Rocks and Minerals/Definitions

=Definitions= Copied shamelessly from the National Audubon Society Field Guide to North American Rocks and Minerals


 * accessory mineral:a mineral that occurs in a rock in minute quantities, and does not affect the way the rock is named for classified


 * acidic rock:a type of igneous rock (i.e. granite) that consists predominatly of light-colored minerals and more than 66% free or combined silica (see basic rock, intermediate rock, and ultrabasic rock)


 * alkaline rock:a rock containing more than average amounts of potassium- and sodium-bearing minerals


 * alteration:any physical or chemical change in a mineral or rock subsequent to original formation; usually results in the formation of new minerals or in textural changes in the rock


 * amorphous:literally, "without form"; applied to rocks and minerals that lack definite crystal structure


 * amphiboles:a group of closely related, dark-colored rock-forming silicate minerals (e.g. actinolite, hornblende, glaucophane, and tremolite)


 * amygdaloidal rock:a volcanic rock containing numerous gas cavities (amygdules) filled with such secondary minerals as calcite, quartz, and zeolites.


 * amygdule:a mineral-filled cavity formed in an igneous rock by escaping gas


 * anthracite coal:a hard, black, lustrous coal


 * aphanitic rock:a rock in which the crystalline constituents are too small to be distinguished with the unaided eye


 * argillaceous:containing or composed largely of clay (e.g. argillaceous shale, etc.)


 * arsenates:minerals (e.g. adamite) in which the arsenate radical (AsO4) is an important constituent


 * basic rock:an igneous rock (e.g. gabbro) with low silica content and a high percentage of pyroxene, hornblende, and labradorite (see acidic rock, intermediate rock, and ultrabasic rock)


 * batholith:a huge body of plutonic rock that has been intruded deep into the earth's crust and later exposed by erosion


 * bedding:the arrangement of sedimentary rocks in approximately parallel layers (strata, or "beds"), which correspond to the orignial sediments that formed the rock


 * bituminous rocks:rocks that contain (and sometimes smell of) asphalt, tar, or petroleum


 * borates:a group of minerals (e.g. borox, colemanite) in which the borate radical (BO3) is an important constituent


 * botryoidal:resembling a bunch of grapes; describes hematite and a number of other minerals in which very small radiating crystals are arranged in massive clumps, giving a surface covered with spherical bulges (from Greek botrys, "bunch of grapes")


 * calcareous:containing calcium carbonate or calcite


 * calcic:containing calcium


 * carbonaceous:composed largely of organic carbon (i.e. carbon derived from plant and animal tissue)


 * carbonates:minerals (e.g. calcite) in which the carbonate radical (CO3) is an important constituent


 * cataclastic rock:a metamorphic rock produced by the crushing and grinding of preexisting rocks, which are still visible as crushed and flattened minerals and as angular fragments (from the Greek klastos, "broken")


 * cataclastic metamorphism:a metamorphism due principally to directed pressure and resulting in rocks with cataclastic texture


 * characterizing accessory mineral:a mineral that gives a specific name to an igneous rock. In hornblende granite, for example, hornblende is the characterizing accessory mineral


 * chemical sedimentary rock:a rock formed by chemical processes; gypsum is a chemical sedimentary rock, formed by chemical precipitation


 * clastic rock:a sedimentary rock that is made up of fragments of preexisting rocks, transported mechanically into the place of deposition


 * clay:any soft sediment or deposit that is very plastic when wet and consists of very fine-grained, micalike materials, mainly hydrous aluminum silicates


 * cleavage:the tendency of some minerals to break along one or more regular, smooth surfaces


 * concordant:describes an intrusive igneous body whose surfaces are parallel to the bedding (or foliation) of the surrounding rocks


 * concretion:an accumulation of mineral matter, formed when particles of silica, pyrite, gypsum, etc. become cemented together into an orderly, rounded, often artificial-looking form


