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Phaneritic

phaneritic

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Due to the different ways that igneous rocks are formed, this gives them a variety of textures which can describe their mode of formation. The phaneritic texture, coined from "phaner" which means "visible" is related to the intrusive igneous rocks which crystallize slowly beneath the surface of the earth. The slow cooling of the magma in the plutonic surrounding gives the minerals time to enlarge and form giant crystals. Each of these crystals is large enough to be seen with the normal eyes, unlike the aphanitic which is small. It may also be applicable to the metamorphic rock that has a similar description. Examples of such phaneritic rocks include granite, gabbro and diorite.

The phaneritic rock texture is just one of six main rock textures. The others include the aphanitic, glassy, pegmatitic, porphyritic and pyroclastic.

· Aphanitic rocks: The ‘a’ in front of the “phaneritic” gives it the meaning “not visible”. Unlike, the phaneritic rocks, which form from the slow cooling of magma. These rocks are formed from lava which speedily crystallizes on or close to the earths surface. This makes the rock extrusive and the minerals do not acquire enough time to produce large crystals.

· The pegmatitic texture is formed during lava cooling when the minerals grow large enough to form massive crystals.

· The glassy texture is vitreous and occurs during eruptions. The lava cools extraordinarily rapidly, and this prevents crystallization. This leads to the formation of amorphous glass having little or no crystals.

· The porphyritic textures occur when the cooling conditions change quickly. The minerals formed during the slow cooling phase would grow large while those formed during the fast phase would be small like the aphanitic rocks.

· The name “pyroclastic” is formed from “pyro” meaning igneous and “clastic” meaning fragment. These textures are formed when the eruptions blast lave into the air which results in fragments of volcano ash when formed.

Geologists use these textures to determine their origin and to classify igneous rocks.

Additional Resources

Orthoclase   Plagioclase   Porphyry

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