Trachyte

Trachyte is a fine-grained and generally light-colored volcanic rock that usually has a rough surface to the touch which is the reason it was given that name (trachys is "rough" in the Greek language).

Trachytes are composed chiefly of alkali feldspar with minor amounts of dark-coloured minerals such as biotite, amphibole, or pyroxene. Compositionally, trachyte is the volcanic equivalent of the plutonic (intrusive) rock syenite. Most 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 commonly associated with other lavas in volcanic regions and is thought to have been formed by the crystallization and abstraction of iron, magnesium, and calcium minerals from a parent basaltic lava.

Two types of trachyte are commonly recognized. In potash, or normal, trachyte, sanidine or orthoclase is the dominant feldspar, and the plagioclase feldspar is generally oligoclase. In soda, or alkali, trachyte, both the alkali feldspar and the dark minerals are rich in sodium. As the ratio of alkali feldspar to plagioclase decreases, trachyte passes into latite. Normal trachytes may contain diopside; alkali types may show pyroxene crystals with diopsidic cores.
Silica-rich trachyte may have small amounts of quartz that is interstitial to feldspar; tridymite and cristobalite are commonly found lining the small cavities that developed as expanding gas bubbles in the molten lava. As the free silica content increases, trachyte passes into rhyolite. Marked decrease in silica favours the formation of such feldspathoids as leucite, nepheline, sodalite, and analcime; as the quantity of feldspathoids increases, trachyte passes into phonolite.

Trachytes and the Petrogeny’s Residua System

Trachytes appear to occupy a paradoxical position within the ternary system NaAlSiO4 (Nepheline)-KalSiO4 (Kalsilite)-SiO2 (Quartz) which bowen has called Petrogeny’s Residua System. On cooling, the composition of many of the common residual magmas end up in this system, and fractional crystallization drives them toward the low-temperature trough, or "Termal valley" (point O-S-U).


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Petrogeny’s Residua System: Cg = Carnegieite, Ne = Nepheline, Ks = Alkali Feldspar, Lc = Leucite, Ab = Albite, Or = Orthose. MU - M - MS = low-temperature trough (pale red), M = central ridge of the low-temperature trough (Red Dot). The two small diagrams, to the right and left of the Petrogeny’s Residua System, represent two termal sections (like geological section but, in place of isolines, we have isotherms) along the line MU - M - MS. Note the morphology of the termal valley with the two subsidiary troughs and the central ridge.



In detail this low-temperature trough is found to contain a central ridge (M) that split the trough into two subsidiary troughs (MS and MU). One minor trough contains liquids of Rhyolite-granite composition (MS), whereas the other contains liquid of Phonolite-nepheline Syenite composition (MU). In this simplified system, trachytic liquids occupy a composition field that is situated on the thermal ridge (M) between the rhyolitic and phonolitic trough. 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).

It has been postulated that the shape of the low-temperature trough in Petrogeny’s residua system may change at higher pressures and/or at higher partial pressure of water vapour. 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 1.0 GPa. 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.

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QAPF Diagram Trachyte field in blue



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Trachyte with sanidine phenocrysts from Gran Canaria. From www.sandatals.org


Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• D’Amico C., Innocenti F. & Sassi F.P. (1987): Magmatismo e metamorfismo. UTET
• Innocenti F., Rocchi S. & Triglia R. (1999:) La classificazione delle rocce vulcaniche e subvulcaniche: schema operativo per il progetto CARG.
• Carmichael I.S.E., Turner F.J. & Verghoogen J. (1974): Igneous Petrology. McGraw-Hill.

Photo
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Sanidine crystals in a Trachyte. XPL image, 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image, 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image, 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image, 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image, 2x (Field of view = 7mm)
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Sanidine crystals in a Trachyte. XPL image, 2x (Field of view = 7mm)
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Aegirine bearing-Trachyte. Aegirine is green. PPL image, 2x (Field of view = 7mm)
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Aegirine bearing-Trachyte. XPL image, 2x (Field of view = 7mm)
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Aegirine bearing-Trachyte. Aegirine is green. PPL image, 2x (Field of view = 7mm)
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Aegirine bearing-Trachyte. XPL image, 2x (Field of view = 7mm)
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Aegirine bearing-Trachyte. Aegirine is green. PPL image, 2x (Field of view = 7mm)
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Aegirine bearing-Trachyte. XPL image, 2x (Field of view = 7mm)
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Aegirine bearing-Trachyte. PPL image, 10x (Field of view = 2mm)
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Aegirine bearing-Trachyte. XPL image, 10x (Field of view = 2mm)
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Aegirine bearing-Trachyte. PPL image, 10x (Field of view = 2mm)
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Aegirine bearing-Trachyte. XPL image, 10x (Field of view = 2mm)
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Aegirine bearing-Trachyte. PPL image, 10x (Field of view = 2mm)
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Aegirine bearing-Trachyte. XPL image, 10x (Field of view = 2mm)
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Aegirine bearing-Trachyte. PPL image, 10x (Field of view = 2mm)
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Aegirine bearing-Trachyte. XPL image, 10x (Field of view = 2mm)
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Aegirine bearing-Trachyte. XPL image, 10x (Field of view = 2mm)
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Sanidine and plagioclase (rimmed by sanidine = antirapakivi texture). XPL image, 10x (Field of view = 2mm)
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Sanidine in a Trachyte. XPL image, 10x (Field of view = 2mm)
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Plagioclase (rimmed by sanidine = antirapakivi texture). XPL image, 10x (Field of view = 2mm)
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Plagioclase crystals (rimmed by sanidine = antirapakivi texture). XPL image, 10x (Field of view = 2mm)
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Trachytic groundmass. XPL image, 10x (Field of view = 2mm)
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Trachytic groundmass. XPL image, 10x (Field of view = 2mm)
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Trachytic groundmass. XPL image, 10x (Field of view = 2mm)