| Igneous rock | |
 A cut sample of trachyte, withphenocrysts ofsanidine, in a wall in Germany | |
| Composition | |
|---|---|
| Classification | Felsic |
| Primary | Alkali feldspar |
| Secondary | Plagioclase,quartz, orfeldspathoid |
| Texture | Aphanitic |
| Equivalents | Intrusive variant issyenite |
Trachyte (/ˈtreɪkaɪt,ˈtræk-/) is anextrusiveigneous rock composed mostly ofalkali feldspar. It is usually light-colored andaphanitic (fine-grained), with minor amounts ofmafic minerals,[1] and is formed by the rapid cooling of lava (or shallow intrusions) enriched with silica andalkali metals.[2][3][4] It is the volcanic equivalent ofsyenite.[5]
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 been used as decorative building stone[12] and was extensively used asdimension stone in theRoman Empire and theRepublic of Venice.[13]
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]
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]
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]
Trachytes are well represented among theCenozoic volcanic rocks of Europe. InBritain they occur inSkye aslava flows and asdikes or intrusions,[23] but they are much more common on the continent of Europe, as in the Rhine district and theEifel, also in Auvergne,Bohemia and theEuganean Hills. In the neighborhood of Rome,Naples and the island ofIschia trachyticlavas andtuffs are of common occurrence.[16] Trachytes are also found on the island ofPantelleria. In the United States, trachytes crop out extensively in theDavis Mountains,Chisos Mountains, andBig Bend Ranch State Park in theBig Bend (Texas) region, as well as southernNevada andSouth Dakota (Black Hills). There is one known voluminous flow from Puʻu Waʻawaʻa on the north flank ofHualālai in Hawaiʻi.[16] Here the trachyte is glassy and black in color.[15] In Iceland, theAzores,Tenerife andAscension there are recent trachytic lavas, and rocks of this kind occur also inNew South Wales (Cambewarra Range), Queensland (Main Range),[24] East Africa,Madagascar,Yemen and in many other districts.[16]
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]
This article incorporates text from a publication now in thepublic domain: Flett, John S. (1911). "Trachyte". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 27 (11th ed.). Cambridge University Press. pp. 116–117.
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