Under this group are comprised all of those granular igneous rocks composed of ferromagnesian minerals alone, or in which the amount of detectible feldspar is so small as to be entirely negligible as a component, and in which the mineral grains are sufficiently large to be determined. The peridotites are ultrabasic rocks without feldspar and composed mainly of olivine, the remaining minerals being one or more of the pyroxene-honblende group together with the iron ore spinel magnetite, or garnet. They occur chiefly as dykes and are not very common in their occurrence. Alkalis are present in small quantities only. Sp. G. 3'0-3'3. By decomposition they give rise to serpentine rocks.
The chemical composition
varies according to the minerals in the rocks but general characters are the
very low silica, the small amount or virtual absence of alkalies and
alumina, and the large quantities of iron and magnesia. A sa result, the
color of these rocks ordinarily varies from dull green to black. The dunites,
which are practically composed of the one mineral olivine, are at times much
lighter. They may show various shades of light green, medium yellow and
light brown, passing into one another, and from these through dull yellowish
green into dark green. They may thus be exceptions to the general rule that
ferromagnesian rocks are dark colored. These may occur alone or in various
mixtures, and according to these the group has been sub-divided into types,
some of the more prominent of which are as follows:
Pyroxenes and Olivine Peridotite.
Hornblende and Olivine Cortlandtite.
Olivine alone Dunite.
Pyroxenes alone Pyroxenite.
Hornblende alone Hornblendite.
The first three, which contain olivine, are comprised under the general name of peridotites, from peridot, the French word for olivine (and used in English to describe the gem version of olivine). But all the different types, while they sometimes occur independently, also occur together, with transition forms grading into one another, and it is difficult, and sometimes impossible, to distinguish them megascopically and therefore they are best treated together as one general group and not as separate rocks. Beside the minerals mentioned, a brown biotite mica sometimes occurs in these rocks, giving rise to the variety called mica peridotite. Additional accessory minerals, some of which are common and some confined to certain occurrences, are titanic iron ore, various spinel minerals, of which the chromium ore chromite is of importance, and garnet.
The peridotites and allied rocks sometimes occur independently as dikes, sheets, laccoliths or small intrusive stocks. In this way, as small isolated occurrences they have been found cutting the Paleozoic rocks, usually in a more or less altered condition, at Syracuse and other localities in New York State, in Kentucky, in Arkansas and elsewhere. But generally speaking they are most liable to occur in connection with greater intrusions of gabbros. Sometimes they form phases of the gabbro mass, with transitions between the two; sometimes they cut the gabbros in dikes or are found in small intrusions in their neighborhood. This dependence upon the gabbros has led to their being held in such cases as products of differentiation of the gabbro magma in which they represent the lamprophyres of other rock groups. In this way a great number of occurrences are known in all parts of the world where gabbros are common rocks.
Dunites occur in masses intrusive in the gneisses of western North and South Carolina and Georgia. Associated with them are smaller amounts of other peridotites and pyroxenite. These occurrences are of importance on account of the deposits of corundum of commercial value associated with them. The mineral is thought to have formed in them in the same manner as described under syenite. Dunite also occurs in considerable masses in New Zealand, especially in the Dun Mountains, from which came the name. Pyroxenite and hornblendite are comparatively rare and of relatively small geologic importance.
Their texture is granitoid or granular; its appearance depends somewhat on the minerals present and their arrangement. When pyroxene or hornblende is the dominant mineral the grain is often very coarse and may exhibit large cleavage surfaces. Dunite is not apt to be coarse grained; it commonly has a sugar-granular texture like many aplites, sandstones, marbles, etc. Porphyritic texture is rare or wanting. A common texture is one in which the cleavage surfaces of the pyroxenes or hornblendes are seen to be spotted with grains of olivine included in the larger crystal. Such a spotting of the shining cleavage surfaces of one mineral by smaller included crystals of another, which have no crystal orientation, either with respect to one another or to their host, is known as the poikilitic texture. It is sometimes well exhibited in these rocks. The included crystals are of course older than their host.
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