 * cross-bedding:an arrangement of layers within a sedimentary rock, such that minor layers lie at an angle to the main layers of sediment; usually a sign of changing wind or water currents action on the orignial sediments forming the rock


 * crystal:a solid mass of mineral, having a regular geometric shape and bounded by smoth, flat surfaces (crystal faces)


 * crystal habit:the actual form of a crystal; determined by the shape and relative proportions of the crystal faces


 * crystal symmetry:the repeat pattern of crystal faces, caused by the ordered internal arrangement of a mineral's atoms


 * detrital sediment:a deposit of mineral and rock fragments that have been transported to their place of deposition


 * differentiation:a process by which different types of igneous rocks are derived from the same parent magma


 * dike:a wall-like body of igneous rock, usually intrusive, that cuts across the layers of surrounding rocks


 * discordant:describes an intrusive igneous body whose margins cut across the bedding (or foliation) of the surrounding rocks


 * dynamothermal metamorphism:metamorphism resulting from the combined effects of heat and pressure; also called regional metamorphism


 * epithermal vein:a vein formed at shalow depths from ascending hot solutions


 * equigranular rock:a rock whose mineral particles are of the same general size


 * essential minerals:the mineral constituents of a rock (usually an igneous rock) that are used to classify and name the rock


 * extrusive rock:an igneous rock that solidifies on the surface of the earth


 * feldspar:a group of abundant rock-forming silicate minerals, including orthosclase and microcline (potash feldspars) and albite, oligoclase, andesine, labradorite, bytownite, and anorthite (the plagioclase, or soda-lime, feldspars)


 * feldspathic rock:a rock that contains feldspar as a principal constituent


 * feldspathoids:a group of minerals (e.g. leucite, nepheline, and sodalite) that are similar in composition to the feldspars, but contain less silica


 * ferruginous:containing iron


 * flow banding:a structure (in some volcanic rocks) consisting of alternating layers of unlike mineralogical composition and formed as a result of flowing lava


 * foliation:the laminated structure present in regionally metamorphosed rocks that results from segregation of different minerals into roughly parallel layers


 * fragmental rock:sedimentary rock consisting of rock and meineral fragments


 * friable:easily crumbled or pulverized


 * fusion:the process of being melted or dissolved by heat


 * gneissose rock:A rock that has the banded appearance of a gneiss but is not formed by metamorphism


 * granulose:a metamorphic texture characterized by granular minerals, such as quartz, feldspars, and garnet, in alterating streaks and bands


 * groundmass:see matrix


 * halides:a group of minerals (e.g. halite and flourite) that are primarily compounds of the halogen elements: bromine, chlorine, flourine, and iodine


 * hardness:resistance of a mineral to abrasion or scratching


 * hydrothermal alteration:an alteration of minerals or rocks by the action of superheated mineral-rich fluids, usually water that has been heated to very high temperatures within a crystallizing magma


 * hydrothermal metamorphism:changes in the structure or composition of rock, caused by the action of hyrdrothermal fluids


 * hydrothermal replacement:a change in a rock or mineral deposit due to the addition or removal of minerals by hydrothermal fluids


 * hypothermal vein:a vein formed at relatively great depth and at relatively high temperatures (300-500 C)


 * igneous:rock formed by the solidification of magma


 * intermediate rock:igneous rock (e.g. syenite or diorite) that is transitional between acidic and basic rock, having a silica content of between 54% and 65% (see acidic rock, basic rock, and ultrabasic rock)


 * intrusive:an igneous rock that formed underground, from magma that was squeezed into cracks or crevices, or between layers of older rocks


 * "Japanese" twins:simple contact twins (in quartz) in which two single crystals, usually boroad flattened prisms, are joined in the same plane at an anble of about 84 degrees


 * laccolith:a lens-shaped body of igneous rock with a dome-shaped upper surface and a flat bottom surface, and with both surfaces parallel to the bedding or foliation of the enclosing rocks


