ALFRED PASIEKA / SCIENCE PHOTO LIBRARY ALFRED PASIEKA / SCIENCE PHOTO LIBRARY
Andesite. Polarised light micrograph of a thin section of andesite. This is an extrusive igneous rock. Its structure shows large phenocrysts (included crystals) surrounded by a fine-grained ground mass. Essential components of andesite include plagioclase and biotite, with accessories such as quartz, magnetite or (as here) pyroxene. Andesite is associated with tectonic margins. It is formed after the melting of rocks subducted at plate boundaries then ejected as magma in volcanoes. Andesite volcanism is associated with porphyry copper ores, the most important source of the metal. Magnification x16 at 6x7cm size. Magnification: x27 at 5x4 inch size.
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