Geology: Minerals, Rocks, and Igneous Processes

Vocabulary practice flashcards defining core terms and concepts in mineralogy, crystalline structures, silicate classification, rock fundamentals, and igneous petrology from Chapters 5, Interlude A, and Chapter 6.

Mineralogist

A geoscientist specializing in the study of minerals, discovering roughly 50 new minerals each year.

Mineralogy

The scientific study of minerals and their characteristics, including critical minerals, ore minerals, and gem minerals.

Mineral

A naturally occurring, solid, inorganic substance with a definable chemical composition and an ordered internal crystalline structure.

Biogenic minerals

Minerals created by living organisms, such as vertebrate bones and teeth, or oyster, mussel, and clam shells.

Unit Cell

The smallest group of atoms making up a unique crystalline pattern that repeats in three dimensions and produces the shape of a crystal.

Constancy of interfacial angles

The rule stating that equivalent faces found on two different-sized samples of the same mineral always bear the exact same angular relationship to one another.

Diffraction

The splitting of light into many tiny beams that interfere with one another, occurring when waves interact with regularly spaced atomic planes whose spacing approaches the wavelength of the waves.

Polymorph

Two or more minerals that share identical chemical compositions but possess completely different internal crystal structures, such as diamond and graphite.

Coordination polyhedra

The geometric packing configurations (such as cubic, tetrahedral, or octahedral) defined by how anions pack tightly around a central cation based on relative ion sizes.

Solidification

A method of mineral formation in which a liquid or melt cools and freezes into a solid as atoms attach to one another.

Solid-state diffusion

The very slow movement of atoms or ions through a heated and/or compressed solid to rearrange into a new crystal structure without melting having occurred.

Biomineralization

A mineral formation process in which living organisms cause minerals to precipitate within, on, or immediately adjacent to their bodies.

Euhedral crystal

A mineral crystal grown in an open cavity with unimpeded space, allowing well-formed, flat crystal faces to develop.

Anhedral crystal

A mineral crystal grown in a confined or tight space where lack of room prevents the development of natural crystal faces.

Streak

The color of a powder produced by pulverizing a mineral or scraping it across an unglazed porcelain plate (applicable for minerals with hardness $< 6$).

Luster

The way a mineral surface scatters or reflects light, classified broadly into metallic and nonmetallic varieties.

Mohs hardness scale

A relative scale of ten reference minerals ranked from 1 (talc) to 10 (diamond) used to measure the scratch resistance of bonds within a crystal structure.

Specific Gravity

The density of a mineral represented by the ratio of the mineral weight relative to the weight of an equal volume of water.

Crystal habit

The characteristic external shape of an individual crystal or aggregate of crystals that grew unimpeded, reflecting internal atomic geometry and directional growth rates.

Cleavage

The tendency of a mineral to break cleanly along preferred planar surfaces where atomic bonds are relatively weaker.

Conchoidal fracture

Smoothly curving, clamshell-shaped fracture surfaces produced when minerals with equal bond strength in all directions break without cleavage.

Silica tetrahedron

The fundamental anionic building block of all silicate minerals, consisting of four oxygen atoms surrounding a single silicon atom ($SiO_4^{4-}$).

Orthosilicates

Silicate minerals (such as olivine and garnet) composed of isolated silica tetrahedra that share no oxygen atoms and are linked together entirely by positive cations.

Single-chain silicates

Silicate minerals (such as pyroxenes) in which each silica tetrahedron links to its neighbors by sharing two oxygen atoms into an extended linear chain.

Double-chain silicates

Silicate minerals (such as amphiboles) in which tetrahedra alternately share two and three oxygen atoms to form parallel double chains.

Sheet silicates

Silicate minerals (such as micas) in which tetrahedra share three oxygen atoms to form two-dimensional flat sheets characterized by one dimension of perfect cleavage.

Framework silicates

Silicate minerals (such as quartz and feldspars) in which all four oxygen atoms of every silica tetrahedron are shared in an interlocking three-dimensional network.

Rock

A coherent, naturally occurring solid consisting of an aggregate of minerals or a body of glass.

Clastic rock

A sedimentary rock composed of preexisting rock and mineral fragments cemented together by natural mineral cement.

Crystalline rock

A rock consisting of mineral grains that grew when a melt solidified or through recrystallization, interlocking together like jigsaw puzzle pieces.

