Trachyte is common wherever alkali magma is erupted, including in late stages of ocean island volcanism[6][7] and in continentalrift valleys,[8] abovemantle plumes,[9] and in areas ofback-arc extension.[10] Trachyte has also been found inGale crater on Mars.[11]
Trachyte has a silica content of 60 to 65% and an alkali oxide content of over 7%. This gives it less SiO2 thanrhyolite and more (Na2O plus K2O) thandacite. These chemical differences are consistent with the position of trachyte in theTAS classification, and they account for the feldspar-rich mineralogy of the rock type.Trachydacite occupies the same field in the TAS diagram as trachyte, but is distinguished from trachyte by anormative quartz content over 20%.[2] Trachydacite is not a recognized rock type in the QAPF classification, where rocks rich in alkali feldspar and with quartz over 20% would be classified as rhyolites.[4]
QAPF diagram with trachyte fields highlightedPolished opal on trachyte
The mineral assemblage of trachytes consists of essential alkali feldspar. Relatively minorplagioclase andquartz or afeldspathoid such asnepheline may also be present.[7] This is reflected in the position of the trachyte fields in theQAPF diagram.Biotite,clinopyroxene andolivine are common accessory minerals. The plagioclase is typically sodium-richoligoclase. The alkali feldspar is typically also sodium-richsanidine (anorthoclase) and is oftencryptoperthitic, with alternating microscopic bands of sodium feldspar (albite) and potassium feldspar (sanidine).[14]
Trachytes are typically fine-grained and light-colored, but can be black if they consist mostly of glass.[15] They are often porphyritic, with large well-shaped crystals of sanidine in agroundmass containing much smaller imperfect sanidine laths.Rhomb porphyry is an example with usually largeporphyriticrhomb shapedphenocrysts embedded in a very fine-grainedmatrix. Some of the best known trachytes, such as the trachyte ofDrachenfels on the Rhine, show striking porphyritic character, having large sanidine crystals of tabular form an inch or two in length scattered through their fine-grained groundmass. In many trachytes, however, the phenocrysts are few and small, and the groundmass comparatively coarse. Theferromagnesian minerals rarely occur in large crystals, and are usually not conspicuous in hand-sized specimens of these rocks. Two types of groundmass are generally recognized: the trachytic, composed mainly of long, narrow, subparallel rods of sanidine, and the orthophyric, consisting of small squarish or rectangular prisms of the same mineral. Sometimes granular augite or spongy riebeckite occurs in the groundmass, but as a rule this part of the rock is highly feldspathic.[16]
Trachytes very often have minute irregularvesicles which make the broken surfaces of specimens of these rocks rough and irregular, and it is from this distinctive texture that they received their name. It was first given to rocks of this class fromAuvergne, and was long used in a much wider sense than that defined above, so that it included quartz-trachytes (now known asliparites andrhyolites) andoligoclase-trachytes, which are now classified asandesites.[17]
Quartz is rare in trachyte, buttridymite (which likewise consists ofsilica) is not uncommon.[14] It is rarely in crystals large enough to be visible without the aid of themicroscope, but inthin sections it may appear as small hexagonal plates, which overlap and form denseaggregates, like a mosaic or like the tiles on a roof. They often cover the surfaces of the larger feldspars or line the vesicles of the rock, where they may be mingled with amorphousopal or fibrouschalcedony. In the older trachytes, secondary quartz from therecrystallization of tridymite is not rare.[17]
Of themafic minerals present,augite is the most common. It is usually of pale green color, and its small crystals are often very perfect in form. Brownhornblende and biotite occur also, and are usually surrounded by black corrosion borders composed ofmagnetite andpyroxene; sometimes the replacement is complete and no hornblende or biotite is left, though the outlines of the cluster of magnetite and augite may clearly indicate from which of these minerals it was derived. Olivine is unusual, though found in some trachytes, for example those of the Arso inIschia. Basic varieties of plagioclase, such aslabradorite, are known also as phenocrysts in some Italian trachytes. Dark brown varieties of augite and rhombic pyroxene (hypersthene orbronzite) have been observed but are not common.Apatite,zircon and magnetite are practically always present as accessory minerals.[18][14]
Occasionally minerals of thefeldspathoid group, such asnepheline,sodalite andleucite, are present in trachytes,[16] and rocks of this kind are known as foid-bearing trachytes.[19] The sodium-bearingamphiboles and pyroxenes so characteristic of the phonolites may also be found in some trachytes;[14] thusaegirine or aegirine augite forms outgrowths ondiopside crystals, andriebeckite may be present in spongy growths among the feldspars of the groundmass (as in the trachyte ofBerkum on theRhine). Glassy forms of trachyte (obsidian) occur, as inIceland, andpumiceous varieties are known (in Tenerife and elsewhere), but these rocks as contrasted with the rhyolites have a remarkably strong tendency to crystallize, and are rarely to any considerable extent vitreous.[16]
The Breadknife is aperalkaline trachytedike in theWarrumbungles of eastern Australia. Iron Pot is one of several trachyte plugs in the Hedlow Creek region west of Yeppoon in Central Queensland.
