GLOSSARY
Needle-shaped, like certain crystals. Also said of sedimentary particles more than three times as long as wide.
A brilliant mineral luster, characteristic of minerals with a high index of refraction, e.g. diamond and cerussite.
Said of a mineral crystal showing no rational faces, or of a detrital grain that shows no crystal outline.
A crystal form consisting of two parallel faces, so oriented as to cut the vertical axis c and to be parallel to the lateral axes a and b.
Having the form of a bunch of grapes. Said of certain minerals, e.g. hematite smithsonite.
1. A mineral compound characterized by a fundamental anionic structure of CO3–2. Calcite and aragonite CaCO3, are examples of carbonates. 2. A sediment formed of the carbonates of calcium, magnesium, and /or iron, e.g. limestone and dolomite.
1. The breaking of a mineral along its crystallographic planes, thus reflecting a crystal structure, e.g. cubic cleavage. 2. The property or tendency of a rock to split along parallel, closely spaced planar surfaces. It is independent of bedding and is produced by deformation or metamorphism.
Said of a type of rock or mineral fracture that it gives a smoothly curved surface. Conchoidal fracture is characteristic of quartz and obsidian. Etymol: like the curve of a conch (seashell).
A hard, compact aggregate of mineral matter, subspherical to irregular in shape, formed by precipitation from water solution around a nucleus, such as a shell or bone, in a sedimentary or pyroclastic rock. Concretions are generally different in composition from the rock in which they occur and represent a concentration of some minor constituent of that rock. Chert, iron oxide, and pyrite are among the common materials that form conretions.
Consisting almost entirely of the fossil skeletal parts of crinoids, esp. the plates of the stem.
Said of the texture of a rock consisting of crystals that are too small to be recognized and distinguished under the ordinary microscope; indistinctly crystalline. Also, said of a rock with such a texture.
A homogeneous, solid body of a chemical element, compound, or isomorphous mixture, having a regularly repeating atomic arrangement that may be outwardly expressed by the plane faces.
A crystal form in the isometric system enclosed by six symmetrically equivalent faces at right angles to one another.
Dendritic (den-drit´-ic)
Said of a mineral that has crystallized in a branching pattern; pertaining to a dendrite.
Pertaining to or formed from detritus; esp. said of minerals occurring in sedimentary rocks which were derived from pre-existing rocks either within or outside the basin of deposition.
A crystal form with 12 faces that are either pentagonal or rhombic. Each face is parallel to one crystallographic axis and intersects the other two at equal distances.
Said of a mineral grain that is completely bounded by its own rational faces, and whose growth has not been restrained or interfered with by adjacent grains.
One of the sediments that are deposited from aqueous solution as a result of extensive or total evaporation. Examples include anhydrite, rock salt, and various nitrates and borates.
1. The planar arrangement of textural or structural features in any type of rock; esp. the planar structure that results from flattening of the constituent grains of metamorphic rock. 2. The layered structure produced in the ice of a glacier by plastic deformation.
1. The way in which a mineral breaks, other than along planes of cleavage, e.g. conchoidal fracture. 2. A crack, joint, fault, or other break in rocks. 3. Any rupture in fast ice or pack ice, from a few meters to many kilometers in length. 4. Deformation due to a momentary loss of cohesion or of resistance to differential stress and a release of stored elastic energy.
Said of a rock or mineral that is easily crumbled, e.g. a poorly cemented sandstone.
1. The process of liquefying a solid by addition of heat; melting. 2. The unification of two or more substances, as by melting together. 3. The combination of two light nuclei to form a heavier nucleus. The reaction is accompanied by the release of a large amount of energy.
Said of a formerly glacier-covered land surface, esp. one that has been modified by the action of a glacier or an ice sheet.
1. A textural term applied to holocrystalline rocks made up of grains of nearly the same size and in the range 2 to 10 mm. 2. Also applied to a sedimentary rock made up of grains or granules.
The characteristic crystal form or combination of forms of a mineral.
One of the six crystal systems, characterized by one unique axis of threefold or sixfold symmetry that is perpendicular and unequal in length to three identical axes that intersect at angles of 120°. This definition includes the trigonal system.
Pertaining to hot water, to the action of hot water, or to the products of this action, such as a mineral deposit precipitated from a hot aqueous solution; also said of the solution itself. Fracture in rocks in which minerals have been deposited from hot magmatic emanations rich in water.
H
A type of oxide characterized by the linkage of a metallic element or radical with the ion OH, such as brucite, Mg(OH)2.
Said of a rock or mineral that solidified from molten material, i.e. from a magma; also, applied to the processes related to the formation of such rocks. Igneous rocks constitute one of three main classes into which rocks are divided, the others being metamorphic and sedimentary. Etymol: Latin ignis, "fire."
A blanket deposit of buff-colored calcareous silt, homogeneous, non-stratified, weakly coherent, porous, and friable. A rude vertical parting allows it to stand in steep or vertical faces. Loess covers wide areas in Northern Europe, eastern China, and the Mississippi Valley. It is considered to be windblown dust of the Pleistocene age.
The reflection of light from the surface of a mineral, described by its quality and intensity; the appearance of a mineral in reflected light. Terms such as metallic or resinous refer to the general appearance; terms such as bright or dull refer to intensity.
