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PERIDOTITE

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Originally appearing in Volume V21, Page 148 of the 1911 Encyclopedia Britannica.
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PERIDOTITE , a plutonic holo-crystalline See also:

rock composed in large See also:part of See also:olivine, and almost or entirely See also:free from feldspar. The rocks are the most basic, or least siliceous plutonic rocks, and contain much See also:iron See also:oxide and See also:magnesia. Hence they have dark See also:colours and a high specific gravity (3.0 and over). They See also:weather readily and are changed to See also:serpentine, in which See also:process See also:water is absorbed and enters into chemical See also: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 See also:good crystalline See also:form, and See also:pale See also:green in See also:colour. Most of the rocks of this See also:group, however, contain other silicates such as See also:augite, See also:hornblende, See also:biotite or rhombic See also:pyroxene, and often two or three of these are See also:present. By the various See also:mineral combinations different See also:species are produced, e.g. See also:mica-periiot:te, hornblende-peridotite, See also:enstatite-peridotite. Of the See also:accessory minerals the commonest are iron oxides and See also:chromite or picotite. In some peridotites these form segregations or irregular masses which are of importance as See also:sources of the ores of See also:chromium. See also:Corundum occurs in small crystals in many See also:North See also:American peridotites and See also:platinum and the See also:nickel-iron See also:compound awaruite are found in rocks of this class in New See also:Zealand. Red See also:garnet (See also:pyrope) characterizes the peridotites of Bohemia.

The See also:

diamond mines of See also:South See also:Africa are situated in pipes or volcanic necks occupied by a peridotite See also:breccia which has been called kimberlite. In this rock in addition to diamond the following minerals are found, See also:hypersthene, garnet, biotite, pyroxene (chromediopside), See also:ilmenite, See also:zircon, &c. Some peridotites have a granular structure, e.g. the dunites, all the crystal grains being of rounded shape and nearly equal See also:size; a few are porphyritic with large individuals of See also:diallage, augite or hypersthene. Some are banded with parallel bands of dissimilar See also:composition, the result probably of fluxion in a magma which was not quite homogeneous. The See also:great See also: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 See also:amphibole succeeded it and caught up its crystals. In See also: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 See also:appearance the name " lustre-mottling " has been given.actinolite are very frequent. Other rocks contain dark See also: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 See also:Wales, See also:Anglesey, See also:Cornwall, See also:Cortland, New See also:York, and many other localities. A well-known peridotite from Schriesheimer Tal in the See also:Odenwald has pale brownish green amphibole in large crystals filled with small grains of olivine which are mostly serpentinized. Very often See also:primary brown hornblende in rocks of this type is surrounded by fringes and outgrowths of colourless See also:tremolite which has formed as a secondary mineral after olivine. See also:Complete pseudomorphs after olivine composed of a See also:matrix of scaly See also:talc and See also:chlorite crossed by a network of tremolite needles, are also very See also:common in some peridotites, especially those which have undergone pressure or shearing: these aggregates are known as pilite.

The peridotites which contain See also:

monoclinic pyroxene may be divided into two classes, those See also: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 See also:kind are known in See also:Hungary, in the Odenwald and in See also:Silesia. In See also:Skye the pyroxenebearing peridotites usually contain green chrome-See also: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 See also:German petrographers under the picrites, but this See also:term has a slightly different signification in the See also:English nomenclature (see See also:PICRITE). The enstatite-peridotites are an important group represented in many parts of the See also: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 See also:

Lizard (Cornwall) See also:Ayrshire and north-western See also:Scotland are of this type. Examples are known also from Baste near See also:Harzburg, New York and See also:Maryland, See also:Norway, See also:Finland, New Zealand, &c. 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 See also:rule very well shown. The lherzolites are rocks, first described from Lherz in the See also:Pyrenees, consisting of olivine, chrome-diopside and enstatite, and accessory picotite or chromite. They are See also:fine-grained, See also:bright green in colour, often very fresh, and may be somewhat granulitic. The dunites are peridotites, similar to the rock of Dun See also:Mountain, New Zealand, composed essentially of olivine in a finely granular See also:condition. Many examples of this type are known in different parts of the world, usually as See also:local facies of other kinds of peridotite. In olivine-basalts of See also:Tertiary See also:age in the See also:Rhine See also:district small nodules of green olivine occur frequently. They are of rounded shapes and may be a See also:foot in See also:diameter. The structure is granular and in addition to olivine they may contain chromite, See also:spinel and See also:magnetite, enstatite and chrome-diopside. Some geologists believe these to be fragments of dunite detached from masses of that rock not exposed at the See also:surface; others consider that they are aggregations of the See also:early minerals of the See also: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 See also:

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 See also: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. (J. S.

End of Article: PERIDOTITE

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PERIER, CASIMIR PIERRE (1777-1832)

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