carries a positive or a negative electric charge as a result of having lost or gained one or more electrons. It may exist in solution, usually in combination with molecules of the solvent, or out of solution; it may be formed during electrolysis and migrate to the electrode of opposite charge, or it may be formed in a gas and be capable of carrying an electric current through the gas.

ion exchange

Reversible exchange of ions contained in a crystal for different ions in solution without destruction of crystal structure or disturbance of electrical neutrality. The process is accomplished by diffusion and occurs typically in crystals possessing one- or two-dimensional channelways where ions are relatively weakly bonded. Also occurs in resins consisting of three-dimensional hydrocarbon networks to which many ionizable groups are attached. Syn: base exchange. See also: cation exchange.

ion-exchange column

A tube packed with particles or beads of resin chosen for their ability to capture specific ions from an aqueous solution as it passes through the column.

ionic activity measurement

Use of an electrode reversible to the ion under test to form a half cell. This is connected by a salt bridge to a reference electrode, and the resulting electromotive force is measured.

ionic bond

a. Electrostatic force holding ions together in a crystal.

b. A chemical bond between atoms, one of which is an electron donor and the other is an electron acceptor.

ionic equilibrium

The situation when, for a prescribed temperature, pressure, concentration of reactants, and pH, the rate of dissociation of molecules into ions is approx. in balance with that of their recombination.

ionic migration

Movement of a charged particle through an electrolyte toward an electrode of opposite charge sign. The losses in a neutral salt around two electrodes during the passage of electric current are in ratio to velocities of ions migrating from these electrodes. Ionic velocities are stated in centimeters per second for a potential gradient of 1 V/cm.

ionic mobility

Velocity in a dilute solution of an ion where the potential difference across this is 1 V/cm. For hydrogen ions, the velocity is 0.00326 cm/s.

ionic transport number

Fraction of total current carried by one ion during electrolysis (ion migration).


A term referring to two different materials found in the Ione Valley of northern California: (1) a clay mineral, possibly a variety of kaolinite, found as scales in the Ione sandstone formation, and (2) an impure fossil hydrocarbon found in lignite, which is brownish-yellow, dissolved by chloroform, and yields a brown, tarry oil on destructive distillation.


The process of adding electrons to, or knocking electrons from, atoms or molecules, thereby creating ions. High temperatures, electrical discharges, and nuclear radiation can cause ionization.

ionization chamber

An instrument that detects and measures ionizing radiation by observing the electrical current created when radiation ionizes gas in the chamber, making it a conductor of electricity.

ionization constant

The ratio of the product of the activities of the ions produced from a given substance to the activity of the undissociated molecules of that substance. See also: activity.

ionizing radiation

Any radiation that directly or indirectly displaces electrons from the outer domains of atoms; e.g., alpha, beta, or gamma radiation.


The outer part of the Earth's atmosphere, beginning at an altitude of about 25 miles (40 km) and extending to the highest parts of the atmosphere: it contains several regions that consist of a series of constantly changing layers characterized by an appreciable electron and ion content.

ion sieve separation

Separation of ions by filtering them through the intermediately sized lattice of a suitable aluminosilicate zeolite, chosen to permit passage only of undersized ions through its rigid structure.


See: wuestite.


A triclinic mineral, Pb (sub 10) Cu(CrO (sub 4) ) (sub 6) (SiO (sub 4) ) (sub 2) (F,OH) (sub 2) ; forms a series with hemihedrite in which zinc substitutes for copper; at the Sebarz Mine, northeast of Anarak, Iran.


The exhibition of interference colors from the surface or interior of a mineral, caused by light interference from thin films or layers of different refractive index. Labradorite and some other feldspars show it. The tarnish on the surface of coal, copper pyrites, etc., is sometimes iridescent. Adj. iridescent.

iridic gold

Said to be a native alloy of gold and iridium carrying 62.1% gold, 30.4% iridium, 3.8% platinum, and 2.1% silver.