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1. Pertaining to a metal. 2. Said of a type of luster that is characteristic of metals.
Any rock derived from pre-existing rocks by mineralogical, chemical, and/or structural changes, essentially in the solid state, in response to marked changes in temperature, pressure, shearing stress, and chemical environment, generally at depth in the earth's crust.
1. Consisting of, containing, or pertaining to mica; e.g. a "micaceous sediment." 2. Resembling mica; e.g. a "micaceous mineral" capable of being easily split into thin sheets, or a "micaceous luster."
One of six crystal systems, characterized by either a single twofold axis of symmetry, a single plan of symmetry, or a combination of the two. Crystals belonging to this system are referred to three unequal crystallographic axes, two of which intersect obliquely and the third perpendicular to the plane formed by the other two.
A small rounded lump of a mineral or mineral aggregate, normally without internal structure, contrasting in composition with the rock matrix in which it was imbedded. 1. Composed of nodules, e.g. a nodular limestone. 2. Having the shape of nodules, e.g. a nodular ore.
A sedimentary rock composed chiefly of nummulitic shells; specif. the "Nummulite Limestone;" a thick, distinctive, and widely distributed Eocene formation stretching from the Alps and northern Africa to China and eastern and southern Asia, composed esp. of the remains of the genus Nummulites.
Mineral cleavage parallel to the faces of the octahedron, as in fluorite.
A sedimentary rock, usually a limestone, made up chiefly of oöliths (one of many small rounded accretionary bodies in a sedimentary rock, resembling fish eggs, with a diameter of 0.25 to 2.0 mm. It is generally formed of calcium carbonate, in concentric layers around a nucleus such as a sand grain.)
cemented together.
The process of combining with oxygen.
1. Sedimentary rock, commonly a limestone, made up chiefly of cemented pisoliths; a coarse-grained oölite. 2. A term often used for pisolith.
1. Said of a sedimentary particle whose length is 1.5 to 3 times its width. 2. Said of a crystal with one dimension markedly longer that the other two; also, said of a metamorphic texture characterized by such crystals.
The deflection of a ray of light or of an energy wave (such as a seismic wave) due to its passage from one medium to another of differing density, which changes its velocity.
Kidney-shaped. Said of a crystal structure in which radiating crystals terminate in rounded masses; also said of mineral deposits having a surface of rounded, kidneylike shapes.
Pertaining to or crystallizing in rhombohedrons. System: a division of the hexagonal system in which the unit cell is a rhombohedron.
A closed crystal form whose faces are scalene triangles; the hexagonal scalenohedron has twelve faces, and the tetragonal scalenohedron has eight.
A strongly foliated crystalline rock, formed by dynamic metamorphism that has well-developed parallelism of more than 50% of the minerals present, particularly those of lamellar or elongate prismatic habit, e.g. mica and hornblende.
A layered rock resulting from consolidation of sediment, (solid material settled from a state of suspension in a liquid; fragmental material transported and deposited by wind, water, or ice, chemically precipitated from solution, or secreted by organisms that forms layers in loose unconsolidated form, e.g. sand, mud, till) e.g. a clastic rock such as sandstone, a chemical rock such as rock salt, or an organic rock such as coal.
A compound whose crystal structure contains SiO4 tetrahedra, either isolated or joined through one or more of the oxygen atoms to form groups, chains, sheets, or three-dimensional structures with metallic elements. Silicates are classified according to crystal structure.
The ratio of the weight of a given volume of a substance to the weight of an equal volume of water.
1. A cylindrical or conical deposit of mineral matter that hangs from the ceiling of a cave, deposited from drops of water. It is usually composed of calcite. 2. A conical formation of lava hanging from the roof of a lava tunnel, developed by the dripping of fluid lava.
The color of a mineral in its powdered form, usually obtained by rubbing the mineral on a streak plate and observing the mark it leaves. Streak is an important characteristic in mineral identification.
One of multiple scratches or minute lines, generally parallel, inscribed on a rock surface by a geological agent, e.g. glaciers, streams, or faulting.
A mineral compound characterized by the linkage of sulfur with a metal, such as galena, PbS, or pyrite, FeS2. Tabular (tab´-u-lar) 1. Said of a feature having two dimensions that are much larger or longer than the third, such as an igneous dike, or of a geomorphic feature having a flat surface, such as a plateau. 2. Said of the shape of a sedimentary body whose width/thickness ratio is greater than 50 to 1, but less than 1000 to 1; blanket deposit. 3. Said of a sedimentary particle or a crystal form that shows one dimension markedly smaller than the other two; prismatic. 4. Said of a metamorphic texture in which a large proportion of grains are tabular and have approximately parallel orientation.
One of the six crystal systems characterized by three mutually perpendicular axes, the vertical one of which is a fourfold rotation or symmetry axis; it is longer or shorter that the two horizontal axes, which are of equal length.
A crystal system of threefold symmetry that is often considered as part of the hexagonal system since the lattice may be either hexagonal or rhombohedral.
A rational intergrowth of two or more single crystals of the same mineral in a mathematically describable manner, so that some lattices are parallel whereas others are in reversed position. The symmetry of the two parts may be reflected about a common plane, axis, or center.
Having the luster and appearance of glass.
In coal, those substances, other than moisture, that are given off as gas and vapor during combustion.
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