1911 Encyclopædia Britannica/Rhyolite

by John Smith Flett

RHYOLITE (Gr. ῥεῖν, to flow, because of the frequency with which they exhibit fluxion structures), the group name of a type of volcanic rock, occurring mostly as lava flows, and characterized by a highly acid composition. They are the most siliceous of all lavas, and, with the exception of the dacites, are the only lavas which contain free primary quartz. In chemical composition they very closely resemble the granites which are the corresponding 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. Hence it has long baffled experimenters to produce rhyolite synthetically by fusion; it is stated that these difficulties have now been overcome, but geologists believe that the presence of steam and other gases in the natural state expedites crystallization. In crucibles these cannot be retained at the temperatures employed; when the rocks are melted the gases escape and on cooling a pure glass is formed. The vitreous forms of rhyolite are known as obsidian, perlite and pumice (qq.v.).

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 potash felspar is sanidine, usually Carlsbad twinned; the soda-lime felspar is a most 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. Glass enclosures, sometimes rectangular with small immobile bubbles, are frequent. The quartz occurs as blebs or sub-rounded grains, which are corroded double hexagonal pyramids. Its glass enclosures are many and nearly always rounded or elliptical in section. No proper cleavage is seen in the quartz, though arcuate (conchoidal) fractures may often be noticed; they may have been produced by strain on cooling. Phenocrysts of micropegmatite are known in some rhyolites; they may have the shape of felspar or of quartz crystals; in the former case Carlsbad twinning is by no means uncommon, but in other cases hour-glass structure is very conspicuous. 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 cossyrite. Like biotite it is eumorphic but often corroded in a marked degree. Augite, which is equally common or more common than the other ferro-magnesian minerals, is always green; its crystals are small and perfectly shaped, and corrosion phenomena are very rarely seen in it. Zircon, apatite and magnetite are always present in rhyolites, their crystals being often beautifully perfect though never large. Olivine is never a normal ingredient, but occurs in the hollow spherulites or lithophysae of some rhyolites with garnet, tridymite, topaz and other minerals which indicate pneumatolytic action. Among the less common accessory minerals of the rhyolites are cordierite in crystals which resemble hexagonal prisms but break up under polarized light into six radiating sectors owing to complicated twinning: they weather to green aggregates of chlorite and Muscovite (pinite); garnet, sphene and orthite may also be met with in rhyolites.

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 microcrystalline. Hence some authorities have proposed to subdivide the group into the vitrophyres, the felsophyres and the granophyres, but this is not now in use, and the last of these terms has obtained a signification quite different from that originally assigned to it. 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 obsidian's, 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 obsidian's are very frequently present (see Obsidian; Perlite). The felsoliparites or liparites with stony ground-mass are especially common among the pre-Tertiary igneous rocks (see Quartz-Porphyry), 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 felsitic along its borders and joint planes, or even along perlitic cracks, and we may assume that the once fibrous 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 yet 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 micro crystalline, 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 “ dome ” which rose as a great tower or column out of the crater of Mont Pelée after the eruption in 1906 contained small crystals of quartz in the ground-mass. The rock was an acid andesite, and it was ascribed by Lacroix to the action of steam retained in the rock under considerable pressure. 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.

The ground-mass of rhyolites is liable to other changes, of which the most important are silicification, kaolinization and sericitization. Among the older rocks of this group it is the exception to find that secondary quartz has not been deposited in some parts of them. Often indeed the matrix is completely replaced by silica in the form of finely crystalline quartz or chalcedony; and these rocks on analysis prove to contain over 90% of silica. In the recent rhyolites of Hungary, New Zealand, &c., the deposit of coarse opal in portions of the rock is a very common phenomenon.

Kaolinization may be due to weathering, and the stony dull appearance of the matrix of many micro crystalline rhyolites is a consequence of the decomposed state of the felspar grains in them; it is even more typically developed by fumarole action, which replaces the felspars with soft, cloudy, white products which belong to a mineral of the kaolin group. Sericitization, or the development of fine white mica after felspar, is usually associated with shearing, and is commonest in the older rhyolites.

Vesicular structure is very common in rhyolites; in fact the pumiceous obsidian's have this character tion than any other rocks (see Pumice); but even the felsorhyolites are very often vesicular. The cavities are usually lined with opal and tridymite; in the older rocks they may be filled with agate and chalcedony. The “ mill-stone porphyries,” extensively used in Germany for grinding corn, are porous rhyolites; the abundance of quartz makes them hard, and their rough surfaces render them peculiarly suitable for this purpose. In some of them the cavities are partly secondary. These rocks are obtained in the Odenwald, Thuringerwald and Fichtelgebirge.

In Britain a pale grey Tertiary rhyolite occurs at Tardree, Antrim (the only British rock containing tridymite), and in Skye. Felsitic rhyolites occur among the Old Red rocks of Scotland (Pentland Hills, Lorne, &c.), in Devonshire, and in large numbers in North Wales. The Carnarvonshire rhyolites are often much altered and silicified; many of them have a nodular structure which is very conspicuous on weathered surfaces. The spheroids may be two or three inches in diameter; some of them are built up of concentric shells. Rhyolites are also known from Fishguard, Malvern, Westmorland and Co. Waterford. One of the oldest volcanic rocks of Britain (pre-Cambrian, Uriconian) is the spherulitic rhyolite of the Lea Rock near Wellington; in Shropshire. It shows bright red spherulites in great numbers and is probably an obsidian completely devitrified. Perlitic structure is also visible in it.