An alloy usually containing 90% or more of platinum. The remaining percentage is of iridium, which is necessary to produce an alloy sufficiently stiff for use in gem mountings.


a. An isometric mineral, native Ir ; Mohs hardness, 6 to 7; sp gr, 22.2 if pure; occurs native (>80% Ir) and alloyed with osmium (iridosmine) or platinum in mafic to ultramafic rocks and derived alluvial deposits; in rare arsenides and sulfides, such as irarsite (Ir,Ru,Rh,Pt)AsS , iridarsenite (Ir,Ru)As (sub 2) , and iridisite-beta (Ir,Cu) (sub 3) S (sub 8) ; placer deposits may include gold.

b. The most corrosion-resistant metal known. Symbol, Ir. It is mainly used as a hardening agent for platinum.


A natural alloy of iridium and osmium, (Ir,Os). Analyses show 43% to 77% iridium, 17% to 49% osmium, and a little rhodium, ruthenium, platinum, iron, and copper. Rhombohedral. Syn: osmite.


A canary-yellow mineral, (UO (sub 2) )(Mo (sub 2) (super 6+) O (sub 7) ).3H (sub 2) O ; luster, vitreous; uneven fracture; does not fluoresce. Syn: priguinite.

Irish coal

Slate, shale, or rock loaded out from a colliery as coal.

Irish dividend

An assessment on mining stock.

iris quartz

A transparent quartz crystal containing minute air-filled or liquid-filled internal cracks that produce iridescence by interference of light. The cracks may occur naturally or be caused artificially by heating and sudden cooling of the specimen. Syn: firestone; rainbow quartz.


a. Iron-base materials not falling into the steel classifications. See also: gray cast iron; ingot iron; malleable cast iron; nodular cast iron; white cast iron; wrought iron.

b. Colloquially, all derrick and drilling equipment above the heads of the workers on the drill platform. c. Any ferrous metal tool or part that must be fished from a borehole. Also called junk. d. An isometric mineral, native alpha-Fe ; metallic steel gray to black; sp gr, 7.3 to 7.9. Occurs as grains in basalt in Disko Island, Greenland; in meteorites, and in placers on South Island, New Zealand; and in Oregon and British Columbia. Nickel is commonly reported in iron in quantities up to several percent; meteoric iron generally contains at least 5%, and up to 25% to 65%, nickel. Because of the instability of iron under oxidizing conditions and the abundance of oxygen in the Earth's crust and atmosphere, practically all terrestrial iron occurs in the divalent (ferrous) or trivalent (ferric) state combined with other metals and nonmetallic elements in silicates, oxides, sulfides, etc. e. Fourth most abundant element, by weight, making up the crust of the Earth. Symbol, Fe. The most common ore is hematite, Fe (sub 2) O (sub 3) , from which the metal is obtained by reduction with carbon. Iron is the cheapest and most abundant, useful, and important of all metals.

iron alum

See: halotrichite.

iron black

Finely divided antimony.

iron clay

See: clay ironstone.

iron formation

A chemical sedimentary rock, typically thin-bedded and/or finely laminated, containing at least 15% iron of sedimentary origin, and commonly but not necessarily containing layers of chert. Various primary facies (usually not weathered) of iron formation are distinguished on the basis of whether the iron occurs predominantly as oxide, silicate, carbonate, or sulfide. Most iron formation is of Precambrian age. In mining usage, the term refers to a low-grade sedimentary iron ore with the iron mineral(s) segregated in bands or sheets irregularly mingled with chert or fine-grained quartz. CF: ironstone; jaspilite. Essentially synonymous terms are itabirite; banded hematite quartzite; taconite; quartz-banded ore; banded iron formation; calico rock; jasper bar.

iron froth

A fine spongy variety of hematite.

iron furnace

A furnace in which iron is smelted or worked in any way.

iron glance

A variety of hematite; specular iron. See also: specularite.

iron hat

a. See: gossan.

b. See: safety hat; hardhat. c. A weathered ironstone outcrop.

iron hypersthene

See: ferrosilite.

iron man

a. An iron worker; a manufacturer of iron; esp. one engaged in the processing of iron.

b. A worker who weighs out ground iron ore and adds it to slurry or dry-ground rock as it goes into the cement kiln. c. An apparatus on wheels for supporting a glassblower's pontil while blowing large cylinders, as for window glass.