 * lamellar:composed of thin layers, plates, or scales


 * laminated rocks:sedimentary rocks that are formed of numerous very thin layers


 * lava:molten rock material extruded onto the surface of the earth


 * lava flow:a body of rock fromed by a single outpouring of lava


 * lenticular:lens-shaped


 * lopolith:a large, lenticular, centrally sunken mass of igneous rock whose surfaces are concordant with the enclosing rocks


 * luster:the surface appearance of a substace, or the manner in which it reflects light


 * magma:molten rock material, beneath the solid crust of the earth, that solidifies to form igneous rocks at or below the earth's surface


 * magmatism:any process by which magma solidifies into volcanic or plutonic rock


 * massive mineral:a mineral that occurs either without any definite external crystal form or in poorly defined masses of small crystals


 * matrix:the fine-grained material (groundmass) that surrounds the larger crystals or particles in a porphyritic or sedimentary rock. Also, any material, such as clay or rock, in which a crystal, fossil, etc. is embedded


 * mesothermal vein:a vein that forms at intermediate depth and temperature


 * metamorphic:any rock (e.g. schist, gneiss, etc.) that was formed in some fashion from a preexisting rock, trough heat, pressure, the effect of superheated fluids, or any combinatino of these forces


 * mica:a group of solf silicate minerals (e.g. biotite, muscovite) that have perfect basal cleavage in one direction and can easily be split into characteristic thin, elastic, pearly sheets


 * microcrystalline rock:a rock whose crystals are too small to be seem without a microscope (see aphanitic rock)


 * mineral environment:the rock (or rock type) in which a mineral or a group of associated minerals forms and occurs. Mineral environments include igneous, sedimentary, and metamorphic rocks as well as several types of veins and replacement depostis


 * mineral stability:the ablility of a mineral to remain unaltered over a stated range of pressure and temperature


 * molybdates:a group of minerals (e.g. wulfenite) in which the molybdate radical (MoO4) is an important constituent


 * nodular:having the shape of or composed of irregular lumps of rock or mineral (e.g. nudular chert)


 * organic compounds:compounds produced in or by plants and animals and containing carbon as the essential ingredient


 * outcrop:a mass of bedrock that is exposed at the surface of the earth


 * oxides:a group of minerals (e.g. cuprite and magnetite) in which oxygen, combined with a metal, is a major constituent


 * oxidized zone:the part of an ore body (usually the upper part) that has been altered by downward percolating groundwater, containing dissolved oxygen and carbon dioxide


 * parting:the tendency of some minerals to separate along certain planes, which are not related to the crystal symmetry of the mineral; usually due to twinning or deformation


 * pegmatite:an igneous rock of extremely coarse grain size. Usually found as dikes within a larger plutonic or metamorphic rock mass, pegmatites are often excellent sources of large, fine crystals, especially of quartz, tourmaline, feldspar, and mica


 * petrography:the branch of geology that deals with the description and classification of rocks


 * petrology:the study of rocks, specifically their composition, origin, and modes of occurence


 * phaneritic rock:an igneous rock in which all of the essential minerals can be distinguished with the unaided eye


 * phenocryst:a prominent crystal (in a porphyritic rock) surrounded by smaller mineral grains


 * phosphates:a group of minerals (e.g. apatite) in which the phosphate radical (PO4) is an improtant constituent


 * pipe:a vertical, cylindrical mass of igneous rock


 * pisolitic:consisting of rounded grains like peas or beans


 * placer:a deposit of heavy mineral particles (e.g. gold) that have weathered out of the bedrock and been concentrated mechanically, usually by the action of streams


 * playa:a desert plain; a shallow basin in which water collects following a rain and is evaporated


 * plug:the solidified core of an extinct volcano


 * pluton:any deep, intrusive igneous body, of any size, whose exact form has not been determined


 * plutonic rock:a granular igneous rock that has solidified at great depth and shows distinct grain texture (e.g. granite, granodiorite)


 * porphyritic rock:an igneous rock in which larger crystals (phenocrysts) are enclosed in a fine-grained groundmass, which may be crystalline or glassy


