Textures of Igneous Rocks

Primary textures

These occur during igneous crystallization and result from interactions between minerals and melt

Secondary textures

These are alterations that take place after the rock is completely solid

Petrography

This is the branch of petrology that deals with the description and classification of rocks with most modern work involving detailed study in thin section

Thin section

These are cut from rock samples cemented to microscope slides and ground down to 0.03 mm thickness so that they readily transmit light

Nucleation

This is a critical initial step in the development of a crystal and requires a critically sized embryonic cluster to overcome surface instability

Undercooling

This is cooling of a melt below the true crystallization temperature and is typically required before stable nuclei can form

Crystal growth

This involves the addition of ions onto existing crystals or nuclei with different faces growing at different rates

Diffusion

This is required for new material and heat to reach or leave the surface of a growing crystal

Homogeneous nucleation

This occurs without a pre-existing surface and needs greater undercooling

Heterogeneous nucleation

This occurs on a pre-existing crystal or surface and is easier

Epitaxis

This is the preferred nucleation of one mineral on another preexisting mineral due to structural similarity

Porphyritic texture

This has a distinctly bimodal distribution in grain size with one size considerably larger than the other

Phenocryst

These are the larger crystals in a porphyritic rock

Groundmass

This is the finer surrounding material also called matrix in porphyritic rocks

Vitrophyric

This has larger crystals set in a glassy finer material

Poikilitic

This occurs when larger crystals contain numerous inclusions of another mineral that they enveloped as they grew

Oikocryst

This is the host crystal in poikilitic texture

Euhedral

Crystals growing free in liquid tend to be this and nicely faceted

Subhedral

This describes crystals with only partially developed faces

Anhedral

This describes crystals with faces entirely absent

Interstitial

The latest minerals to form are this filling the spaces between the earlier ones

Cumulate texture

Early-forming crystals of a single mineral accumulate to the extent that they are in mutual contact with the remaining liquid occupying the spaces between the crystals

Orthocumulate

This has other minerals occupying the interstitial areas with little exchange

Adcumulate

This allows components to escape so early minerals fill most available space

Mesocumulate

This is intermediate between ortho and ad types

Heteradcumulate

This is a type of adcumulate with large enclosing crystals that nucleate poorly

Compositional zoning

This is common and occurs when a mineral changes composition as it grows during cooling

Normal zoning

This has a more anorthite-rich core toward a more albite-rich rim

Reverse zoning

This is the opposite with more sodic inner and calcic outer zones

Oscillatory zoning

This is the most common type because a regular decrease rarely dominates the full crystallization period

Ophitic texture

This refers to the envelopment of plagioclase laths by larger clinopyroxenes

Subophitic

This has plagioclase laths that are larger and only partially enclosed by pyroxene

Intergranular

This has small discrete grains of pyroxene olivine filling the interstices in a random network of larger plagioclase laths

Intersertal

This occurs when glass or alteration products occupy the spaces between plagioclase laths

Hyalo-ophitic

This has an intersertal texture in which a larger amount of glass is present than pyroxene

Hyalopilitic

This has a large amount of glass with crystals occurring only as tiny microlites

Granophyric texture

This has a texture in which the quartz and feldspars penetrate each other as feathery irregular intergrowths

Graphic texture

This is a coarser variation where the cuneiform nature of the quartz rods in the feldspar host is readily seen

Rapakivi texture

This involves plagioclase overgrowths on orthoclase

Spherulitic texture

This in silicic volcanics has needles of quartz and alkali feldspar growing radially from a common center

Variolitic texture

This of radiating plagioclase laths occurs in some basalts

Comb structure

Growth of elongated crystals with c-axes normal to vein walls results in this structure

Crescumulate texture

This describes the parallel growth of elongated non-equilibrium arrangements of crystals that appear to nucleate on a wall or layer

Trachytic texture

This consists of microlites aligned due to flow

Pilotaxitic

Random or non-aligned microlites are called this or felty

Flow banding

This alternating layers of different composition can result from flow near chamber walls or mingling