Person who conducts or manages the founding or manufacture of iron, esp. on a large scale.

iron mica

A micaceous hematite. See: biotite.

iron molybdate

See: ferrimolybdite.

iron monarch

An important iron ore (chiefly hematite) deposit in the Middleback Ranges area of South Australia.

iron olivine

See: fayalite.

iron ore

Ferruginous rock containing one or more minerals from which metallic iron may be profitably extracted. The chief ores of iron consist mainly of the oxides: hematite, Fe (sub 2) O (sub 3) ; goethite, alpha -FeO(OH); magnetite, Fe (sub 3) O (sub 4) ; and the carbonate, siderite or chalybite, FeCO (sub 3) . See also: bog iron ore; limonite; kidney iron ore; magnetite; siderite; hematite; prereduced iron-ore pellet.

iron oxide

a. A common ore of iron, sometimes prepared as a fine powder for use by drillers as a drill-mud heavy loader.

b. A common compound of iron and oxygen; e.g., rust.

iron oxides

The basic constituent of the ferromagnetic spinels or ferrites. FeO, Fe (sub 2) O (sub 3) , and Fe (sub 3) O (sub 4) have melting points ranging from that of FeO at 1,420 degrees C to that of Fe (sub 2) O (sub 3) at 1,565 degrees C. Used extensively for producing colors in glasses, glazes, and enamels. Iron and iron oxides produce brown or reddish colors in ceramic mixtures if they are fired in an oxidizing atmosphere, and greenish or bluish colors if fired in a reducing atmosphere. Iron oxides are fluxing and coloring materials. Larger particles produce brown or black spots, which, particularly in whiteware, are undesirable. Much care is taken to remove iron and iron oxides from the raw materials and from the bodies used for whiteware manufacture.

iron pan

A general terrm for a hardpan in a soil in which iron oxides are the principal cementing agents; several types of iron pans are found in dry and wet areas and in soils of widely varying textures.

iron phosphate

See: vivianite.

iron piler

A laborer who removes iron from cars, sometimes breaks it, and piles and classifies it according to grade.

iron portland cement

Mixture of portland cement and granulated blast furnace slag.

iron pyrite

See: pyrite.

iron-reduction process

A process used in the treatment of lead ores. See also: precipitation process.

iron refining

The reduction of prepared ores of iron to metallic iron, as in the blast furnace. The reduction and purification of semirefined iron, such as pig iron, or a mixture of pig iron, scrap iron, and scrap steel, to form substantially pure iron or steel, as in the electric furnace, open- hearth furnace, puddling furnace, or Bessemer converter.

iron runner

The spout by which iron flows from the taphole of a blast furnace.

iron sampler

In the iron and steel industry, a laborer who obtains samples of iron ore as it is brought into the plant, or samples of semifinished or finished metal products, such as iron and steel sheets, rails, rods, or bars, and carries them to the laboratory for routine tests. Also called sampler; test carrier.

iron sand

A sand containing particles of iron ore (usually magnetite), as along a coastal area.

iron schefferite

See: pyroxene.

iron series

See: magnetite.


a. Said of a mineral that is streaked, speckled, or marked with iron or an iron ore.

b. Containing small nodules or oolitic bodies of limonite or hematite; e.g., an ironshot rock in which the ooliths are essentially composed of limonite. A limonitic oolith in an ironshot rock.


A worker in iron, as a blacksmith.

iron spar

See: siderite.

iron stain

Strongly colored yellowish, reddish, or brownish deposit of iron oxides.

iron steel

A material formed of iron between steel surfaces, or of steel-located iron.


Any rock containing a substantial proportion of an iron compound, or any iron ore from which the metal may be smelted commercially; specif., an iron-rich sedimentary rock, either deposited directly as a ferruginous sediment or resulting from chemical replacement. The term is customarily applied to a hard, coarsely banded or nonbanded, and noncherty sedimentary rock of post-Precambrian age, in contrast with iron formation. The iron minerals may be oxides (limonite, hematite, magnetite), carbonate (siderite), or silicate (chamosite); most ironstones containing iron oxides or chamosite are oolitic. See: clay ironstone; banded ironstone. CF: iron formation.

ironstone cap

A surficial or near-surface sheet or cap of concretionary clay ironstone.

ironstone clay

See: argillaceous hematite; red hematite.

iron sulfides

See: chalcopyrite; pyrite; marcasite; pyrrhotite.

iron talc

See: minnesotaite.

iron vitriol

See: melanterite.


Person engaged in manufacturing iron or ironwork.


