Plutonic, Coarse-grained Rocks
Granite. A granite is a medium- to coarse-grained, light colored rock having an even texture and consisting chiefly of quartz and a feldspar. Frequently both orthoclase and a pagiolclase feldspar, and usually also small amounts of mica or hornblende are frequently the orthoclase in colored flesh-color or red, while the soda-lime feldspar in usually white or smoky-gray in color and is found in irregular grains filling up the interstices between the other minerals. The mica, which may be either muscovite or biotite, is to be recognized by its cleavage. Granite is a common rock type.
Syenite. A syenite is a medium- to coarse-grained light-colored rocks with an even texture and much like a granite in appearance. It is to be distinguished from granite, however, by the fact that it contains little or no quartz. It chief minerals are the feldespars, with more or less hornblende, mica or pyroxene. A variety, know as nephelite, is characterized by the presence of considerable amount of nephelite. Another variety, called anorthosite, is composed chiefly of labradorite. The feldspars, mica and hornblende, may be distinguished as described under granite. The pyroxene resembles horblende in appearance, but does not show as good a prismatic cleavage. Nephelite is recognized by its lack of a distinct cleavage and its oily and greasy luster. Syenite are not very common.
Diorite. Diorite is a medium-to coarse-grained dark gray or greenish color rock having an even texture and consisting chiefly of hornblende and a feldspar, in which the hornblende predominates. Often fine grains of iron ore may be observed, and frequently considerable amounts of biotite. It is a common rocks type.
Gabbro. Gabbro is a medium-to coarse -grained dark gray to greenish black rock with an even texture composed chiefly of piroxene and a feldspar. It is closely similar to diorite, the distinction lying in the fact that it contains pyroxene instead of amphibole. These two mineral, as they occur in these rocks, cannot always be told apart by a megascopic examination. The pyroxene is usually in small crystal grains with rather poor prismatic cleavages which are at nearly right angles to each other. Hornblende is more liable to be in longer prismatic crystals and shows better cleavages, the angle of which is about 125°. It is a common rock.
Dorelity. This is a name given to those varieties of diorite and gabbro which are too fin-grained in character to enable one to tell megascopically whether the dark-colored minerals which they contain is hornblende or pyroxene.
Peridotite. A peridotite is a medium- to coarse-grained dark green to black rock with an even texture which consists wholly of ferromagnesian minerals. These are chiefly olivine, piropxene and hornblende. As one or the other of these minerals predominates, various variety names are used, such as dunite for an olivine rock and pyroxenite and horblendite for respectively pyroxene and hornblende rocks. Common accessory minerals found in these rocks are ilmenite, chromite and garnet. The periodites are not very common in their occurrence.
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