Peridotite
PERIDOTITE, a plutonic holo-crystalline rock composed in large part of olivine, and almost or entirely free from feldspar. The rocks are the most basic, or least siliceous plutonic rocks, and contain much iron oxide and magnesia. Hence they have dark colours and a high specific gravity (3-0 and over). They weather readily and are changed to serpentine, in which process water is absorbed and enters jnto chemical combination with the silicates of magnesia and iron. In some peridotites, such as the dunites, olivine greatly preponderates over all other minerals. It is always in small, rather rounded crystals without good crystalline form, and pale green in colour. Most of the rocks of this group, however, contain other silicates such as augite, hornblende, biotite or rhombic pyroxene, and often two or three of these are present. By the various mineral combinations different species are produced, e.g. mica-peridot. : te, hornblende-peridotite, enstatite-peridotite. Of the accessory minerals the commonest are iron oxides and chromite or picotite. In some peridotites these form segregations or irregular masses which are of importance as sources of the ores of chromium. Corundum occurs in small crystals in many North American peridotites and platinum and the nickel-iron compound awaruite are found in rocks of this class in New Zealand. Red garnet(pyrope) characterizes the peridotites of Bohemia. The diamond mines of South Africa are situated in pipes or volcanic necks occupied by a peridotite breccia which has been called kimberlite. In this rock in addition to diamond the following minerals are found, hypersthene, garnet biotite, pyroxene ( chromediopside), ilmenite, zircon, etc.
Some peridotites have a granular structure, e.g. the dunites, all the crystal grains being of rounded shape and nearly equal size; a few are porphyritic with large individuals of diallage, augite or hypersthene. Some are banded with parallel bands of dissimilar composition, the result probably of fluxion in a magma which was not quite homogeneous. The great majority of the rocks of this group are poikilitic, that is to say, they contain olivine in small rounded crystals embedded in large irregular masses of pyroxene or hornblende. The structure is not unlike that known as ophitic in the dolerites, and arises from the olivine having first separated out of the liquid magma while the pyroxene or amphibole succeeded it and caught up its crystals. In hand specimens of the rocks the smooth and shining cleavage surfaces of hornblende and augite are dotted over with dull blackish green spots of olivine; to this appearance the name " lustre-mottling " has been given.
Mica-peridotites are not of frequent occurrence. A well-known rock from Kaltes Thai, Harzburg, contains much biotite, deep brown in thin section. Other examples are found in India and in Arkansas. Poikilitic structure is rarely well developed in this group. The " blue-ground " of Kimberley which contains the diamonds is a brecciform biotite-hypersthene-peridotite with augite. In the north of Scotland, in several places in Sutherland and Ross, there are peridotites with silvery yellow green biotite and large plates of pale green hornblende: these have been called scyelites. In the hornblende-peridotites lustre-mottling is often very striking. The amphibole may be colourless tremolite in small prisms, as in some varieties of serpentine from the Lizard (Cornwall); or pale green hornblende as in scyelite. In both these cases there is some probability that the hornblende has developed, partly at least, from olivine or augite. In sheared peridotites tremolite and actinolite are very frequent. Other rocks contain dark brown hornblende, with much olivine; there may also be augite which is often intergrown perthitically with the hornblende. Examples of this type occur in North Wales, Anglesey, Cornwall, Cortland, New York, and many other localities. A well-known peridotite from Schriesheimer Tal in the Odenwald has pale brownish green amphibole in large crystals filled with small grains of olivine which are mostly serpentinized. Very often primary brown hornblende in rocks of this type is surrounded by fringes and outgrowths of colourless tremolite which has formed as a secondary mineral after olivine. Complete pseudomorphs after olivine composed of a matrix of scaly talc and chlorite crossed by a network of tremolite needles, are also very common in some peridotites, especially those which have undergone pressure or shearing: these aggregates are known as pilite.
The peridotites which contain monoclinic pyroxene may be divided into two classes, those rich in diallage and those in which there is much augite. The diallage-peridotites have been called wehrlites; often they show excellent lustre-mottling. Brown or green hornblende may surround the diallage, and hypersthene may occur also in lamellar intergrowth with it. Some of these rocks contain biotite, while a little feldspar (often saussuritic) may often be seen in the sections. Rocks of this kind are known in Hungary, in the Odenwald and in Silesia. In Skye the pyroxenebearing peridotites usually contain green chrome-diopside (a variety of augite distinguished by its pale colour and the presence of a small amount of chromium). The augite-peridotites are grouped by German petrographers under the picrites, but this term has a slightly different signification in the English nomenclature (see Pic RITE).
The enstatite-peridotites are an important group represented in many parts of the world. Their rhombic pyroxene is often very pale coloured but may then be filled with platy enclosures which give it a metallic or bronzy lustre. These rocks have been called saxonites or harzburgites. When weathered the enstatite passes into platy masses of bastite. Picotite and chromite are common accessory minerals and diallage or hornblende may also be present. Many of the serpentine rocks of the Lizard (Cornwall) Ayrshire and north-western Scotland are of this type. Examples are known also from Baste near Harzburg, New York and Maryland, Norway, Finland, New Zealand, etc. Often the enstatite crystals are of large size and are very conspicuous in the hand specimens. They may be porphyritic, or may form a coarsely crystalline matrix enclosing innumerable olivine grains, and then lustre-mottling is as a rule very well shown.
The Iherzolites are rocks, first described from Lherz in the Pyrenees, consisting of olivine, chrome-diopside and enstatite, and accessory picotite or chromite. They are fine-grained, bright green in colour, often very fresh, and may be somewhat granulitic. The dunites are peridotites, similar to the rock of Dun Mountain, New Zealand, composed essentially of olivine in a finely granular condition. Many examples of this type are known in different parts of the world, usually as local facies of other kinds of peridotite. In olivine-basalts of Tertiary age in the Rhine district small nodules of green olivine occur frequently. They are of rounded shapes and may be a foot in diameter. The structure is granular and in addition to olivine they may contain chromite, spinel and magnetite, enstatite and chrome-diopside. Some geologists believe these to be fragments of dunite detached from masses of that rock not exposed at the surface; others consider that they are aggregations of the early minerals of the basalt magma, which were already crystallized before the liquid rock was emitted.
The great majority of stony or lithoidal meteorites (aerolites) are rich in olivine and present many analogies to the terrestrial peridotites. Among their minerals are hypersthene (enstatite) augite and chrome-diopside, chromite, pyrite and troilite, nickeliferous iron and basic plagioclase feldspar. The structure of these meteorites is described as " chondritic " ; their minerals often occur as small rounded grains arranged in radiate clusters; this has very rarely been observed in ordinary peridotites.
Although many peridotites are known in which the constituent minerals are excellently preserved, the majority show more or less advanced decomposition. The olivine is especially unstable and is altered to serpentine, while augite, hornblende and biotite are in large measure fresh. In other cases the whole rock is changed to an aggregate of secondary products. Most serpentines (q.v.) arise in this way. (] S. F.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