An establishment for the manufacture of iron or of heavy ironwork.


Exposure to radiation, as in a nuclear reactor.

irregular polygon

Polygon in which neither the sides nor the angles are equal.


Not respirable; not fit to be breathed. Said of mine gases.

irrespirable atmosphere

In a coal mine, atmosphere containing poisonous gases or a lack of sufficient oxygen as a result of combustible gases explosions, coal-dust explosions, combined gas and dust explosions, or mine fires, and which can only be entered by persons wearing breathing apparatus.

irrotational wave

See: P wave; compressional wave.


A nearly obsolete syn. of intrusion.

isanomalic line

Line of equal value of an anomaly.

Isbell table

Obsolete type of shaking table.


See: ilmenorutile.


An orthorhombic mineral, (U,Fe,Y,Ca)(Nb,Ta) (sub 4) (?) ; opaque black; at Ishikawa, Iwaki Province, Japan.


Transparent sheet mica, commonly muscovite, principally from pegmatite dikes. See also: mica; muscovite.

island arc

A group of islands having a curving, arclike pattern. Most island arcs lie near the continental masses, but inasmuch as they rise from the deep ocean floors, they are not a part of the continents proper.

isoanthracite lines

a. A term for lines of equal volatile content (now called isovols) drawn on a map or diagram.

b. Lines of equal C:H ratio in coal drawn on the map or diagram.


An imaginary line or a line on a map or chart connecting or marking places on the surface of the Earth where the height of the barometer reduced to sea level is the same either at a given time or for a certain period.


A term used for a line that connects all areas of equal uplift or depression; it is used esp. in Quaternary geology as a means for expressing crustal movements related to postglacial uplift.


a. A line on a map or chart that connects points of equal water depth. Syn: bathymetric contour; depth contour.

b. An imaginary line on a land surface along which all points are the same vertical distance above the upper or lower surface of an aquifer or above the water table.


Lines of equal calorific value in coal drawn on a map or diagram.


On a map or diagram, a line connecting points of equal fixed-carbon content in coal. See also: isocarbon map.

isocarbon map

A map showing, by contours, the areas having an equal quantity of carbon within an assumed interval of stratigraphic section. See also: isocarb.


a. A line drawn on a map through points of equal drilled thickness for a specified subsurface unit. Thickness figures are uncorrected for dip in vertical wells, and corrected for hole angle, but not for dip, in deviated wells. CF: isopach.

b. In a phase diagram, a line connecting points of constant volume.

isochromatic lines

A map showing drilled thickness of a given stratigraphic unit by means of isochores. Syn: convergence map. 2 ��B ��B �� � � � DICTIONARY TERMS:isochromatic lines In stress analysis by the photoe In stress analysis by the photoelastic method, lines of equal difference of principal stress, appearing as colored streaks. See also: photoelasticity.

isochrome map

A contour map that depicts the continuity and extent of color stains on geologic formations.


See: isochronous.

isochrone lines

Lines connecting points of equal times. When the relative seismic velocities are known, the isochrones can be translated into depth contours. See also: refraction shooting.


a. Equal in duration or uniform in time; e.g., an isochronous interval between two synchronous surfaces, or an isochronous unit of rock representing the complete rock record of an isochronous interval.

b. A term frequently applied in the sense of synchronous, such as an isochronous surface having everywhere the same age or time value within a body of strata. Syn: isochronal. CF: synchronous.


Adj. of isocline.

isoclinal fold

A fold whose limbs are parallel. Syn: isocline.


See: isoclinal fold.


A line (in a stressed body) at all points on which the corresponding principal stresses have the same direction.

isoclinic line

a. A line drawn through all points on the Earth's surface having the same magnetic inclination. The particular isoclinic line drawn through points of zero inclination is given the special name of aclinic line.

b. An isomagnetic line connecting points of equal magnetic inclination.


Characteristic of a crystal structure with bond strengths roughly equal in all directions.


Refers to the hexagonal, trigonal, and tetragonal crystal systems; i.e., having the lateral crystal axes a (sub 1) and a (sub 2) of equal length. CF: isometric; anisometric.


