Igneous Petrology Terms

These definitions are from Principles of Igneous and Metamorphic Petrology, 3rd ed. (2022) by Anthony Philpotts and Jay Ague

Phenocryst

A large crystal set in a fine matrix

Microphenocryst

A microscopic crystal that is larger than the remainder of the groundmass

Megacryst

An unusually large crystal, either a phenocryst or a xenocryst

Poikilitic

The state of a host phenocryst containing many inclusions of other minerals

Oikocryst

A host phenocryst in a poikilitic texture

Groundmass (Matrix, Mesostasis)

The glassy or finer-grained element in the porphyritic texture

Cumulophyric

Having phenocrysts of the same or different minerals occurring in clusters

Glomeroporphyritic

Synonymous with cumulophyric (used by some to specify that only one mineral is involved)

Hiatial porphyritic

Having a pronounced difference in size between phyric and groundmass phases

Seriate

Having a continuous gradation in size

Aphyric

Not porphyritic (having no phenocrysts)

Euhedral (idiomorphic)

Completely bounded by crystal faces

Subhedral (subidiomorphic)

Having crystal faces that are only partially developed

Anhedral (allotriomorphic)

Having crystal faces that are entirely absent

Corroded (embayed)

Subhedral or anhedral and produced by partial melting (resorption) of phenocrysts by the melt

Panidiomorphic

Having a majority of euhedral grains (rare)

Hypidiomorphic

Consisting predominantly of subhedral grains; common in many granitic rocks

Allotriomorphic

Having a majority of anhedral grains (common)

Sutured

Characterized by articulation along highly irregular interpenetrating boundaries (common in recrystallized deformed rocks)

Mosaic

A texture of polygonal equigranular crystals

Host (oikocryst)

A large mineral that includes others in poikilitic texture

Guest

The included mineral in poikilitic texture

Poikilitic

Containing several small discrete crystals of another mineral; refers to growth phenomena, not exsolution or replacement

Graphic

Having an intergrowth in which the guest shows angular wedge-like forms; usually occurs with quartz in microcline

Micrographic

Having graphic texture that is visible only under the microscope

Granophyric

Having a texture in which the quartz and feldspars penetrate each other as feathery irregular intergrowths; resembles micrographic texture, but is more irregular

Exsolution lamellae

Lamellar bands of a phase exsolved from a host phase

Perthitic

Having irregular veins, patches, lenses of sodic plagioclase in an alkali feldspar host; usually results from exsolution

Antiperthitic

Having exsolution lamellae of alkali feldspar in a plagioclase host; usually much thinner than perthite

Symplectite

A replacement texture in which a mineral is replaced by an intergrowth of one or more minerals

Myrmekite

A secondary texture consisting of irregular “wormy” blebs or rods of quartz in a plagioclase host adjacent to alkali feldspar grains

Spherulitic

Having a radial intergrowth of fibrous minerals; commonly alkali feldspar and quartz in devitrified silicic volcanics

Axiolitic

Similar to spherulitic, but with fibers occurring in a layer and oriented normal to its walls

Variolitic

Spherulitic and consisting of divergent plagioclase fibers; applies to certain basalts

Coalescent

Having anhedral texture developed by simultaneous growth of two mineral grains in contact

Ophitic

Having large pyroxene grains enclosing small, random plagioclase laths

Subophitic

Having plagioclase laths that are larger and only partially enclosed by the pyroxene

Nesophitic

Having a plagioclase that is larger, with interstitial pyroxenes

Intergranular

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

Intersertal

Having glass, cryptocrystalline material, or alteration products occupying the spaces between plagioclase laths

Hyalo-ophitic

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

Hyalopilitic

Having a larger amount of glass, with pyroxene occurring only as tiny, random microlites

Diktytaxitic

The texture of certain volcanics in which bounding crystals protrude into abundant angular interstitial gas cavities

