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Very finely crystalline extrusive rock of volcanic origin composed largely of plagioclase feldspar (oligoclase or andesine) with smaller amounts of dark-colored mineral (hornblende, biotite, or pyroxene), the extrusive equivalent of diorite.



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.

References in periodicals archive ?
Basaltic to andesitic rocks of the East Bay Hills Group are typically aphanitic and massive, with slight to moderate porphyritic textures.
Found at the interface between Layers C1 and C3 with a concentration of other water-rolled andesitic and siltstone pebbles, and Stegodon bone, including a complete femur and ribs.
Significant quantities of garnet must be left in the residue to produce andesitic melts by partial melting of basaltic eclogite (2).
In addition to strong values within the veins, sampling has shown that proximal to the veins the host andesitic volcanic rocks often host silver mineralization in the order of 15 to 50 grams per tonne.
ELA 2203 is underlain largely by andesitic volcanic rocks of the ancestral Pliocene Crater Mountain stratovolcano which grew to an immense size before undergoing caldron collapse on a ring fracture system 20 kilometres in diameter, followed by a long period of deep erosion which continued until recent renewed andesitic - dacitic volcanic activity formed a string of smaller parasite cones within and east of the northeast quadrant of the collapse structure.
Initial drilling has intercepted alteration in andesitic host rocks.
The most favourable lithologies appear to be propylitically altered andesitic flow and pyroclastics, with higher grade gold values being associated within restricted zones of sericite alteration.
The zone is entirely within argillites along the east contact with andesitic volcanics.
The Tambang Tinggi Region is underlain by intercalated andesitic volcanics and limestone.
Additional potential within the Rosales area is demonstrated by widely spaced samples consisting of altered and mineralized andesitic host rocks that are in contact with structures and/or veins.
The Silver Eagle target is an apparently flat lying silicified body that alters the upper parts of a conglomerate unit that overlies andesitic volcanic rocks.