- Trachyte
Trachyte is an
igneous ,volcanic rock with anaphanitic toporphyritic texture. The mineral assemblage consists of essentialalkali feldspar ; relatively minorplagioclase andquartz or afeldspathoid such asnepheline may also be present. (See theQAPF diagram ).Biotite ,clinopyroxene andolivine are common accessory minerals.Chemically, trachyte contains less SiO2 than
rhyolite 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.Trachytes usually consist mainly of
sanidine feldspar. Very often they have minute irregular steam cavities which make the broken surfaces of specimens of these rocks rough and irregular, and from this character they have derived their name. It was first given to certain rocks of this class from Auvergne, and was long used in a much wider sense than that defined above, in fact it included quartz-trachytes (now known as liparites andrhyolite s) and oligoclase-trachytes, which are now more properly assigned toandesite s. The trachytes are often described as being the volcanic equivalents of the plutonic syenites. Their dominant mineral, sanidine feldspar, very commonly occurs in two generations, i.e. both as large well-shaped porphyritic crystals and in smaller imperfect rods or laths forming a finely crystalline groundmass. With this there is practically always a smaller amount of plagioclase, usually oligoclase; but the potash felspar (sanidine) often contains a considerable proportion of the sodium feldspar (albite), and has rather the characteristics of anorthoclase or cryptoperthite than of pure sanidine.Rhomb porphyry is an example with usually largeporphyritic rhomb shapedphenocrysts embedded in a very fine grained matrix.Quartz is typically rare in trachyte, but
tridymite (which likewise consists ofsilica ) is by no means uncommon. It is rarely in crystals large enough to, be visible without the aid of themicroscope , but in thin sections it may appear as small hexagonal plates, which overlap and form dense aggregates, like a mosaic or like the tiles on a roof. They often cover the surfaces of the larger feldspars or line the steam cavities of the rock, where they may be mingled with amorphousopal or fibrouschalcedony . In the older trachytes, secondary quartz is not rare, and probably sometimes results from therecrystallization of tridymite.Of the
mafic 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 of magnetite and pyroxene; Sometimes the replacement is complete and no bornblende or biotite is left, though the outlines of the cluster ofmagnetite and augite may clearly indicate from which of these minerals it was derived. Olivine is unusual, though found in some trachytes, like those of the Arso in Isthia. Basic varieties of plagioclase, such as labradorite, 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.The trachytes being very rich in potash feldspar, necessarily contain considerable amounts of alkali; in this character they approach the
phonolite s. Occasionally minerals of thefeldspathoid group, such asnepheline ,sodalite andleucite , occur, and rocks of this kind are known as phonolitic trachytes. The sodium-bearingamphibole s and pyroxenes so characteristic of the phonolites may also be found in some trachytes; thusaegirine or aegirineaugite forms outgrowths on diopside crystals, andriebeckite may be present in spongy growths among the feldspars of the groundmass (as in the trachyte of Berkum on theRhine ). Trachytic rocks are typically porphyritic, and some of the best known examples, such as the trachyte of Drachenfels on the Rhine, show this character excellently, 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. The ferromagnesian minerals rarely occur in large crystals, and are usually not conspicuous in hand 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. Glassy forms of trachyte (obsidian ) occur, as inIceland , andpumice ous varieties are known (inTeneriffe 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.Trachytes are well represented among the
Tertiary and recent volcanic rocks ofEurope . In Britain they occur in Skye aslava flow s 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 neighborhoord ofRome ,Naples and the island ofIschia trachyticlava s andtuff s are of common occurrence. In theUnited States trachytes are less frequent, being known inSouth Dakota (Black Hills ). InIceland , theAzores , Teneriffe and Ascension there are recent trachytic lavas, and rocks of this kind occur also inNew South Wales (Cambewarra range),East Africa ,Madagascar ,Aden and in many other districts.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 for Tertiary 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 ofScotland . The latter differ in no essential respect from their modern representatives inItaly and theRhine valley, but their augite and biotite are often repiaced by chlorite and other secondary products. Permian trachytes occur also in Thuringia and the Saar district inGermany .Closely allied to the trachytes are the keratophyres, which occur mainly in
Palaeozoic strata in the Harz (Germany), in the Southern Uplands of Scotland, inCornwall , 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.See also
*
igneous rock s
*list of rock types
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