Trachyte

TRACHYTE (Gr. Tp ax in, rough), in petrology, a group of volcanic rocks which consist mainly of sanidine (or glassy ortho clase) felspar. Very often they have minute irregular steam cavities which make the broken surfaces of specimens of these rocks rough and irregular; whence comes their name, which was first given by }Tally to certain rocks of this class from Auvergne, and long included quartz-trachytes (now known as liparites and rhyolites) and oligoclase-trachytes, which are now more properly assigned to andesites.

Mineral Composition.

The trachytes are often described as being the volcanic equivalents of the plutonic syenites. Their dominant mineral, sanidine felspar, 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 ground mass. 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 soda felspar, and has rather the characteristics of anorthoclase or cryptoperthite than of pure sanidine.

Quartz is typically absent from the trachytes, but tridymite (which likewise consists of silica) is by no means uncommon in them. It is rarely in crystals large enough to be visible without the aid of the microscope, but in thin slides it may appear as small hexagonal plates, which overlap and form dense aggregates ; they often cover the surfaces of the larger felspars or line the steam cavities of the rock, where they may be mingled with amorphous opal or fibrous chalcedony. In the older trachytes secondary quartz is not rare, and probably sometimes results from the recrystallization of tridymite.

Of the ferromagnesian minerals present augite is the most common. It is usually pale green, and its small crystals are often very perfect in form. Brown hornblende and biotite occur also, and are usually surrounded by black corrosion borders composed of magnetite and pyroxene. Sometimes the replacement is com plete 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, like those of the Arso in Ischia. 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 or bronzite) have been observed but are not common. Apatite, zircon and magnetite are practically always present as unimportant accessory minerals.

The trachytes being very rich in potash felspar, necessarily contain considerable amounts of alkalis ; in this character they approach the phonolites. Occasionally minerals of the felspathoid group, such as nepheline, sodalite and leucite, occur, and rocks of this kind are known as phonolitic trachytes. The soda-bearing

amphiboles and pyroxenes so characteristic of the phonolites may also be found in some trachytes ; thus aegirine or aegirine-augite forms outgrowths on diopside crystals, and riebeckite may be present in spongy growths among the felspars of the ground mass (as in the trachyte of Berkum on the Rhine). Trachytic rocks are typically porphyritic, and some of the best-known examples, such as the trachyte of Drachenfels, show this character excel lently, having large sanidine crystals of tabular form 1-2 in. in length scattered through their fine-grained ground mass.

Two types of ground mass are generally recognized : the tra chytic, composed mainly of long, narrow, sub-parallel rods of sanidine, and the orthophyric, consisting of small, squarish or rec tangular prisms of the same mineral. Sometimes granular augite or spongy riebeckite occurs in the ground mass, but as a rule this part of the rock is highly felspathic. Glassy forms of trachyte (obsidians) occur, as in Iceland, and pumiceous varieties are known (in Teneriffe and elsewhere), but as contrasted with the rhyolites, trachytes have a strong tendency to crystallize, and are rarely vitreous.

Distribution.

Trachytes are well represented among the Ter tiary and Recent volcanic rocks of Europe. In Britain they occur in Skye as lava flows and as dikes or intrusions, but they are much more common on the Continent, as in the Rhine district and the Eifel, also in Auvergne, Bohemia and the Euganean Hills. In the neighbourhood of Rome, Naples and the island of Ischia trachytic lavas and tuffs are of common occurrence. In America trachytes are less frequent, but occur in S. Dakota (Black Hills). In Ice land, the Azores, Teneriffe and Ascension there are Recent tra chytic lavas, and rocks of this kind occur also in New South Wales (Cambewarra range), East Africa, Madagascar, Aden, etc.

In England there are Permian trachytes in the Exeter district, and Carboniferous trachytes in many parts of the central valley of Scotland. The latter differ in no essential respect from their modern representatives in Italy and the Rhine valley, but their augite and biotite are often replaced by chlorite and other second ary products. Permian trachytes occur also in Thuringia and the Saar district.

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 felspar (anor thoclase principally, but also albite and orthoclase), with a small quantity of chlorite and iron oxides. Many of them are lavas, but others are probably dikes or thin intrusions. As the analyses given above will show, they differ from trachytes mainly in being richer in soda. (J. S. F.)