PHONOLITE (Gr. φωνή, sound, λίθος, 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 ground-mass.
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
hauyne 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
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I. Phonolite, Wolf Rock, Cornwall...................
II. Phonolite, Teplitzer, Schlossberg, Bohemia
III. Leucite-phonolite, Rocca Monfina, Italy.....
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in the ground-mass, and these rocks form transitions to the
nephelinites (nephelinitoid phonolites) (see Petrology, Plate III.
fig. 1), in others it is scarce and the rocks resemble trachytes
containing a little nepheline (trachytoid phonolites). The
felspathoid minerals, sodalite, hauyne and nosean, which
crystallize in isometric dodecahedra, are very frequent components
of the phonolites; their crystals are often corroded or partly
dissolved and their outlines may then be very irregular. Small
rounded enclosures of glass are often numerous in them. The
pyroxenes may be pale green diopside, dark green aegirine-augite,
or blackish green aegirine (soda iron pyroxene), and in many
cases are complex, the outer portions being aegirine while the
centre is diopside. Fine needles of aegirine are often found in
the ground-mass. The commonest hornblende is dark brown
barkevicite. Biotite and olivine are not really frequent in
these rocks, and usually have been affected by resorption.
The ordinary accessory minerals of igneous rocks, apatite,
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
rcspects to the nepheline-syenites of which they provide the effusive
types. (J. S. F.)