Trachyte is the usual silica-rich end member of thealkaline magma series, in whichalkaline basaltic magma experiencesfractional crystallization while still underground. This process removes calcium, magnesium, and iron from the magma to give it a composition close to that of alkali feldspar.[6][7] As a result, trachyte is common wherever alkali magma is erupted, including late eruptions of ocean islands[6][7] and in continentalrift valleys andmantle plumes.[9] Only rarely does magmatic differentiation proceed beyond trachyte tophonolite or even more evolved alkaline magmas.[7] Trachyte also occurs in areas ofback-arc extension, such as the northernAegean Sea[9] and theAeolian arc of Italy.[20] The Aeolian back-arc includes theCampi Flegrei volcanic field,[21] where trachytes have been erupted.[22]
Among the older volcanic rocks trachytes also are not scarce, though they have often been described under the names orthophyre and orthoclase-porphyry, while trachyte was reserved forTertiary and recent rocks of similar composition. In England there arePermian trachytes in the Exeter district, andCarboniferous trachytes are found in many parts of the central valley of Scotland. The latter differ in no essential respect from their modern representatives in Italy and theRhine valley, but their augite and biotite are often replaced bychlorite and other secondary products. Permian trachytes occur also inThuringia and theSaar district in Germany.[16]
Alkaline rocks such as trachyte are rare in theArchean, but become common in theProterozoic. Alkaline rocks with an age close to 570 million years are common around the perimeters of manycontinental shields and are evidence of worldwide rifting at that time.[25]
Closely allied to trachyte is the rock type calledkeratophyre, which is the sodium-rich-plagioclase equivalent of trachyte.[26]
^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 140.ISBN9780521880060.
^Pe-Piper, G.; Piper, D.J.W. (2005). "The South Aegean active volcanic arc: relationships between magmatism and tectonics".Developments in Volcanology.7:113–133.doi:10.1016/S1871-644X(05)80034-8.ISBN9780444520463.
^Sautter, Violaine; Toplis, Michael J.; Beck, Pierre; Mangold, Nicolas; Wiens, Roger; Pinet, Patrick; Cousin, Agnes; Maurice, Sylvestre; LeDeit, Laetitia; Hewins, Roger; Gasnault, Olivier; Quantin, Cathy; Forni, Olivier; Newsom, Horton; Meslin, Pierre-Yves; Wray, James; Bridges, Nathan; Payré, Valérie; Rapin, William; Le Mouélic, Stéphane (June 2016). "Magmatic complexity on early Mars as seen through a combination of orbital, in-situ and meteorite data".Lithos.254–255:36–52.Bibcode:2016Litho.254...36S.doi:10.1016/j.lithos.2016.02.023.
^Germinario, Luigi; Siegesmund, Siegfried; Maritan, Lara; Mazzoli, Claudio (November 2017). "Petrophysical and mechanical properties of Euganean trachyte and implications for dimension stone decay and durability performance".Environmental Earth Sciences.76 (21): 739.Bibcode:2017EES....76..739G.doi:10.1007/s12665-017-7034-6.S2CID133942939.
^abcdBlatt, Harvey; Tracy, Robert J. (1996).Petrology : igneous, sedimentary, and metamorphic (2nd ed.). New York: W.H. Freeman. pp. 55–56.ISBN0716724383.
^Stevens, Neville (September 1996).The Main Range(PDF). Brisbane, Queensland: Geological Society of Australia. Archived fromthe original(PDF) on 19 July 2008.