The Galleries:. Photo Gallery. Silicon Zoo. Chip Shots. DNA Gallery. Amino Acids. Religion Collection. Cocktail Collection. Screen Savers. Win Wallpaper. Closely allied to the trachytes are the keratophyres, which occur mainly in Palaeozoic strata in the Harz Germany , in the Southern Uplands of Scotland, in Cornwall, etc. They are usually porphyritic and fluidal; and consist mainly of alkali feldspar anorthoclase principally, but also albite and orthoclase , with a small quantity of chlorite and iron oxides.
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Additional recommended knowledge. Topics A-Z. All topics. To top. About chemeurope. We can say that the definition of this rock type is narrower now and it is not very abundant although its occurrences are widespread. Trachyte usually contains lots of K-feldspar sanidine but plagioclase, anorthoclase, and sometimes feldspathoids are common as well.
However, feldspathoid-bearing rocks are often named foid-trachytes if we use QAPF diagram instead of TAS diagram and trachytes that contain lots of plagioclase may be more properly described as trachyandesite known also as latite. Volcanic rocks that are similar to this rock are rhyolite contains more quartz , phonolite more feldspathoids , trachyandesite more plagioclase and dark minerals , and dacite more quartz and plagioclase.
See TAS diagram below to see how these rock types differ in their chemistry. Trachyte is a volcanic equivalent of syenite. Syenite is a feldspar-rich plutonic rock which is similar to granite but lacks or contains very little quartz. Mafic minerals in trachyte are usually biotite, amphiboles hornblende or arfvedsonite , and pyroxenes diopside , augite, aegirine. Aegirine and arfvedsonite occur in the rock when it is rich in alkali metals compositionally close to phonolite.
In this simplified system, trachytic liquids occupy a composition field that is situated on the thermal ridge M between the rhyolitic and phonolitic minima. Thus the evolution of trachytic liquids pose the problem of how can a liquid remain critically saturated with regards to silica during fractional crystallization, and finally congeal on a thermal ridge, and not move towards either silica oversaturation the rhyolites or silica undersaturation the phonolites.
Experimental studies have, however, shown that under hydrous conditions the general shape of the low-temperature trough is modified, but the rhyolite-granite and phonolite-nepheline syenite subsidiary troughs are preserved at pressures up to 1. Such increase in pressure do, however, contract the field occupied by leucite, and this helps to explain why leucite is not normally found in plutonic rocks. An introduction to the rock-forming minerals p. Igneous rocks. A classification and glossary of terms, 2.
Cambridge University Press. Magmas and magmatic rocks: an introduction to igneous petrology. Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations. Photo Sanidine crystals in a Trachyte. Aegirine is green.
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