TONALITE, in petrology, a rock of the diorite class, first described from Monte Adamello near Tonale in the Eastern Alps. It may be described as a quartz-diorite containing biotite and hornblende in nearly equal proportions. The principal felspar is plagioclase, but orthoclase occurs also, usually in small amount. With increase in silica, the tonalites pass into the granodiorites (see GRANITE).
The hornblende of the diorites is green, sometimes with a tinge of brown; the biotite is always brown and strongly pleochroic. Often these two minerals are clustered together irregularly or in parallel growths. They have generally a fairly strong tendency to idiomorphism, but may sometimes enclose plagioclase fel spar in ophitic manner. Both of them decompose to chlorite, epidote and carbonates. The plagioclase felspar, which may form more than one-half of the rock, is andesine or oligoclase; simple crystals are rare, the majority being complex growths with centres of felspar rich in lime, while in the external zones the proportion of soda felspar increases greatly. The inner portions have often well-defined, but very irregular, boundaries, and are sometimes sponge-like, with the cavities filled up with a later, more acid, deposit. This seems to indicate that growth has taken place in stages, alternating with periods when the crystal lized felspar was corroded or partly dissolved. The orthoclase
sometimes forms irregular plates enclosing individuals of plagio clase. Quartz occurs both in irregular simple grains and as micropegmatite. Occasionally pale green pyroxene is visible in the centre of crystals of dark green hornblende. The accessory minerals apatite, magnetite and zircon are always present, and very common also are orthite in coffee-coloured zonal prisms practically always encircled by yellow epidote, and reddish-brown crystals of sphene, simple or twinned.
Externally tonalites are very like granites but are usually darker. Towards their margins the larger alpine masses of ton alite often assume banded or gneissic facies, due apparently to movement during intrusion. In the south of Scotland (Galloway district) they accompany hornblende- and biotite-granites, horn blende- and augite-diorites. The newer granites of the Highlands of Scotland in many places pass into tonalites, especially near their margins, and similar rocks occur in Ireland. Grano-diorites have been described from California, and rocks of similar char acter occur in the kndes, Patagonia and Lesser Antilles. Tonalites are also said to be frequent among the igneous rocks of Alaska.
(J. S. F.)