Andesite(redirected from Andesites)
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an extrusive, dark-colored (such as dark gray, brown, or black) mountain rock. Its structure is porphyritic or hyalopilitic; in the latter case the microlites resemble felt saturated with glass. It is formed as a result of the freezing, at the top surface or close to it, of volcanic lava containing 56–60 percent silicic acid and much magnesium, calcium, and iron. Andesite is usually a fresh rock composed of crystals of medium plagioclases, andesine, and pyroxenes and less frequently of magnetite, hornblende, and other minerals immersed in glass. Andésite is widely spread throughout contemporary and ancient volcanic regions such as Kamchatka, the Caucasus, Middle Asia, and the Mediterranean. Together with basalt, it constitutes the principal mass of extrusive rocks. It is used as an acid-proof material and for special glazing.