Texture of Igneous Rocks Five common textures in igneous rocks include; 1) Coarse Grain (Phaneritic) describes the appearance of an igneous rock, based on the size, shape and arrangement of interlocking crystals.  gives clues to the type of environment in which the molten rock crystallized.  2) Fine Grain (Aphanitic) 3) Porphyritic 4) Glassy 5) Vesicular Reference: Tarbuck and Lutgens Pages

Texture Igneous textures Texture a. size b. shape c. arrangement of interlocking crystals

crystal size Slow cooling cooling rate larger crystals Fast coolingsmall or no crystals Igneous textures

Coarse Grain Texture forms deep beneath the surface in a plutonic environment where the molten rock cools slowly.  also called Phaneritic texture.  minerals can be identified with the unaided eye.  Quartz Orthoclase Feldspar Amphibole Examples: Granite Gabbro Diorite  larger crystals of uniform size.  Igneous textures

Fine Grain Texture minerals are not identified with the unaided eye.  Examples: Basalt Andesite Rhyolite  forms at or near the Earth’s surface in volcanic environment where the molten rock cools quickly.  very small crystals (only seen under microscope)  may contain vesicles (gas bubbles)  also called Aphanitic texture.  Igneous textures

Porphyritic Texture this texture results when magma with crystals already formed escape to the surface and cools quickly forming a fine grained igneous rock with large crystals inside.  this texture is a result of two stages of cooling; 1) slow cooling forming the larger crystals. 2) rapid cooling forming the finer crystals.  Examples: Andesite Porphyry  Igneous textures

minerals can not identified with the unaided eye.  Glassy Texture forms when the ions in the molten rock do not have time to organize into an orderly pattern to form crystals because it cools very, very quickly.  the result is volcanic glass.  Examples: Obsidian  Igneous textures

Vesicular Texture forms when gas bubbles escape from molten rock and are trapped as it cools and crystallizes.  this texture can form near the top of lava flows.  Examples: Scoria  minerals can not identified with the unaided eye.  Igneous textures

Fine-grainedCoarse-grained Large crystals in matrixGlassy Igneous textures

Fine-grained Coarse-grained PorphyriticGlassy fast cooling magma/lava u fast cooling magma/lava u forms at or near surface u sometimes holes present u can’t see individual crystals forms far u forms far below surface below surface u slow cooling u inter-grown crystals crystals magma cooled slowly for a u magma cooled slowly for a while then erupted while then erupted minerals crystallize at minerals crystallize at different temperatures and different temperatures and or rates or rates very rapid u very rapid cooling cooling u ions unable to unite in orderly unite in orderly crystalline crystalline structure structure Igneous textures

Sample Problem An igneous rock is found to contain both large and small crystals, as shown below. What conditions were necessary for this rock to form? Answer: This texture is a result of two stages of cooling; 1) slow cooling forming the larger crystals. 2) rapid cooling forming the finer crystals.