Fundamental Minerals

Sialic Minerals

Feldspar

Feldspathoid

Quartz

Femic Minerals

Accessory Minerals

Very Rare Minerals

Secondary Minerals

Texture

Alteration Products

Glass and related Texture

Volcanic Rocks

Komatiites

Lamprophyres

Horneblende-bearing 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.

Feldspar and pyroxene crystals in a andesite. Yellowstone, northwestern Wyoming, USA. From James St. John

Porphyritic andesite from Lyon County, western Nevada, USA. From James St. John

Porphyritic andesite from Sugarloaf Hill, western Nevada, USA. From James St. John

Andesite from the 1997 lava from Soufriere Hills Volcano, Montserrat, Lesser Antilles Volcanic Arc, eastern Caribbean Sea. From James St. John

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 2x (Field of view = 7mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 2x (Field of view = 7mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. XPL image, 2x (Field of view = 7mm)
Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 2x (Field of view = 7mm)	Green Honblende, clinopyroxene and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 2x (Field of view = 7mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 2x (Field of view = 7mm)
Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 2x (Field of view = 7mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)
Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)
Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)
Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. XPL image, 10x (Field of view = 2mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. XPL image, 10x (Field of view = 2mm)	Green Honblende and plagioclase crystals in a Andesite from Grenada, Antilles. PPL image, 10x (Field of view = 2mm)
Cristalli di plagioclasio e orneblenda bruna immersi in una pasta di fondo vetrosa. Immagine a N//, 2x (lato lungo = 7mm)	Cristalli di plagioclasio e orneblenda bruna immersi in una pasta di fondo vetrosa. Immagine a N//, 2x (lato lungo = 7mm)	Cristalli di plagioclasio e orneblenda bruna immersi in una pasta di fondo vetrosa. Immagine a N//, 2x (lato lungo = 7mm)
Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. PPL image, 2x (Field of view = 7mm)	Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. PPL image, 2x (Field of view = 7mm)	Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. PPL image, 2x (Field of view = 7mm)
Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. XPL image, 2x (Field of view = 7mm)	Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. XPL image, 2x (Field of view = 7mm)	Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. XPL image, 2x (Field of view = 7mm)
Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. XPL image, 2x (Field of view = 7mm)	Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. XPL image, 2x (Field of view = 7mm)	Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. PPL image, 10x (Field of view = 2mm)
Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. XPL image, 10x (Field of view = 2mm)	Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. PPL image, 10x (Field of view = 2mm)	Brown Honblende and plagioclase crystals in a Andesite with glassy groundmass. PPL image, 10x (Field of view = 2mm)