Dacite (devitrified)

Dacite, from Dacia, a Roman province, Transylvania, Romania, is a volcanic rock that may be considered a quartz-bearing variety of andesite. Dacite is primarily associated with andesite and trachyte and forms lava flows, dikes, and sometimes massive intrusions in the centers of old volcanoes. Like andesite, dacite consists mostly of plagioclase feldspar with biotite, hornblende, augite, or enstatite and generally has a porphyritic structure; additionally, however, it contains quartz as rounded, corroded crystals or grains, or as a constituent of the groundmass. The feldspar content of dacite ranges from oligoclase to andesine and labradorite; sanidine occurs also in some dacites and when abundant gives rise to rocks transitional to the rhyolites.

Dacitic magma is formed by the subduction of young oceanic crust under a thick felsic continental plate. Oceanic crust is hydrothermally altered causing addition of quartz and sodium. As the young, hot oceanic plate is subducted under continental crust, the subducted slab partially melts and interacts with the upper mantle through convection and dehydration reactions. The process of subduction creates metamorphism in the subducting slab. When this slab reaches the mantle and initiates the dehydration reactions, minerals such as talc, serpentine, mica and amphiboles break down generating a more sodic melt. The magma then continues to migrate upwards causing differentiation and becomes even more sodic and silicic as it rises. Once at the cold surface, the sodium rich magma crystallizes plagioclase, quartz and hornblende. The formation of dacite provides a great deal of information about the connection between oceanic crust and continental crust. It provides a model for the generation of felsic, buoyant, perennial rock from a mafic, dense, short-lived one.

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Porphyritic dacite California, USA. From James St. John.

Hornblende dacite California, USA. From James St. John.

Dacite pumice from the 15 June 1991 eruption of Mt. Pinatubo, Luzon Volcanic Arc, western Luzon Island, northern Philippines. 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.

Foto
Brown Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)	Spherulites in a Devitrified Dacite from New Zeland. XPL image, 2x (Field of view = 7mm)	Brown Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)
Brown Spherulites in a Devitrified Dacite from New Zeland. PPL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. XPL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. XPL image, 10x (Field of view = 2mm)
Brown Spherulites in a Devitrified Dacite from New Zeland. PPL image, 10x (Field of view = 2mm)	Brown Spherulites around pyroxene in a Devitrified Dacite from New Zeland. PPL image, 10x (Field of view = 2mm)	Spherulites around pyroxene in a Devitrified Dacite from New Zeland. XPL image, 10x (Field of view = 2mm)
Brown Spherulites in a Devitrified Dacite from New Zeland. PPL image, 10x (Field of view = 2mm)	Brown Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)	Spherulites around plagioclase in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)
Brown Spherulites in a Devitrified Dacite from New Zeland. PPL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. XPL image, 10x (Field of view = 2mm)	Brown Spherulites around plagioclase in a Devitrified Dacite from New Zeland. Note the flow structure under the Spherulites. PPL image, 10x (Field of view = 2mm)
Brown Spherulites around plagioclase in a Devitrified Dacite from New Zeland. Note the flow structure under the Spherulites. XPL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)	Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)
Spherulites in a Devitrified Dacite from New Zeland. PPL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. PPL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. XPL image, 10x (Field of view = 2mm)
Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)	Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)	Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)
Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)	Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)	Spherulites in a Devitrified Dacite from New Zeland. PPL image, 2x (Field of view = 7mm)
Spherulites in a Devitrified Dacite from New Zeland. XpL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. XpL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. XpL image, 10x (Field of view = 2mm)
Spherulites in a Devitrified Dacite from New Zeland. XpL image, 10x (Field of view = 2mm)	Spherulites in a Devitrified Dacite from New Zeland. XpL image, 10x (Field of view = 2mm)