Cumulate

Displaying interstitial growth of a mineral between earlier ones that are all in contact and give the distinct impression that they accumulated at the bottom of a magma chamber

Orthocumulate

Having cumulate texture, with other minerals occupying the interstitial areas

Adcumulate

Having cumulate texture in which the early cumulate minerals grow to fill the pore space

Mesocumulate

Having a texture that is intermediate between ortho- and adcumulate

Pseudomorph

A replacement texture in which one or more minerals replace another, retaining the form of the original mineral

Symplectite

A replacement texture in which a mineral is replace by an intergrowth of one or more minerals

Uralitization

Replacement of pyroxene by amphibole

Saussuritization

Replacement of plagioclase by epidote

Biotitization

Replacement of pyroxene or amphibole by biotite

Chloritization

Replacement of any mafic mineral by chlorite

Seritization

Replacement of feldspar or feldspathoid by fine white micas

Interstitial

Having one mineral filling the interstices between earlier crystallized grains

Crystallites

Minute, inchoate crystals in the earliest stages of formation; they are isotropic and cannot be identified under the microscope

Microlites

Tiny needle- or lath-like crystals of which at least some properties are microscopically determinable

Felty (pilotaxitic)

Consisting of random microlites

Trachytic

Consisting of (feldspar) microlites aligned due to flow

Embayed

Having embayments due to reaction with the melt (resorption)

Skeletal

Having crystals that grew as, or have been corroded to, a skeletal framework with a high proportion of internal voids

Sieve

Crystals filled with channelways (appearing as holes) due to resorption

Epitactic

Oriented nucleation of one mineral on another of a different kind

Rapakivi

Overgrowths of plagioclase on alkali feldspar

Vesicular

Containing gas bubbles

Scoriaceous

Highly vesicular (think scoria)

Pumiceous

Having a frothy vesicular structure characteristic of pumice

Miarolitic

Having gas cavities into which euhedral minerals protrude; applies to certain plutonic rocks

Pipe vesicles

Tubelike elongate vesicles that result from rising gases

Vesicular pipes

Cylindrical bodies that are highly charged with vesicles

Holocrystalline

Consisting entirely of crystal (default term, not commonly used)

Hypocrystalline

Containing both crystals and glass

Holohyaline (holovitric)

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 fined-grained to distinguish microscopically

Fine-grained

Having an average crystal diameter less than 1mm

Medium-grained

Having an average crystal diameter 1-5mm

Coarse-grained

Having an average crystal diameter greater than 5mm

Very coarse-grained

Having an average crystal diameter greater than 50mm

Pegmatitic

Being very coarse-grained (historically associated with very coarse granitoid rocks: pegmatites)

Saccharoidal

Being fine- to medium-grained xenomorphic and equigranular (looking like sugar)

Aplitic

A synonym for saccharoidal, but typically restricted to leucocratic granitoid rocks

Equigranular

Having grains that are all approximately the same size

Inequigranular

Having grains that vary considerably in size

Porphyritic

Having approximately bimodal size distribution (usually requires a great difference)

Megaporphyritic

Having a porphyritic texture that can be seen in hand specimen (rarely used)

Microporphyritic

Having a porphyritic texture that is visible only under the microscope

Phyric (-phyric)

An adjective (or suffix) referring to porphyritic texture

Amygdaloidal

Having vesicles that are completely or partially filled with secondary minerals

Lithophysae

Large ovoid structures representing gas bubbles in devitrified rhyolitic glass

Foliation

Planar parallelism

Banding

Alternating planar layers

Lineation

Linear parallelism

Xenolith

An inclusion of country rock (“strangers”)

Xenocryst

A single-crystal foreign inclusion

Perlitic

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

Pyroclastic

Composed of fragments

Ocelli

Ovoid blobs created by liquid immiscibility, mingled magmas, or filled vesicles

Orbicules

Ovoid masses of radiating crystals, commonly concentrically banded, found in some granites