Rhyolite

RHYOLITE, the group name of a type of volcanic rock, occurring mostly as lava flows, characterized by a highly acid composition, and so called from Gr. bimE, to flow (because of the frequency with which they exhibit fluxion structures). They are the most siliceous of all lavas, and, with the exception of the dacites, are the only lavas with free primary quartz. In chemical composition they very closely resemble the granites, the cor responding rocks of plutonic or deep-seated origin ; their minerals also present many points of similarity to those of granite though they are by no means entirely the same. Quartz, orthoclase and plagioclase felspars, and biotite are the commonest ingredients of both rocks, but the quartz of rhyolites is full of glass enclosures and the potash felspar is pellucid sanidine, while the quartz of granite contains dust-like fluid cavities of very minute size and its potash felspar is of the turbid variety which is properly called orthoclase. The granites also are holocrystalline, while in the rhyolites there are usually porphyritic crystals floating in a fine ground-mass. Rhyolites have also been called liparites because many of the lavas of the Lipari Islands are excellent examples of this group. Above all rocks they have a disposition to assume vitreous forms, as when fused they crystallize with great difficulty; the vitreous forms are known as obsidian, perlite and pumice (qq.v.). • Mineral Constituents.—The minerals of the first generation, or phenocrysts, of rhyolite are generally orthoclase, oligoclase, quartz, biotite, augite, or hornblende. The felspars are usually glassy clear, small but of well-developed crystalline form : the pot ash felspar is sanidine, usually Carlsbad twinned ; the soda-lime felspar is almost always oligoclase, with characteristic polysynthetic structure. Both of these may be corroded and irregular in their outlines; their cleavage and twinning then distinguish them readily from quartz. The quartz occurs as blebs or sub-rounded grains, which are corroded double hexagonal pyramids. In some rhyolites apparent crystals of quartz or felspar are found under the microscope to consist of a micrographic intergrowth of the two. Biotite is always deep brown or greenish brown, in small hexagonal tablets, generally blackened at their edges by magmatic corrosion. Muscovite is not known in rhyolites. Hornblende may be green or brown; in the quartz-pantellarites it sometimes takes the form of strongly pleochroic brown crossyrite. Like biotite it is idiomorphic but often corroded in a marked degree. Augite, which is equally common or more common than the other ferro-magnesian min erals, is always green; its crystals are small and perfectly shaped, and corrosion phenomena are very rarely seen in it. Zircon, apa tite and magnetite are always present in rhyolites, their crystals being often beautifully perfect though never large. Olivine (faya

lite) is never a normal ingredient, but occurs in the hollow spher ulites or lithophysae of some rhyolites with garnet, tridymite, topaz and other minerals which indicate pneumatolytic action.

Types of Texture.

The ground-mass of rhyolitic rocks is of three distinct types which are stages in crystalline development, viz., the vitreous, the felsitic or cryptocrystalline, and the micro crystalline. Mixtures of the different kinds occur ; thus a vitreous rhyolite has often felsitic areas in its ground-mass, and in the same lava flow some parts may be vitreous while others are felsitic. The vitreous rhyolites are identical in most respects with the obsidians, from which they can only be separated in an artificial classification; and in their glassy base the banded or eutaxitic, spherulitic and perlitic structures of pure obsidians are very frequently present (see OBSIDIAN : PERLITE). The felso-liparites or liparites with stony ground-mass are specially common among the pre-Tertiary igneous rocks, as liparite glass is unstable and experiences devitrification in course of time. Many of these felsites have fluxion banding, spherulites and even perlitic cracks, which are strong evidence that they were originally glassy. In other cases a hyaloliparite, obsidian or pitchstone becomes fel sitic along its borders and joint planes, or even along perlitic cracks, and we may assume that the once glassy rock has changed into felsite under the action of percolating moisture or even by atmospheric decomposition. In many rhyolites the felsite is original and represents an incipient crystallization of the vitreous material which took place before the rock was cold. The felsite in turn is liable to change; it becomes a fine mosaic of quartz and alkali felspar; and in this way a matrix of the third type, the microcrystalline, may develop. This is proved by the occurrence of the remains of spherulitic and perlitic structures in rocks which are no longer felsitic or glassy. Many microcrystalline rhyolites have a ground-mass in which much felsitic matter occurs; but as this tends to recrystallize in course of time, the older rocks of this group show least of it. Whilst no quartz bearing rhyolites are known to have been erupted in recent years, Lacroix proved that portions of the "spine" which rose as a great tower or column out of the crater of Mont Pelee after the eruption in 1906 contained small crystals of quartz in the ground-mass. The microcrystalline ground-mass of rhyolites is never micro graphic as in the porphyries (granophyres) ; on the other hand it is often micropoikilitic, consisting of small felspars, often sub rectangular, embedded in little rounded or irregular plates of quartz.