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AN EASY GUIDE TO THE COMMON IGNEOUS ROCKS MAGMA TYPE SILICA CONTENT VOLCANIC ROCK(S) PLUTONIC ROCK COMMON MINERALS ACCESSORY MINS. Felsic > 65% Rhyolite GRANITE Sanidine (volc.) Orthoclase (plut.) Microcline (plut.) Na-rich plagioclase Biotite and/or hornblende Quartz Zircon magnetite muscovite Intermediate 55-65% ANDESITE Diorite Na-Ca plagioclase pyroxene and/or hornblende NO QUARTZ biotite magnetite ilmenite (FeTiO3) Mafic 45-55% BASALT Gabbro Olivine Pyroxene Ca-rich plagioclase magnetite ilmenite (FeTiO3) Ultramafic < 45% Komatiite (rare) Dunite ----------> Peridotite ------> Olivine Olivine + Pyroxene Kimberlite General rules that will help put order in the system: magnetite chromite ilmenite olivine, pyroxene, biotite, ilmenite, serpentine, diamonds (rocks in bold will be seen in the lab!) Silica content and mineral content: 1. Igneous rocks low in silica will consist mostly of minerals at or near the top of Bowen's Reaction Series. 2. Igneous rocks high in silica will consist mostly of minerals near the bottom of Bowen's Reaction Series. 3. Intermediate igneous rocks will consist of minerals in the middle of Bowen's Reaction Series. Crystal size (texture) and origins: 1. Igneous rocks with a coarse-grained texture (generally, all or almost all grains at least sand-sized, over 1 mm in diameter) are going to be plutonic in origin - having crystallized slowly, underground. 2. Igneous rocks with a fine-grained texture (grains generally tiny or too small to see) are volcanic in origin. 3. Igneous rocks with a porphyritic texture (large grains - phenocrysts - surrounded by a finer-grained groundmass) cooled in a two-stage process, first slowly and then more rapidly; this occurs in both plutonic and volcanic rocks. The grain size in the groundmass indicates whether the rock as a whole had a volcanic or plutonic origin. (See reverse for more details on textures.) TEXTURES IN IGNEOUS ROCKS Aphanitic Fine-Grained Porphyritic Pyroclastic Coarse-Grained Glassy textures in volcanic rocks result from rapid cooling. There are few if any mineral grains; the rock is chiefly or entirely comprised of natural volcanic glass, or obsidian. Most obsidian is felsic (high in silica), but it also forms from other magma types as well. (For example, in Hawai'i, basaltic lavas flow into the ocean and chill into basaltic glass.) Frequently, there are tiny phenocrysts, called microlites, in glasses. Over time, these glasses are unstable and will recrystallize into aphanitic volcanic rocks. Volcanic glass that is frothy as well, from trapped gas bubbles, is called pumice. Aphanitic textures characterize volcanic rocks that have cooled rapidly but nonetheless have allowed for mineral crystals to form. However, the individual crystals are too small to be seen or identified with the naked eye. These can be differentiated from glassy textures by the fact that they are far less "glassy" or shiny in appearance; the rock surface may be smooth, but still is dull. Fine-Grained textures are very common in volcanic rocks, particularly basalts. In these, all the rock is crystallized, but the individual grains are very small (typically in the range of 0.1-0.5 mm across, the size of fine to medium sand, and sometimes even smaller). Porphyritic textures may be the most common volcanic texture, especially for volcanic rocks on the continents, and is surprisingly common in plutonic rocks as well. In these, large grains, called phenocrysts are embedded in a finer groundmass. Most commonly, the phenocrysts are grains that began crystallizing while the magma was still deep underground, then became entrapped in the fine-grained groundmass that formed either as the magma was intruded into cooler country rocks, or erupted from a volcano, and cooled more quickly. Phenocrysts are commonly minerals higher in Bowen's Reaction Series than dominant groundmass minerals, and in volcanic rocks may even include minerals such as hornblende or even biotite, which may not occur, or be much less common, in the groundmass. Plagioclase feldspar phenocrysts will commonly be more calcium-rich than the plagioclase feldspars in the groundmass. Pyroclastic textures in volcanic rocks result from the fusion of solid volcanic ejecta that have been welded into a solid, coherent mass. These often include shards of glass and many irregular rock fragments, ash, etc. Coarse-Grained textures occur only in plutonic rocks, and result from slow, gradual cooling. Individual crystals are commonly >2mm in diameter, commonly much larger. Note that all grains are intergrown in the texture.