Everything about Trachyte totally explained
Trachyte is an
igneous,
volcanic rock with an
aphanitic to
porphyritic texture. The mineral assemblage consists of essential
alkali feldspar; relatively minor
plagioclase and
quartz or a
feldspathoid such as
nepheline may also be present. (See the
QAPF diagram).
Biotite,
clinopyroxene and
olivine are common accessory minerals.
Chemically, trachyte contains less SiO
2 than
rhyolite and more (Na
2O plus K
2O) than
dacite. These chemical differences are consistent with the position of trachyte in the
TAS classification, and they account for the feldspar-rich mineralogy of the rock type.
Trachytes usually consist mainly of
sanidine feldspar. Very often they've minute irregular steam cavities which make the broken surfaces of specimens of these rocks rough and irregular, and from this character they've 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 and
rhyolites) and oligoclase-trachytes, which are now more properly assigned to
andesites. 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, for example both as large well-shaped porphyritic crystals and in smaller imperfect rods or laths forming a finely crystalline groundmass. With this there's 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 large
porphyritic rhomb shaped
phenocrysts embedded in a very fine grained matrix.
Quartz is typically rare in trachyte, but
tridymite (which likewise consists of
silica) is by no means uncommon. It is rarely in crystals large enough to, be visible without the aid of the
microscope, 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 amorphous
opal or fibrous
chalcedony. In the older trachytes, secondary quartz isn't rare, and probably sometimes results from the
recrystallization 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. Brown
hornblende 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 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 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 or
bronzite) 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
phonolites. Occasionally minerals of the
feldspathoid group, such as
nepheline,
sodalite and
leucite, occur, and rocks of this kind are known as phonolitic trachytes. The sodium-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 feldspars of the groundmass (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 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 in
Iceland, and
pumiceous varieties are known (in
Teneriffe 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 of
Europe. In
Britain they occur in
Skye as
lava flows and as
dikes or intrusions, but they're 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 of
Rome,
Naples and the island of
Ischia trachytic
lavas and
tuffs are of common occurrence. In the
United States trachytes are less frequent, being known in
South Dakota (
Black Hills). In
Iceland, the
Azores, Teneriffe and
Ascension there are recent trachytic lavas, and rocks of this kind occur also in
New 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've 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 are
Permian trachytes in the Exeter district, and
Carboniferous 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 the
Rhine 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 in
Germany.
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 feldspar (anorthoclase principally, but also albite and orthoclase), with a small quantity of chlorite and iron oxides.
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