In mineralogy, both isomorphous and dimorphous; said of certain groups of minerals.

isodynamic line

Any line joining points of equal magnetic intensity. Applicable to the total intensity or the vertical, horizontal, north-south, or east-west components. So used in terrestrial magnetism literature, esp. in British and Canadian writings.

isoelectric point

Zero potential or point of electrical neutrality; the hydrogen-ion exponent at which particles in aqueous suspension are neutral and best able to flocculate. Also called zero point of change.


a. Pertaining to rocks belonging to the same metamorphic facies and having reached equilibrium under the same set of physical conditions. Syn: isograde.

b. Pertaining to rocks belonging to the same sedimentary facies; e.g., an isofacial line on a map, along which the thickness of stratum of the same lithologic composition is constant.


In gravity prospecting, a contour line of equal gravity values.


In magnetic prospecting, a contour line of equal magnetic values.

isogam map

A chart showing contour lines of equal magnetic field intensity and employed in the magnetic methods of geophysical prospecting. Also called isogal map in gravity surveys.


A line or surface within the Earth connecting points of equal temperature. Syn: isogeothermal line; geotherm; geoisotherm.

isogeothermal line

See: isogeotherm.


See: isogonic line.

isogonic line

An isomagnetic line connecting points of equal magnetic declination. See also: agonic line. Syn: isogon.


A line on a map joining points at which metamorphism proceeded at similar values of pressure and temperature as indicated by rocks belonging to the same metamorphic facies. Such a line represents the intersection of an inclined surface with the Earth's surface corresponding to the boundary between two contiguous facies or zones of metamorphic grade, as defined by the appearance of specific index minerals; e.g., garnet isograd, staurolite isograd.


See: isofacial.


A general term for a line on a map or chart connecting points having an equal numerical value of some physical quantity (such as temperature, pressure, or rainfall); an isopleth.


A line constructed on a map, somewhat similar to a contour line, but connecting points of equal moisture content of coal in the bed.


A line connecting points of equal precipitation.


A culture of an organism isolated by selection procedures.

isolated consignment

A particular case of a single consignment where the sampling is to be carried out without prior knowledge of a coal's sampling characteristics other than its presumed ash content and size.


Part of a circuit that can be removed from it in order to break the circuit when there is no current flowing.


See: isopleth.


a. An imaginary line connecting points of similar lithology and separating rocks of differing nature, such as of color, texture, or composition.

b. An imaginary line of equal aggregate thickness of a given lithologic facies or particular class of material within a formation, measured perpendicular to the bedding at selected points (which may be on outcrops or in the subsurface).

isolith map

A map that depicts isoliths; esp. a facies map showing the net thickness of a single rock type or selected rock component in a given stratigraphic unit.

isomagnetic line

A line connecting points of equal value of some magnetic element; e.g., isogonic line; isodynamic line; isoporic line.


a. One of two or more substances composed of the same molecular formula, but differing in chemical or physical properties owing to the arrangement of the atoms in the molecule.

b. In nuclear science, one of two or more nuclides with the same numbers of neutrons and protons in the nucleus, but having different energy.


Of, relating to, or exhibiting isomerism.


a. A system of crystallization with three axes at right angles and of equal length; nine planes of symmetry; singly refracting. CF: anisodesmic.

b. Characterized by equality of measure. c. The crystal system characterized by three orthogonal axes of equal length. Syn: cubic; equant. CF: anisometric; isodiametric.

isometric line

See: isopleth.

isometric projection

In technical drawing, a three-dimensional view of an object can be drawn to scale with three perpendicular edges at 120 degrees to each other, and with the vertical edges vertical. See also: oblique projection; axonometric projection.


The name given to chemical compounds that have analogous composition, similar crystal structures, and closely related crystal forms; e.g., carbonate minerals of the aragonite group--aragonite, witherite, strontianite, and cerrusite--in which the metal ions are different but the several minerals crystallize in the orthorhombic system in closely similar forms. Adj. isomorphous. Noun, isomorph. CF: polymorphism.


