Andesite


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Related to Andesite: Andesite line

andesite

[′an·də‚zīt]
(petrology)
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.

Andesite

 

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 ?
The two cycles of andesite eruption within Miri Volcano are light grey in colour and form secondary craters.
Calcite is a relatively rare mineral in andesite cavities at Lonavala, forming yellow, transparent crystals to a few centimeters in length.
Andesite in the Cheviots is a dark grey, or sometimes slightly pink, rock.
The dacite contains significantly fewer quartz phenocrysts than the rhyolite but is otherwise difficult to distinguish from it; dacite is distinguished from andesite (see below) by the presence of quartz phenocrysts (Gill 1981).
Volcanic rocks of the Ross Island Formation range from basalt to andesite in composition.
This mineralization takes place as breccia matrix fill, disseminations and veinlets in altered andesites.
Follow-up reconnaissance-style mapping has identified a broad area of quartz veining hosted by a mid-Tertiary aged andesite volcanic formation.
Exposed in the trenches are several strongly altered dikes up to 20 meters in width that have intruded chloritized andesite that contains up to 5% pyrite.
2 grams/tonne silver from surface to a depth of 73 meters true thickness, below which unmineralized andesite was encountered.
Three or more mineralization trends in quartz breccia and silicified andesite breccia extend across the deposit.
Drilling has delineated a core zone of copper-gold mineralization developed along the contact between a granodiorite stock and andesite country rocks.
This fault and fracture zone is also the contact between Tertiary age strongly altered andesite and rhyolite flows which have been intruded by a dacite dike and cut by stockwork quartz veins and veinlets.