Synneusis

This is a process in which suspended phenocrysts cluster together and adhere by surface tension

Cumulophyric

Multiple-grain clusters of adhering phenocrysts is called this texture

Glomeroporphyritic

This is synonymous with cumulophyric and sometimes used when only one mineral is involved

Sieve texture

This or deep and irregular embayments

Skeletal crystals

These resulting forms have rapid growth at edges and corners

Dendritic

This is a tree-like branching form when diffusion is slower than growth

Spinifex texture

This may develop spectacular elongated olivine crystals up to 1 m long in some quenched ultramafic lavas

Swallow-tail plagioclase

The corners of quenched plagioclase tend to create this shape

Resorption

This is the term applied to re-fusion or dissolution of a mineral back into a melt

Reaction rim

Other reactions may result from dropping pressure or compositional changes producing a mantle of new mineral

Uralitization

This is the replacement of pyroxene by amphibole

Biotitization

This is a similar process of replacement producing biotite

Chloritization

This is the alteration of any mafic mineral to chlorite

Seritization

This is the process by which felsic minerals are hydrated to produce fine white mica

Saussuritization

This is the alteration of plagioclase to produce an epidote mineral

Myrmekite

This is an intergrowth of dendritic quartz in a single crystal of plagioclase

Symplectite

This is a term applied to fine-grained intergrowths resulting from the combined growth of two or more minerals as they replace another mineral

Devitrification

This is the secondary crystallization of glass to fine-grained mineral aggregates

Felsitic texture

This has crystal form entirely suppressed and looks very much like that of chert

Spherulites

These may be found imbedded in a felsite matrix

Lithophysae

These are large cavities bordered by spherulitic growth and probably represent late volatile releases

Palagonite

This may replace water-quenched basaltic glass

Perthite

This results when unmixing produces albite lamellae in a potassic host

Antiperthite

This has lamellae of K-feldspar in a sodic host

Transformation twinning

These are caused when a high-temperature crystal structure inverts to a low-temperature polymorph

Deformation twinning

These can occur on the albite law but usually lack the straight lamellar form and are wedge shaped

Undulose extinction

This is a waviness in the optical extinction pattern due to minor bending of the crystal lattice

Ostwald ripening

This is a process of annealing of crystals in a static environment where grain boundaries migrate toward their centers of curvature

Polygonal mosaic

This equilibrium texture has similarly sized grains having straight approximately 120 triple-grain intersections

Pseudomorph

This may form when the replacement polymorph assumes the form of the original

Amygdule

These are vesicles filled with later mineral growth typically secondary zeolite carbonate or opal

Vesicular

These create subspherical voids in volcanics

Scoriaceous

This has increasing vesicle content in basalt

Pumiceous

This is typically light and frothy in silicic rocks

Holocrystalline

Consisting entirely of crystals

Hypocrystalline

Containing both crystals and glass

Holohyaline

Consisting entirely of glass

Aphanitic

Having minerals too fine grained to see with the naked eye

Phaneritic

Having minerals coarse enough to see with the naked eye

Cryptocrystalline

Having minerals too fine grained to distinguish microscopically

Equigranular

Having grains that are all approximately the same size

Inequigranular

Having grains that vary considerably in size

Hiatal porphyritic

Having a pronounced difference in size between the larger and finer phases

Seriate

Having a continuous gradation in size

Aphyric

Having no phenocrysts

Panidiomorphic

Having a majority of euhedral grains

Hypidiomorphic

Consisting predominantly of subhedral grains

Allotriomorphic

Having a majority of anhedral grains

Sutured

Characterized by articulation along highly irregular interpenetrating boundaries

Miarolitic

Having gas cavities into which euhedral minerals protrude

Perlitic

Having a concentric fracture pattern resulting from contraction of some volcanic glasses

Fiamme

Compressed pumice fragments in a tuff

Accretionary lapilli

These form when ash falling through moist air accumulates successive layers on a single nucleus

Eutaxitic

These structures caused by compression and deformation in hot ash accumulations