Originally defined (Mitscherlich, 1819) as having similar crystalline form, but now generally restricted to compounds that form solid solutions by isomorphous substitution; i.e., by the replacement of one ion for another in a crystal structure without alteration in the crystal form. CF: isotypic. Syn: allomeric.

isomorphous mixture

a. A solid solution of two or more isomorphous substances.

b. A type of solid solution in which mineral compounds of analogous chemical composition and closely related crystal habit crystallize together in various proportions. CF: solid solution.

isomorphous replacement

A characteristic of some minerals where substitution for one or more elements by others does not change the crystal structure. An example is the substitution of iron for zinc in sphalerite, wherein the iron content can range up to more than 15% without changing the sphalerite structure. Similarly, iron, manganese, and magnesium ions can replace each other in the calcite structure common to siderite, rhodochrosite, and magnesite.

isomorphous series

Descriptive of two minerals with the same crystal structure but different end-member compositions which may show partial or complete crystal miscibility (solid solution) between them. One mineral may belong to more than one isomorphous series; e.g., the garnet grossular forms a series with andradite, with hibschite and katoite, and with uvarovite. There are many isomorphous series among minerals; e.g., plagioclase feldspars, monoclinic pyroxenes, and the spinel and garnet groups. CF: solid solution.

isontic line

See: isopleth.


A line drawn on a map through points of equal true thickness of a designated stratigraphic unit or group of stratigraphic units. CF: isochore. Syn: isopachyte; isopachous line; thickness line; thickness contour.

isopach map

A map indicating, usually by means of contour lines, the varying thickness of a designated stratigraphic unit. Also called isopachous map.


Of, relating to, or having an isopach; e.g., an isopachous contour. Not recommended usage.

isopachous line

See: isopach.

isopachous map

A nonrecommended syn. of isopach map.


British term for isopach.

isophysical series

A series comprising rocks of different chemical composition that were metamorphosed under identical physical conditions.


Said of sedimentary rocks of the same facies, or said of facies characterized by identical or closely similar rock types. The rocks may be formed in different sedimentation areas or at different times or both, but the lithologies are the same; e.g., a facies repeated in vertical succession. Also, said of a map depicting isopic facies or rocks. CF: heteropic.


Relating to synchronous deposits that exhibit the same facies.


Constant value of pressure on a surface of the sea.

isopiestic line

See: equipotential line.


a. In a strict sense, a line or surface on which some mathematical function has a constant value. It is sometimes distinguished from a contour by the fact that an isopleth need not refer to a directly measurable quantity characteristic of each point in the map area; e.g., maximum temperature of a particular point.

b. A general term for a line, on a map or chart, along which all points have a numerically specified constant or equal value of any given variable, element, or quantity (such as abundance or magnitude), with respect to space or time; esp. a contour. Etymol. Greek isos, equal, + plethos, fullness, quantity, multitude. Syn: isogram; isoline; isontic line; isometric line.


See: isoporic line.

isoporic line

A line drawn through points whose annual change in magnetic declination is equal. Syn: isopor.


A line connecting points of equal density, particularly of ocean water. A line connecting points of equal atmospheric density may be called an isostere.


Lines joining points of equal radioactivity, drawn from geiger- or scintillation-counter data to form an isorad map.

isoresistivity plan

A plan showing lines of equal resistivity at a certain selected depth. It is prepared from data obtained by the resistivity method of geophysical prospecting.


See: isoseismal line.


A line on the surface of the Earth joining points of equal seismic disturbance due to any single earthquake. Syn: isoseismal line.

isoseismal line

A line connecting points on the Earth's surface at which earthquake intensity is the same. It is usually a closed curve around the epicenter. Syn: isoseism; isoseismal.


See: kesterite.


The condition of equilibrium, comparable to floating, of the units of the lithosphere above the asthenosphere. Crustal loading, as by ice, water, sediments, or volcanic flows, leads to isostatic depression or downwarping; removal of load leads to isostatic uplift or upwarping. Two differing concepts of the mechanism of isostasy are the Airy hypothesis of constant density and the Pratt hypothesis of constant thickness. See also: isostatic compensation. Adj. isostatic.


