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Texture of Igneous Rocks 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. Five common textures in igneous rocks include; 1) Coarse Grain (Phaneritic) 4) Glassy 2) Fine Grain (Aphanitic) 5) Vesicular 3) Porphyritic Reference: Tarbuck and Lutgens Pages 37 - 38 Igneous textures Texture a. size b. shape c. arrangement of interlocking crystals Igneous textures cooling rate crystal size Slow cooling larger crystals Fast cooling small or no crystals Igneous textures Coarse Grain Texture forms deep beneath the surface in a plutonic environment where the molten rock cools slowly. larger crystals of uniform size. also called Phaneritic texture. minerals can be identified with the unaided eye. Examples: Orthoclase Feldspar Quartz Granite Gabbro Diorite Amphibole Igneous textures Fine Grain Texture 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. minerals are not identified with the unaided eye. Examples: Basalt Andesite Rhyolite 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 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. minerals can not identified with the unaided eye. 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. minerals can not identified with the unaided eye. Examples: Scoria Igneous textures Fine-grained Large crystals in matrix Coarse-grained Glassy Igneous textures fast cooling magma/lava forms at or near surface sometimes holes present can’t see individual crystals Fine-grained magma cooled slowly for a while then erupted minerals crystallize at different temperatures and or rates Porphyritic forms far below surface slow cooling inter-grown crystals Coarse-grained very rapid cooling ions unable to unite in orderly crystalline structure Glassy 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.