Andesite

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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 compositions of the analyzed Ebeko andesites are given in Table 1 (the average composition of 12 samples) and in Supplementary Table S3 (full results).
Also in this south sector (Cordon del Portillo) there is a plutonic association of gabbros and tonalites to granodiorites, and a volcanic sequence of andesites and dacites to rhyodacites and rhyolites (Polanski, 1972).
The Eocene lavas are mostly andesites, trachyandesites, trachybasalts and analcime-bearing tephrites with shoshonitic to alkaline geochemical signatures.
Granite, basalt, andesite and trachyte are used as coating materials for buildings.
Measurements from various orbiters and landers show that most Martian volcanic deposits are similar to terrestrial basalts and andesites. NASA's rovers Spirit and Opportunity found plenty of evidence of volcanic rocks and even ash--the layers Spirit found at the Columbia Hills resemble ash fall or ash flow deposits.
the andesites), especially along highly fractured, brecciated areas that form chimney-like structures as described by (among others) Kothavala (1991).
This is supported by widespread petrographic evidence of textural disequilibrium in andesites (e.g.
Classification of samples with respect to rock types and alteration stage Rock type Alteration stage Unaltered Smectite/ Mixed Chlorite zeolites layer clay Basaltic Lavas 122 44 22 2 Hyaloclastites 30 21 8 3 Intrusions 24 12 6 Andesites 5 Acid rocks 25 Sediments 2 3 Total 154 122 42 11 Rock type Alteration stage Chlorite/ Epidote/ Total epidote amphibole Basaltic Lavas 42 36 268 Hyaloclastites 6 1 69 Intrusions 30 36 108 Andesites 2 2 9 Acid rocks 11 12 48 Sediments 1 1 7 Total 92 88 509 Table 2.
low-silica andesite in the lower part of the Porten Road Formation is highly enriched in LREE and LFSE.