Igneous Rock Textures

These flashcards cover key vocabulary related to the textures of igneous rocks, assisting with understanding the characteristics, structures, and classifications of these geological formations.

Texture

Describes the size, shape, and orientation of the grains constituting a rock.

Holocrystalline

Consisting entirely of crystals.

Hypocrystalline

Containing both crystals and glass.

Porphyritic

Having approximately bimodal size distribution in crystal sizes.

Phenocryst

A large crystal set in a fine matrix.

Aphyric

Not porphyritic; having no phenocrysts.

Euhedral

Completely bounded by crystal faces.

Cryptocrystalline

Having minerals too fine grained to distinguish microscopically.

Mafic

Relating to dark-colored minerals rich in magnesium and iron.

Xenolith

An inclusion of country rock within an igneous rock.

Pseudomorph

A replacement texture retaining the form of the original mineral.

Spherulitic

Having a radial intergrowth of fibrous minerals.

Pegmatitic

Very coarse-grained texture often associated with granitoids.

Glomeroporphyritic

Refers to cumulophyric texture with clusters of phenocrysts.

Graphitic

Having an intergrowth where a guest mineral shows angular wedge-like forms.

Vesicular

Containing gas bubbles.

Aplite

A synonym for saccharoidal, typically found in leucocratic granitoid rocks.

Seriate

Having a continuous gradation in grain size.

Intergrowth

A structure formed by two or more minerals growing together.

Trachytic

Consisting of feldspar microlites aligned due to flow.

Scoriaceous

Highly vesicular texture, often found in volcanic rocks.

Phaneritic

A texture where individual crystals are large enough to be seen with the naked eye.

Aphanitic

A texture where the crystals are too small to be seen without magnification.

Glassy

A non-crystalline texture resulting from extremely rapid cooling of magma.

Subhedral

Partly bounded by crystal faces and partly by surfaces formed against preexisting grains.

Anhedral

Lacking well-developed crystal faces or having no faces at all.

Amygdaloidal

A texture in which secondary minerals fill gas vesicles (amygdules).

Poikilitic

A texture where large crystals (oikocrysts) enclose many smaller crystals of other minerals.

Equigranular

A texture where all the component crystals are approximately the same size.

Felsic

Igneous rocks enriched in lighter elements such as silicon, oxygen, aluminum, sodium, and potassium.

Ultramafic

Igneous rocks with very low silica content (usually < 45\%) and high levels of magnesium and iron.

Pyroclastic

Texture formed from the consolidation of individual rock fragments ejected during volcanic eruptions.