Subjected to equal pressure from every side; being in hydrostatic equilibrium; relating to or characterized by isostasy. See: stress trajectory.

isostatic adjustment

See: isostatic compensation.

isostatic anomaly

a. The difference between the observed value of gravity at a point after applying to it the isostatic correction and the normal value of gravity at the point.

b. Anomaly on a map of observed gravity anomalies after applying the isostatic correction. Negative isostatic anomalies indicate undercompensation, implying a tendency to rise; positive isostatic anomalies connote overcompensation and a tendency to sink. c. A gravity anomaly calculated on a hypothesis that the gravitational effect of masses extending above sea level is approx. compensated by a deficiency of density of the material beneath those masses; the effect of deficiency of density in ocean waters is compensated by an excess of density in the material under the oceans. See also: anomaly.

isostatic compensation

The adjustment of the lithosphere of the Earth to maintain equilibrium among units of varying mass and density; excess mass above is balanced by a deficit of density below, and vice versa. See also: isostasy. Syn: isostatic adjustment; isostatic equilibrium.

isostatic correction

The adjustment made to values of gravity, or to deflections of the vertical, observed at a point, to take account of the assumed mass deficiency under topographic features for which a topographic correction is also made.

isostatic equilibrium

The shifting of the rock beneath the Earth's crust in response to the shifting in the weight above the Earth's crust. Syn: isostatic compensation.


a. Refers to minerals that are closely similar in crystallographic, physical, and chemical properties but have little tendency for isomorphous substitution; same as isotypic.

b. Said of minerals that have the same ionic or molecular crystal structure. Isostructure is more rigorous than isomorphism; the latter requires similar crystal forms, the former a one-to-one correspondence of atomic particles. Isostructural minerals may differ markedly in chemical composition and physical properties, e.g., fluorite and uraninite, or may be closely similar, e.g., the calcite group of carbonates.


A line connecting points of equal temperature. Isotherm maps are often used to portray surface temperature patterns of water bodies.


a. A change taking place at a constant temperature.

b. Pertaining to the process of changing the thermodynamic state of a substance, such as its pressure and volume, while maintaining the temperature constant.

isothermal compression

a. Reduction in the volume of a fluid without any change in its temperature.

b. Compression in which there is no change in the temperature of the air; used as a standard against which the conditions of actual compression may be checked.

isothermal expansion

The expansion of air under constant temperature. Since the air does work on expanding, it loses heat; consequently, heat must be added to the air to maintain it at constant temperature.

isothermal layer

A water column through which a constant temperature exists.


A descriptive term applied to igneous rocks with an orbicular texture in which the nuclei of the orbicules are composed of the same rock as the groundmass. CF: crystallothrausmatic; homeothrausmatic; heterothrausmatic.

isotropic mass

A mass having the same property (or properties) in all directions.


One-dimensional polymorphism; e.g., alternate stacking of identical layers of micas or clays. Also called isotypism. CF: polymorphism.


The phenomenon of sound being the same in all parts of a given water column.


Lines constructed on a map of a coalbed connecting points of equal volatile matter, delineating the distribution of volatile matter of the coal.

Istrian stone

A marble found near Trieste, from which Venice is largely built.


A laminated, metamorphosed oxide-facies iron formation in which (1) the original chert or jasper bands have been recrystallized into megascopically distinguishable grains of quartz and (2) the iron is present as thin layers of hematite, magnetite, or martite (Dorr & Barbosa, 1963). The term was originally applied in Itabira, Brazil, to a high-grade, massive specular-hematite ore (66% iron) associated with a schistose rock composed of granular quartz and scaly hematite. The term is now widely used outside Brazil. CF: jacutinga; canga. Syn: banded-quartz hematite; hematite schist.


A micaceous sandstone or a schistose quartzite that contains interstitial, loosely interlocking grains of mica, chlorite, and talc; exhibits flexibility when split into thin slabs. Type locality is Itacolumi Mountain in Minas Gerais, Brazil. Syn: flexible sandstone; articulite.

Italian asbestos

A name often given to tremolite asbestos to distinguish it from Canadian or chrysotile asbestos; extensively quarried in Piedmont and Lombardy, Italy.


The fine-grained, calcareous creamy-white dentine forming most of the tusks of elephants and the teeth or tusks of certain other large animals, such as the walrus; it has long been esteemed for a wide variety of ornamental articles. Elephant ivory is now illegal to use. It is often simulated by bone.

ivory turquoise

Odontolite; fossil tooth or bone colored by a blue phosphate of iron. Syn: bone turquoise.

Iwan-pattern earth auger

A dry sampler equipped with an Iwan auger shoe or cutterhead. See: post-hole digger.


A monoclinic mineral (Ta,Nb,Sn,Fe,Mn) (sub 4) O (sub 8) ) .