MONZONITE, the group-name of a type of rocks which have acquired it from their most celebrated occurrence, that of Mon zoni in Tirol. The rocks are of granitic appearance, usually rather dark grey in colour and fine to moderately coarse grained. The special characteristic which distinguishes them from granites and ordinary syenites is the presence of basic plagioclase and ortho clase felspars in nearly equal amounts. Labradorite and andesine are present, usually in well-shaped crystals, often zoned ; ortho clase forms large irregular plates in which the other minerals are embedded. There is rarely any considerable amount of quartz, though in a few of these rocks this mineral occurs (the quartz monzonites). Other features are the abundance of augite, and of large bronze-coloured plates of biotite which are of quite irregular shapes and full of enclosures. Hypersthene or bronzite is less common, but hornblende is sometimes abundant. Olivine also may be present ; when the rock contains this in notable quantity it may be called an olivine-monzonite.
The monzonites of Tirol show a great variability in appearance, structure and the relative proportions of their minerals. They tend to pass into iolesites and gabbros, and near the margins of the outcrop occur facies very rich in pyroxene (pyroxenites).
Rocks of monzonitic facies occur also in Norway, where they have been described as akerites. They contain quartz, ortho clase and plagioclase, augite and biotite; hornblende and hyper sthene also may be present. Some of them have porphyritic
rather than granitic texture, especially near the margins of the laccoliths. From a study of these and other occurrences Brogger proposed to define the monzonites as orthoclase-plagioclase rocks in which the two chief classes of felspar occur in nearly equal quantities (as distinguished from the orthoclase rocks or granites and syenites and the plagioclase rocks or diorites and gabbros).
At Yogo Peak and Beaver Creek, Montana, there are masses of granitoid rock which bear a close resemblance to the monzonites of Tirol. Two main types occur: (a) yogoite, which differs little from monzonite, and (b) shonkinite, which is a more basic rock richer in plagioclase and augite ; this rock contains olivine and in places passes into dark pyroxenites. In shonkinite also a little nepheline may be present. In the west of Scotland intrusive bosses are known which consist of an olivine-bearing rock closely related to monzonite. It has been called kentallenite because it is quarried at Kentallen in Argyllshire.
The following analyses show the chemical peculiarities of the principal rocks of the monzonite group :— Si02 A1202 Fe2O3 FeO MgO CaO K20 Na20 Monzonite, Monzoni . . 54.20 3.67 5.40 3.4o 8.5o 4.42 Yogoite, Yogo Peak . . 54.42 14.28 3.32 4.13 6.12 7.72 4.22 3.44 Kentallenite, Argyllshire . 52.09 7.11 12.48 7.84 3.01 2.04 (C. E. T.)