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 that consists predominantly of light-colored minerals and more than 66% free or combined silica.

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 resulting in the formation of new minerals.

amorphous

Applied to rocks and minerals that lack definite crystal structure.

amphiboles

A group of closely related, dark-colored rock-forming silicate minerals.

amygdaloidal rock

A volcanic rock containing numerous gas cavities filled with secondary minerals.

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.

arsenates

Minerals in which the arsenate radical (AsO4) is an important constituent.

basic rock

An igneous rock with low silica content and a high percentage of pyroxene, hornblende, and labradorite.

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.

bituminous rocks

Rocks that contain asphalt, tar, or petroleum.

borates

A group of minerals in which the borate radical (BO3) is an important constituent.

botryoidal

Resembling a bunch of grapes; describes certain minerals with radiating crystals arranged in massive clumps.

calcareous

Containing calcium carbonate or calcite.

calcic

Containing calcium.

carbonaceous

Composed largely of organic carbon.

carbonates

Minerals in which the carbonate radical (CO3) is an important constituent.

cataclastic rock

A metamorphic rock produced by the crushing and grinding of preexisting rocks.

cataclastic metamorphism

A metamorphism due to directed pressure, resulting in rocks with cataclastic texture.

characterizing accessory mineral

A mineral that gives a specific name to an igneous rock.

chemical sedimentary rock

A rock formed by chemical processes.

clastic rock

A sedimentary rock that is made up of fragments of preexisting rocks.

clay

A soft sediment or deposit that is very plastic when wet.

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 of the surrounding rocks.

concretion

An accumulation of mineral matter formed into an orderly, rounded form.

cross-bedding

An arrangement of layers within a sedimentary rock at an angle to the main layers.

crystal

A solid mass of mineral with a regular geometric shape bounded by smooth surfaces.

crystal habit

The actual form of a crystal determined by the shape and proportions of the crystal faces.

crystal symmetry

The repeat pattern of crystal faces caused by the ordered internal arrangement of atoms.

detrital sediment

A deposit of mineral and rock fragments 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 that cuts across the layers of surrounding rocks.

discordant

Describes an intrusive igneous body whose margins cut across the bedding of surrounding rocks.

dynamothermal metamorphism

Metamorphism resulting from the combined effects of heat and pressure.

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 used to classify and name it.

extrusive rock

An igneous rock that solidifies on the surface of the earth.

feldspar

A group of abundant rock-forming silicate minerals.

feldspathic rock

A rock that contains feldspar as a principal constituent.

feldspathoids

A group of minerals similar to feldspars but containing less silica.

ferruginous

Containing iron.

flow banding

A structure in volcanic rocks consisting of alternating layers of unlike mineral composition.

foliation

The laminated structure present in regionally metamorphosed rocks.

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 in alternating streaks and bands.

groundmass

The fine-grained material that surrounds larger crystals in a porphyritic or sedimentary rock.

halides

A group of minerals that are primarily compounds of halogen elements.

hardness

Resistance of a mineral to abrasion or scratching.

hydrothermal alteration

An alteration of minerals or rocks by superheated mineral-rich fluids.

hydrothermal metamorphism

Changes in the structure or composition of rock, caused by hydrothermal fluids.

hydrothermal replacement

A change in a rock or mineral deposit due to addition or removal of minerals by hydrothermal fluids.

hypothermal vein

A vein formed at relatively great depth and high temperatures.

igneous

Rock formed by the solidification of magma.

intermediate rock

Igneous rock that is transitional between acidic and basic rock.

intrusive

An igneous rock that formed underground from magma.

"Japanese" twins

Simple contact twins in quartz joined in the same plane at an angle.

laccolith

A lens-shaped body of igneous rock with a dome-shaped upper surface.

lamellar

Composed of thin layers, plates, or scales.

laminated rocks

Sedimentary rocks 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.

luster

The surface appearance of a substance.

magma

Molten rock material beneath the solid crust of the earth.

magmatism

Any process by which magma solidifies into volcanic or plutonic rock.

massive mineral

A mineral that occurs without any definite external crystal form.

matrix

The fine-grained material that surrounds larger crystals or particles in a rock.

mesothermal vein

A vein that forms at intermediate depth and temperature.

metamorphic

Any rock that was formed from a preexisting rock through heat, pressure, or fluids.

mica

A group of soft silicate minerals that can easily be split into thin sheets.

microcrystalline rock

A rock whose crystals are too small to be seen without a microscope.

mineral environment

The rock type in which a mineral forms and occurs.

mineral stability

The ability of a mineral to remain unaltered over a range of pressure and temperature.

molybdates

A group of minerals in which the molybdate radical is an important constituent.

nodular

Having the shape of or composed of irregular lumps of rock or mineral.

organic compounds

Compounds produced in or by plants and animals containing carbon.

outcrop

A mass of bedrock that is exposed at the surface.

oxides

A group of minerals in which oxygen combined with a metal is a major constituent.

oxidized zone

The part of an ore body that has been altered by downward percolating groundwater.

parting

The tendency of some minerals to separate along certain planes.

pegmatite

An igneous rock of extremely coarse grain size, often excellent for large crystals.

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 essential minerals can be distinguished with the unaided eye.

phenocryst

A prominent crystal surrounded by smaller mineral grains in a porphyritic rock.

phosphates

A group of minerals in which the phosphate radical 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 concentrated mechanically.