Rocks and Minerals Vocabulary

accessory mineral

a mineral that occurs in a rock in minute quantities, and does not affect the way the rock is named or 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 original 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 original sediments forming the rock

crystal

a solid mass of mineral, having a regular geometric shape and bounded by smooth, 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 shallow 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 mineral 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 alternating 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, fluorine, 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 broad flattened prisms, are joined in the same plane at an angle 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 formed 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 substance, 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 combination of these forces

mica

a group of soft 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 deposits

mineral stability

the ability 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. nodular 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 occurrence

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 important 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 produced 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 tremendously 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 sulfur 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