Andesite

The term Andesite was used for the first time by Leopold von Buch in 1835 to describe a group of rocks found in the Andes of Bolivia and Chile and also in the Kamchatka in Russia. Andesite is recorded as being an important rock type associated with 422 of the 721 active volcanoes of the Earth; most of the modern Andesite occur above the Benioff seismic zone and the mean chemical composition of andesite is broadly similar to the estimated mean chemical composition of the continental crust of the Earth. This is one of the reasons why it is often suggested that andesitic magma has, and is, playing a significant role in the evolution of the continental crust; for example Brown and Musset (1981) have proposed that the continental crust is at present growing at a rate of almost 0.5 Km3 /a and that most of this new material is essentially andesitic in composition.

Andesite lavas usually have porphyritic, or vitrophyric textures, and the phenocrysts often provide a remarkable record of at least part of the evolutionary history of the rocks. They color index (M) generally range from 20 to 40.
Plagioclase and pyroxene are the normal essential minerals and plagioclase is the usually the most abundant phenocryst. This abundance support the observation that plagioclase is usually a liquidus phase in andesitic magma eruption.

Plagioclase crystals are usually complexly zoned and according to Gill (1981), the complexly-zoned crystals in andesite usually include one or more of the following features:

A homogeneous internal region, frequently the core; patchy-zoned, inclusions rich zones, regions of oscillatory zoning, abrupt, non-oscillatory changes in compositions of 10 to 30 mol% An, often accompanied by resorption of the inner feldspar, a clear normally zoned mantle and thin (<20 micron) rim, usually similar in composition to groundmass microlites. Abrupt non-oscillatory changes probably indicate abrupt changes in the magmatic environment in which such crystals evolved. Reverse zoning in which calcic plagioclase surrounds a sodic core is found in many Andesite, and this phenomenon is sometime attributed to magma mixing or assimilation.

Augite is the second most abundant type of phenocrysts in Andesite, the core of the Augite crystals in Andesite usually have a composition the lie within the range Wo38-50 En7-20 Fs7-20; Augite is also the most abundant crystals in the groundmass and these small crystals have a compositions similar to the phenocrysts.

Orthopyroxene is found in many Andesite, and in basaltic Andesite, the appearance of orthopyroxene usually heralds the disappearance of Olivine. Usually Orthopyroxene composition range from En85 to En60 (Bronzite and Hypersthene).

Green to brown Hornblende is the typical amphibole found in Andesite, it normally does not found in the groundmass but it is found as a Phenocryst. Experimental studies indicate the Andesitic magmas that precipitate Hornblende should normally carry at least 3%H2O and such magmas are usually relatively rich in Alkali and have a high Fe2O3/FeO ratio. Many Hornblende crystals show evidence of resorption and in some Andesite the outer rim is replaced by a corona of plagioclase, pyroxene and magnetite.

Olivine occurs in minor (less than 1%) amounts in many SiO2-poor Andesite. As the modal Olivine content generally correlate with the MgO content in the rock, it is possible that much of this Olivine is an accumulative phase.

Both Ilmenite and titanomagnetite occur as primary minerals in Andesite and they occur both in phenocryst and in the groundmass.

Garnet is a rare mineral found in some Andesite and the distribution of such Andesite is normally restricted to continental margins that contain pelitic sediments. Garnet occur as euhedral, usually between 1 and 2 mm, crystals. Some have plagioclase-magnetite and chlorite corona that indicate reaction between magma and crystals, followed by the precipitation of a rind of minerals that are stable at lower pressures.

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QAPF Diagram Andesite field in Blue.





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Andesite sample. Plagioclase (white), pyroxene and biotite (dark). From R.Weller-Cochise College





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Andesite sample. Plagioclase (white), Hornblende (dark). From R.Weller-Cochise College



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Highly Porphyritic Andesite. Plagioclase (white), pyroxene (dark)



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Honeblende Andesite sapmple.




Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• D’Amico C., Innocenti F. & Sassi F.P. (1987): Magmatismo e metamorfismo. UTET
• Innocenti F., Rocchi S. & Triglia R. (1999:) La classificazione delle rocce vulcaniche e subvulcaniche: schema operativo per il progetto CARG.
• Carmichael I.S.E., Turner F.J. & Verghoogen J. (1974): Igneous Petrology. McGraw-Hill.

Photo
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. XPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. XPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. XPL image, 2x (Field of view = 7mm)
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Hornblende, biotite and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende, biotite and plagioclase crystals in a Andesite. XPL image, 2x (Field of view = 7mm)
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Hornblende, biotite and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende, biotite and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende, biotite and plagioclase crystals in a Andesite. XPL image, 2x (Field of view = 7mm)
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Hornblende, biotite and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Plagioclase (with thin sieve texture on the rim) and Hornblende crystals in a Andesite. XPL image, 2x (Field of view = 7mm)
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Plagioclase (with thin sieve texture on the rim) and Hornblende crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Plagioclase (with thin sieve texture on the rim) and Hornblende crystals in a Andesite. XPL image, 2x (Field of view = 7mm)
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Hornblende crystals. PPL image, 10x (Field of view = 2mm)
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Zoned plagioclase. PPL image, 10x (Field of view = 2mm)
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Plagioclase (with thin sieve texture on the rim) and Hornblende crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende, clinopyroxene and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite. PPL image, 2x (Field of view = 7mm)
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Hornblende crystals in a Andesite. PPL image, 10x (Field of view = 2mm)
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Hornblende crystal (with opacitic core) in a Andesite. PPL image, 10x (Field of view = 2mm)
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Hornblende crystals in a Andesite. PPL image, 10x (Field of view = 2mm)
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Hornblende crystals in a Andesite. PPL image, 10x (Field of view = 2mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Cristalli di orneblenda bruna e plagioclasio in una andesite. Immagine a N// 2x (lato lungo = 2mm)
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Cristalli di orneblenda bruna e plagioclasio in una andesite. Immagine a N// 2x (lato lungo = 2mm)
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Cristalli di orneblenda bruna e plagioclasio (con tessitura sieve) in una andesite. Immagine a N// 2x (lato lungo = 2mm)
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Hornblende and plagioclase (with sieve texture) crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Hornblende and plagioclase (with sieve texture) crystals in a Andesite from Equador. PPL image, 2x (Field of view = 7mm)
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Clinopyroxene, plagioclase and magnetite crystals. PPL image, 2x (Field of view = 7mm)
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Clinopyroxene, plagioclase and magnetite crystals. XPL image, 2x (Field of view = 7mm)
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Clinopyroxene, plagioclase and magnetite crystals. PPL image, 2x (Field of view = 7mm)
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Clinopyroxene, plagioclase and magnetite crystals. PPL image, 2x (Field of view = 7mm)
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Clinopyroxene, plagioclase and magnetite crystals. PPL image, 2x (Field of view = 7mm)
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Clinopyroxene, plagioclase and magnetite crystals. XPL image, 2x (Field of view = 7mm)