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

Dacite

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.

dacite2020(3).jpg

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.

Dacite (pale brown) and andesite (dark) banded pumice from Katmai National Park, Alaska, USA. From Wikipedia.

Banded pumice with light dacite and dark andesite, eruptions of Lassen Peak 1915, Californa, USA. From Axeleratio.

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
Plagiolcase, Biotite and clinopyroxene in a dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)	Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)	Plagiolcase and Biotite in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase and Biotite (with plagioclase inclusions) in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
Plagiolcase and Biotite (with plagioclase inclusions) in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase and clinopyroxene in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
Plagiolcase and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
Plagiolcase and Hornblende in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)	Plagiolcase quartz (rounded) and Hornblende in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
Plagiolcase and Hornblende in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)	Plagiolcase and Hornblende in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
Plagiolcase and Hornblende in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)	Plagiolcase quartz (rounded) and Hornblende in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Plagiolcase and Hornblende in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 2x (Field of view = 7mm)	Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 2x (Field of view = 7mm)	Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)
Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)	Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)	Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)
Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)	Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)	Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)