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Trachyte

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Trachyte is a extrusive igneous rock that light coloured is composed mostly of alkali feldspar with minor amount of dark coloured minerals such as biotite, amphibole, or pyroxene. Trachyte is the volcanic equivalent of rock Syenite. Generally trachytes show porphyritic texture in which abundant, large, well-formed crystals (phenocrysts) of early generation are embedded in a very fine-grained matrix (groundmass). The phenocrysts are usually sanidine, a glassy potash feldspar commonly rich in sodium, and may range up to two to five centimetres (one to two inches) across; smaller phenocrysts of other minerals may also occur. Rapid cooling and solidification of trachytic lava produces the fine texture of the groundmass, and cooling may be so rapid locally that small quantities of glass are formed.

Trachyte is associated with other lavas in volcanic regions and it have been formed by the crystallization and abstraction of iron, magnesium, and calcium minerals from a parent basaltic lava.

Volcanic Equivalent: Syenite

Group: volcanic.

Colour: variable but often light coloured, generally light coloured phenocrysts.

Texture: Usually porphyritic (can be trachytic), sometimes aphanitic.

Mineral Content: Orthoclase phenocrysts in a groundmass of orthoclase with minor plagioclase,biotite, hornblende, augite etc..

Silica (SiO 2) content – 60%-65%.

Trachytic Texture

Trachytic is an extruded rock texture composed of small tabular crystals consisting of micoid microliths, in which the mass of the soil contains a small amount of volcanic glass. The microliths are parallel, creating flow lines around the lava flow direction and flows. Trachytic texture occurs in alkaline rich rocks; therefore, the vitreous mass of the rocks has a relatively low viscosity.

Trachytic texture is especially characteristic of trachytes and rocks similar to trachytes. Trachytic textures are often attributed to flow orientation, however, there is little evidence to support this. Macroscopic trachytic textures visible with the naked eye are sometimes called trachytoid textures.

Classification

QAPF diagram as a means to visualize and memorize the classification of igneous rocks based on their content of the minerals quartz, alkali-feldspar, plagioclase, and foids.

Foid-bearing-trachyte appears in field 7′ in the QAPF Diagram

Chemical Composition of Trachyte

Chemically, trachyte contains 60 to 65% silica content; less SiO2 than rhyolite and more (Na2O plus K2O) than dacite. These chemical differences are consistent with the position of trachyte in the TAS classification, and they account for the feldspar-rich mineralogy of the rock type.

Formation of the Rock

Trachyte is probably a product of magmatic differentiation. Its parent magma was perhaps basaltic but it evolved (its composition became enriched in alkalies and silica) by the removal of mafic minerals. It may be associated with phonolite, latite, rhyolite, etc. which means that the same volcano has extruded magmas with slightly different composition. Trachyte is not necessarily volcanic in the strict sense. It may also form underground but still relatively close to the surface because its grain size is fine. Coarse-grained rocks with a trachytic composition are know as syenites as said before. Magma with a trachytic composition may also solidify as obsidian or pumice.

Where is It Located

Trachytes are well represented among the Cenozoic volcanic rocks of Europe. In Britain they occur in Skye as lava flows and as dikes or intrusions, but they are much more common on the continent of Europe, as in the Rhine district and the Eifel, also in Auvergne, Bohemia and the Euganean Hills. In the neighborhood of Rome, Naples and the island of Ischia trachytic lavas and tuffs are of common occurrence. Trachytes are also found on the island of Pantelleria. In the United States, trachytes crop out extensively in the Davis Mountains, Chisos Mountains, and Big Bend Ranch State Park in the Big Bend (Texas) region, as well as southern Nevada and South Dakota (Black Hills). There is one known voluminous flow from Pu’u Wa’awa’a on the north flank of Hualalai in Hawaii. In Iceland, the Azores, Tenerife and Ascension there are recent trachytic lavas, and rocks of this kind occur also in New South Wales (Cambewarra Range), Queensland (Main Range), East Africa, Madagascar, Yemen and in many other districts.

Uses of The Rock

  • Decorative Aggregates, Flooring, Homes, Interior Decoration
  • As Building Stone, As Facing Stone, Paving Stone, Garden Decoration, Office Buildings
  • Curbing
  • As Dimension Stone, Building houses or walls, Cement Manufacture, Construction Aggregate, for Road Aggregate, Landscaping, Making natural cement, Manufacture of Magnesium and Dolomite Refractories
  • Artifacts, Monuments, Sculpture, Small Figurines
  • Cemetery Markers, Creating Artwork

References

Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.

Wikipedia contributors. (2019, January 20). Trachyte. In Wikipedia, The Free Encyclopedia. Retrieved 00:54, April 11, 2019, from https://en.wikipedia.org/w/index.php?title=Trachyte&oldid=879352410

Cite this article as: Geology Science. (2019). Trachyte. [online] Available at: http://geologyscience.com/rocks/igneous-rocks/trachyte/ [9th December 2019 ]
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