In other parts of Europe rhyolites have a fairly wide distribution though they are not very numerous. In Hungary (Hlinik, &c.) there are many well-known examples of this class. They extend along the margin of the Carpathians and are found also in Siebenburgen. In Italy they occur in the Euganean Hills and in the Lipari Islands; the latter being the principal source of pumice at the present day. Rhyolites of Recent age occur in Iceland (Myvatn, &c.), where they are characterized by the frequent absence of quartz, and th e presence of much plagioclase and pyroxene. Some of these rocks have been called trachyte-obsidians, but they seem to be rhyolites which contain an exceptionally large amount of soda. The older rhyolites, which are generally called quartz-porphyries in Germany, are mostly of Permian or Carboniferous age and are numerous in the Vosges, Odenwald, Thuringerwald, &c. They are often accompanied by basic rocks (melaphyres). Permian rhyolites occur also at Lugano in Italy. Rhyolites are known also in Asia Minor and the Caucasus, in New Zealand, Colorado, Nevada and other parts of western North America. In the Yellowstone National Park there is a well-known cliff of obsidian which shows remarkably perfect columnar jointing. Some of the rhyolites of Nevada are exceedingly rich in porphyritic minerals, so that they appear at first sight to be holocrystalline rocks, since the ground-mass is scanty and inconspicuous. To this type the name nevadite has been given, but it is rare and local in its distribution.

In the island, of Pantellaria, which lies to the south-west of Sicily, there are rocks of rhyolitic affinities which present so many unusual features that they have been designated pantellarites. They contain less silica and alumina and more alkalis and iron than do ordinary rhyolites. Their felspars are of the anorthoclase group, being rich insoda together with potash, and are very variable in crystalline development. Aegirine-augite and forms of soda-amphibole are also characteristic of these rocks: dark brown aenigmatite or cossyrite often occur in them. Quartz is not very plentiful; other ingredients are olivine, arfvedsonite and tridymite. The ground-mass varies much, being sometimes quite vitreous, at other times a glass filled with swarms of microliths, while in certain pantellarites it is a micro crystalline aggregate of quartz and alkali felspar. The absence of plagioclase and biotite are marked distinctions between these rocks and the rhyolites, together with the scarcity of quartz and the prevalence of soda-bearing pyroxenes and amphiboles.

Among the Palaeozoic volcanic rocks of Germany there is a group of lavas, the quartz-keratophyres, which are of acid composition and rich in alkali felspar. Their dominant alkali is soda: hence their felspars are albite and cryptoperthite, not sanidine as in rhyolites. Quartz occurs sometimes as corroded phenocrysts, but is often scarce even in the ground-mass. Porphyritic biotite or augite are very rare, but occur in the matrix along with felspars and quartz. Micropegmatite is not infrequent in these rocks, and they may be silicified like the rhyolites. As quartz-keratophyres mostly occur in districts where there has been a good deal of folding, they are often crushed and more or less sericitized. They are best known from the Devonian rocks of Westphalia and the Harz, but are also. found in Queensland, and similar rocks have been described (as soda-felsites) from Ireland. The rocks which they accompany are usually diabases and spilites.

The other group of rhyolitic rocks rich in alkali felspars and soda pyroxenes and amphiboles are the comendites. They are often porphyritic, with crystals of quartz, sanidine, microperthite or albite: the ground-mass is microcrystalline or rarely micrographic, and often filled with spongy growths of aegirine and rieieckite. They are known from the recent eruptive districts of East Africa, from Sardinia and Texas, and very similar rocks occur as intrusive masses which may be grouped with the porphyries.

The following analyses show the composition of some of the principal types of rhyolites:—

	SiO2	Al2O3	Fe2O3	FeO	CaO	MgO	K2O	Na2O	H2O
I.	76·34	13·22	1·93		1·85	0·21	3·67	2·84	0·61
II.	72·15	13·50	3·12		0·93	0·16	4·54	4·20	0·85
III.	77·59	12·75	0·67	n.f.	0·04	0·16	3·99	2·56	1·54
IV.	67·48	9·70	7·24	2·21	1·45	0·77	2·84	7·21	0·96
V.	70·97	13·84	3·21	0·78	1·26	0·20	1·57	6·27	0·74
VI.	74·76	11·60	3·50	0·19	0·07	0·18	4·92	4·35	0·64

I.	Rhyolite, Telki Banya, Hungary.
II.	Rhyolite, do. Mafahlid, Iceland.
III.	Rhyolite, do. Omahu, New Zealand.
IV.	Pantellarite, Pantellaria.
V.	Quartz-keratophyre, Muhlenthal, Hara.
VI.	Comendite, Sardinia.

We note in the rhyolites I.–III. the very high silica, with alkalis and alumina also in considerable amount, while lime, magnesia and iron are very low. In the pantellarite, keratophyre and comendite the silica tends to be less abundant, while the alkalis, especially soda, increase; they have less alumina but are richer in iron and magnesia. It is easy to see why the latter types contain less quartz, felspars often very rich in soda, and femic minerals which contain iron and alkalis in notable amounts such as aegirine, riebeckite and arfvedsonite.  (J. S. F.)