The mineral assemblage of trachytes consists of essential alkali feldspar. Relatively minor
plagioclase and
quartz or a
feldspathoid such as
nepheline may also be present. This is reflected in the position of the trachyte fields in the
QAPF diagram.
Biotite,
clinopyroxene and
olivine are common accessory minerals. The plagioclase is typically sodium-rich
oligoclase. The alkali feldspar is typically also sodium-rich
sanidine (
anorthoclase) and is often
cryptoperthitic, with alternating microscopic bands of sodium feldspar (
albite) and potassium feldspar (sanidine). Trachytes are typically fine-grained and light-colored, but can be black if they consist mostly of glass. They are often porphyritic, with large well-shaped crystals of sanidine in a
groundmass containing much smaller imperfect sanidine laths.
Rhomb porphyry is an example with usually large
porphyritic rhomb shaped
phenocrysts embedded in a very fine-grained
matrix. Some of the best known trachytes, such as the trachyte of
Drachenfels 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. The
ferromagnesian 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. Trachytes very often have minute irregular
vesicles 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 from
Auvergne, and was long used in a much wider sense than that defined above, so that it included quartz-trachytes (now known as
liparites and
rhyolites) and
oligoclase-trachytes, which are now classified as
andesites. Quartz is rare in trachyte, but
tridymite (which likewise consists of
silica) is not uncommon. 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). Glassy forms of trachyte (
obsidian) occur, as in
Iceland, and
pumiceous 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. ==Geographic distribution==