Bedrock

Solid rock that remains directly attached to the Earth's crust underneath soil or surficial debris.

Outcrop

A surface exposure of bedrock visible on the Earth's surface.

Equant and inequant grains

Descriptions of mineral grain geometries, where equant grains exhibit equal dimensions in all directions and inequant grains have unequal dimensions.

Bedding

Layering or stratification visible in sedimentary rocks, typically defined by differences in grain size or mineral composition.

Foliation

A planar fabric defined by parallel surfaces or compositional layers developed in a rock in response to metamorphism.

Thin Section

A $0.03\,\text{mm}$-thick slice of rock mounted on a glass slide and examined with a petrographic microscope under plane-polarized and cross-polarized light.

Extrusive igneous rock

Igneous rock formed when molten rock cools rapidly at or near the surface, typically precluding the growth of large crystals.

Intrusive igneous rock

Igneous rock formed when molten magma slowly crystallizes underground at depth, allowing large mineral crystals to grow.

Geotherm

A curving graph profile illustrating how temperature steadily increases with increasing depth and pressure in the Earth's interior.

Decompression melting

The generation of melt caused when hot mantle rock rises to shallower depths and lower pressures without significant loss of heat.

Flux melting

The melting of hot solid rock that occurs when volatile substances (especially water) inject into the mantle rock, lowering its melting temperature.

Heat-transfer melting

Melting of continental or crustal rocks triggered when rising hot mantle magma transfers its thermal energy directly into the surrounding wall rock.

Magma categories

The four compositions of magma divided by silica weight percentage: felsic ($67\text{--}76\%$), intermediate ($53\text{--}66\%$), mafic ($46\text{--}52\%$), and ultramafic ($38\text{--}45\%$).

Partial melting

The incomplete melting of a rock where minerals with lower melting temperatures (silica-rich) melt first, producing a liquid more felsic than the source residue.

Assimilation

The process of magma contamination in which blocks of country wall rock fall into a magma chamber and dissolve, changing the magma's chemistry.

Magma mixing

The combination and mutual dissolution of two chemically distinct magmas within a common magma chamber to produce a single intermediate composition.

Viscosity

The resistance of a liquid to flow, controlled in magma by temperature, silica concentration, and dissolved volatile content.

Fractional crystallization

The process during magma cooling where early-formed mafic crystals (olivine, Ca-rich plagioclase) settle or separate, leaving the remaining melt progressively enriched in silica.

Dike

A tabular, discordant igneous intrusion that cuts vertically or obliquely across preexisting wall rock layering.

Sill

A nearly horizontal tabular igneous intrusion injected parallel to the preexisting bedding or stratification of country rock.

Laccolith

A blister-shaped igneous intrusion formed when magma injects between sedimentary layers and bows the overlying strata upward into a dome.

Pluton

An irregular or blob-shaped intrusive igneous body ranging from tens of meters to tens of kilometers in size.

Stoping

The intrusive mechanism by which magma forces its way upward as shattered blocks of wall rock break off and sink into the melt.

Xenolith

A relict piece of foreign country rock enveloped and preserved within an igneous intrusion after the magma freezes.

Batholith

An enormous composite igneous body formed by the coalescence of multiple plutons, extending up to hundreds of kilometers in length.

Aphanitic texture

A fine-grained igneous rock texture formed by rapid cooling in extrusive volcanic settings where individual crystals are too small to see with the naked eye.

Phaneritic texture

A coarse-grained igneous rock texture formed by slow cooling in subsurface intrusive plutonic settings where mineral grains are large enough to be identified unassisted.

Porphyritic texture

An igneous rock texture featuring two distinct crystal sizes: large phenocrysts formed during initial slow cooling enclosed within a fine-grained groundmass formed by subsequent rapid cooling.

Tuff

A fragmental pyroclastic rock formed by the welding or consolidation of fine volcanic ash deposited during an explosive eruption.

Obsidian

A solid glassy felsic rock devoid of vesicles, formed by rapid cooling of silica-rich lava.

Large igneous province (LIP)

A vast geographic region where extraordinarily large volumes of mafic lava or pyroclastic ash erupted over relatively short geologic intervals, typically driven by mantle plume heads.

Flood basalts

Extensive sheets of low-viscosity basaltic lava erupted in huge volumes from fissures or volcanic vents that spread over vast continental surface areas.