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Rock Keys - KEY TO IGNEOUS ROCKS I If rock is...

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KEY TO IGNEOUS ROCKS I. If rock is phaneritic [Grains can be seen with the naked eye (macroscopic) and are approximately the same size] A. If light-colored with minerals such as quartz and/or orthoclase feldspar ( felsic composition ) is a GRANITE B. If gray-colored with amphibole and gray plagioclase ( intermediate composition ) is a DIORITE C. If dark-colored with augite and/or black plagioclase and sometimes with minor olivine ( mafic composition ) is a GABBRO II. If rock is aphanitic [with microscopic grains] A. If pink-colored (felsic composition) is a RHYOLITE B. If gray-colored (intermediate composition) is a ANDESITE C. If black-colored (mafic composition) is a BASALT III. If rock is porphyritic [contains two distinct sizes of grains (with large grains or phenocrysts in a fine-grained groundmass or matrix )] A. If pink-colored (felsic composition) is a RHYOLITE PORPHYRY B. If intermediate-colored (intermediate composition) is an ANDESITE PORPHYRY C. If dark-colored (mafic composition) is a BASALT PORPHYRY IV. If rock is vesicular [contains holes] A. If basaltic composition and color is a VESICULAR BASALT V. If rock is glassy (with a felsic composition but is usually dark-colored) is OBSIDIAN VI. If rock is amygdaloidal [with holes infilled with mineral precipitates] and is dark colored is an AMYGDALOIDAL BASALT VII. If rock is pyroclastic [made of tephra - with grains of volcanic material] A. If made from large, rounded grains is AGGLOMERATE B. If made from large, angular grains is VOLCANIC BRECCIA C. If made from volcanic ash is TUFF
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KEY TO SEDIMENTARY ROCKS I. If rock is aggregate of rock or mineral particles ( clastic or detrital sedimentary rock ): A. If particles greater than 2 millimeters (gravel size): 1. If particles are rounded rock is a CONGLOMERATE 2. If particles are angular rock is a BRECCIA B. If the particles are between 1/16 and 2 millimeters (sand size) the rock is a
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