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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 ?
Similar REE characteristics of ore-bearing quartz and pyrites and volcanic rocks, together with oxygen and hydrogen isotope studies of quartz, suggest that the ore-forming fluids of the Gaosongshan gold deposit were mainly magmatic origin which was associated with andesitic magma and was partly mixed with atmospheric water.
It includes the World Meteoric Trend line, the range of andesitic water as proposed by Giggenbach (1992).
In terms of their major and trace element abundances, the Ebeko andesites are similar to other Quaternary andesitic lavas from the Northern Kuriles [24].
The geometry of this structure is masked and obliterated by many dacitic and andesitic subvolcanic bodies and pyroclastic flows (Leveratto, 1968), their ages ranging from 18 to 7 Ma (Leveratto, 1976; Bercowski et al., 1986, 1993; Verges et al., 2001).
In this area the copper mineral with secondary minerals filling cavities of andesitic rocks (Fig.
leptocaulis which happened to be found with high frequencies in sampled sites, but with zero presence in Quaternary andesitic rocks and granite.
After that, in the middle Pleistocene (~0.5 Ma), another volcanic event produced andesitic domes, being labeled as Ajusco period.
Stage 1 is huge masses of ignimbrites and volcanic ashes associated with interbedded andesitic to trachyandesitic lavas, erupted in early stages.
Next, Kebobutak formation material is andesitic breccias, tuff, lapili tuff, agglomerate and intercalations of andesitic lava flows.