The Textures of Igneous Rocks

Spitzkoppe granite rocks, Namibia

 Marco Bottigelli / Getty Images

The texture of a rock refers to the details of its visible character. This includes the size and quality and interrelations of its grains and the fabric they form. Larger scale features, such as fractures and layering, are considered rock structures in comparison.

There are nine main types of igneous rock textures: Phaneritic, vesicular, aphanitic, porphyritic, poikilitic, glassy, pyroclastic, equigranular, and spinifex. Each kind of texture has a variety of different characteristics that make them unique.

Properties of Igneous Rock Textures

What determines igneous rock texture? It all comes down to the rate at which the rock cools. Other factors include the diffusion rate, which is how atoms and molecules move through the liquid. The rate of crystal growth is another factor, and that's how quickly new constituents come to the surface of the growing crystal. New crystal nucleation rates, which is how enough chemical components can come together without dissolving, is another factor affecting the texture.

Texture is comprised of grains, and there are a few main types of igneous rock grains: Equant grains are those with boundaries of equal lengths; rectangular tablet shapes are known as tabular grains; acicular grains are slender crystals; long fibers are known as fibrous grains, and a grain that is prismatic is one that has different types of prisms.

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Aphanitic Texture

Porphyritic andesite

 James St. John/Flickr

Aphanitic ("AY-fa-NIT-ic") rocks have mineral grains that are mostly too small to be seen with the naked eye or a hand lens, like this rhyolite. Basalt is another igneous rock with the aphanitic texture.

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Equigranular Texture

Brachinite (NWA 3151 Meteorite) 3

James St. John/Flickr 

Rocks with equigranular ("EC-wi-GRAN-ular") have mineral grains that are generally the same size. This example is a granite.

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Glassy Texture

Obsidian volcanic glass

 

Michael Szönyi / Getty Images

Glassy (or hyaline or vitreous) rocks have no or almost no grains at all, as in this quickly chilled pahoehoe basalt or in obsidian. Pumice is another type of igneous rock with a glassy texture.

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Phaneritic Texture

Quartz monzonite (Butte Quartz Monzonite, Late Cretaceous, 68-78 Ma; Interstate 90 outcrop, southeast of Butte, Montana, USA)

 James St. John/Getty Images

Phaneritic ("FAN-a-RIT-ic") rocks have mineral grains that are large enough to be seen with the naked eye or a hand lens, like this granite.

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Poikilitic Texture

Vesicular olivine diabase (Lafayette Bluff Sill, Proterozoic; Lafayette Bluff Tunnel, northeastern Minnesota, USA)

James St. John/Getty Images 

Poikilitic ("POIK-i-LIT-ic") texture is one in which large crystals, like this feldspar grain, contain small grains of other minerals scattered inside them.

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Porphyritic Texture

Illustration of andesite, a volcanic rock

 Dorling Kindersley / Getty Images

Rocks with porphyritic ("POR-fi-RIT-ic") texture like this andesite have larger mineral grains, or phenocrysts ("FEEN-o-crists"), in a matrix of smaller grains. In other words, they display two distinctive sizes of grains that are visible to the naked eye.

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Pyroclastic Texture

volcanic tuff ring

 

Mangiwau / Getty Images 

Rocks with pyroclastic ("PY-ro-CLAS-tic") texture are made of pieces of volcanic material that are created in an explosive eruption, like this welded tuff.

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Spinifex Texture

Spinifex metakomatiite (serpentinite)

 James St. John/Flickr

Spinifex texture, found only in komatiite, consists of large crisscrossing platy crystals of olivine. Spinifex is a spiny Australian grass.

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Vesicular Texture

Vesicular basalt

 James St. John/Flickr

Rocks with vesicular ("ve-SIC-ular") texture are full of bubbles. It always indicates a volcanic rock, like this scoria.