 * precipitation:the process by which a suspended or dissolved solid is separated out of a liquid


 * psuedomorph:a mineral that has taken the outward crystal form of a different mineral


 * pyroclastic:an extrusive rock made up of material ejected explosively from a volcanic vent


 * pyroelectricity:an electric charge produed in a crystal by heat


 * pyroxenes:a group of closely related, dark-colored rock-forming minerals (e.g. augite, diopside, and acmite)


 * radical:a group of oxygen atoms clustered about a nonmetallic atom (e.g. silicon, phosphorus, sulfur) so as to form a "structural unit" that behaves like a single atom


 * replacement:a process by which one mineral replaces another, while often retaining the physical form of the first mineral


 * secondary minerals:minerals that are formed by the alteration of preexisting (primary) minerals


 * sedimentary:a layered rock, formed through the accumulation and solidification of sediments, which may originally be made up of minerals, rock debris, or animal or vegetable matter


 * silica:silicon dioxide (SiO2), a tremendoulsy abundant mineral that occurs widely an in many forms, including quartz, chalcedony, opal, and chert


 * silicates:a group of minerals (e.g. quartz, and orthoclase) composed essentially of SiO4 tetrahedra in different arrangements


 * sill:a tabular, sheetlike body of intrusive igneous rock, which has been injected between layers of sedimentary or metamorphic rock


 * slaty cleavage:a variety of foliation typical of slates and characterized by parallel arrangement of clay minerals


 * stock:a small, irregularly shaped body of intrusive igneous rock with a surface area of less than 65 square kilometers


 * streak:the color of the powder of a mineral produced by rubbing the mineral over the surface of a piece of unglazed, white porcelain


 * striations:minute parallel grooves or narrow channels on crystal faces


 * structure:large features of rock masses, such as flow banding and bedding; in minerals, structure refers to the shapes and forms of crystal groups and masses


 * sulfates:a group of minerals (e.g. gypsum and barite) in which the sulfate radical (SO4) is an important constituent


 * sulfides:a group of minerals (e.g. pyrite, galena, and sphalerite) in which sulfur is in combination with one or more metals (copper, iron, or zinc)


 * sulfosalts:a group of minerals in which sulfer combines with the semi-metals (arsenic, anitmony, and bismuth) to form what are called negative ions/anions, which then combine with the metals (lead, silver, copper, and zinc) to form such minerals as enargite, tetrahedrite, and pyrargyrite


 * tenacity:the ability of a substance to resist separation


 * texture:the surface appearance of a homogeneous rock or mineral aggregate. The degree of crystallization, the size of the crystals, and the shape and interrelations of the crystals or other components all contribute to the texture of a rock


 * tungstates:a group of minerals (e.g. scheelite) in which the tungstate radical (WO4) is an important constituent


 * twin:a specimen that consists of two or more single crystals of the same mineral, intergrown in a definite systematic arrangement


 * ultrabasic rock:any plutonic igneous rock (e.g. peridotite) with very low silica content (less than that of a basic rock) (see acidic rock, basic rock, and intermediate rock)


 * vanadates:a group of minerals (e.g. vanadinite) in which the vanadate radical (VO4) is an important constituent


 * vein:a tabular or sheetlike body of mineral matter (e.g. quartz) cutting across preexisting rock (e.g. granite or gneiss)


 * vesicle:a small cavity in a volcanic rock


 * vitreous:glasslike in appearance and texture


 * volcanism:the movement of molten rock, by way of volcanoes, fissures, vents, etc., to the earth's surface, where it cools into extrusive (volcanic) igneous rocks


 * volcano:a vent or hole in the earth's crust through which magma (in the form of lava), gases, ashes, and other products escape to the surface


 * zeolites:a group of hydrated aluminosilicates of potassium, sodium, and calcium that can lose part or all of their water (reversibly) without changing crystal structure. This open structure allows the zeolites to absorb other compounds, making them commercially valuable as purifiers and water softeners