Phonolite
PHONOLITE (Gr. ifavv, sound, and Xt0os, stone), in petrology, a group of volcanic lavas containing much nepheline and sanidine felspar. The term " clinkstone " was formerly given by geologists to many fine grained compact lavas, which split into thin tough plates, and gave out a ringing sound when struck with the hammer. Some of these clinkstones were phonolites in the modern sense, but as the name clinkstone was used for a large variety of rocks, many of which have no close affinities with one another, it has been discarded and " phonolite " is substituted for it. The group includes rocks which are rich in alkalis with only a moderate percentage of silica; hence they contain no free quartz but much alkali felspar (sanidine and anorthoclase) and nepheline. Large plates of sanidine are often visible in the rocks; the nepheline is usually not obvious to the unaided eye. Most phonolites show fluxion structure, both in the orientation of their phenocrysts and in the smaller crystals which make up the ground-mass; and this determines to a large extent the platy jointing. Although vitreous and pumiceous forms are known they are rare, and in the great majority of cases these rocks are finely crystalline with a dull or shimmering lustre in the groundmass. Marked characteristics are the readiness with which they decompose, and the frequency of veins and cavities occupied by natrolite, analcite, scolecite and other zeolites. Small black grains of augite or hornblende and sometimes blue specks of hatiyne may be seen in the rocks when they are fresh.
The dominant minerals are sanidine, nepheline, pyroxene, amphibole, various felspathoids and iron oxides. The sanidine is usually in two generations, the first consisting of large crystals of flattened and tabular shape, while the second generation is represented by small rectangular prisms arranged in parallel streams in the ground-mass; these felspars are nearly always simply twinned on the Carlsbad plan. They contain often as much soda as potash. The nepheline takes the form of hexagonal prisms with flat ends, and may be completely replaced by fibrous zeolites, so that it can only be recognized by the outlines of its pseudomorphs. In some phonolites it is exceedingly abundant magnetite and zircon occur in the phonolites, and sphene is often rather common. Another mineral which is more frequent in phonolites than in many other rocks is brown melanite garnet
The majority of the rocks of this group are of Tertiary or Recent age, but in Scotland Carboniferous phonolites occur in several localities, e.g. Traprain in Haddingtonshire, also in the Eildon Hills and in Renfrewshire. In Brazil phonolites belonging to the same epoch are also known. There are several districts in Europe where Tertiary or Recent phonolites occur in considerable numbers, as in Auvergne (Mont Dore), the Eifel, and Bohemia. The Wolf Rock which lies off the south coast of Cornwall, and is the site of a well-known lighthouse, is the only mass of phonolite in England; it is supposed to be the remains of a Tertiary lava or intrusion. The Canary Islands, Cape Verde Islands, Sardinia, Aden, British East Africa and New Zealand contain many types of phonolites; they are known also in New South Wales, while in the United States phonolites occur in Colorado (at Cripple Creek) and in the Black Hills of South Dakota.
Leucite occurs in place of nepheline in a small group of phonolites (the leucite-phonolites), known principally from Rocca Monfina and other places near Naples. Blue hauyne is rather a conspicuous mineral in some of these rocks, and they also contain a good deal of sphene. When sanidine, nepheline and leucite all occur together in a volcanic rock it is classed among the leucitophyres (see PETROLOGY, Plate III. fig. 2).
The chemical analyses of phonolites given below show that these rocks are very rich in alkalis and alumina with only a moderate amount of silica, while lime, magnesia and iron oxides are present only in small quantity. They have a close resemblance in these respects to the nepheline-syenites of which they provide the effusive types. (J. S. F.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
