the Week of Proper 26 / Ordinary 31
Click here to join the effort!
Bible Encyclopedias
Peridotite
1911 Encyclopedia Britannica
or 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. o and over). They weather readily and are changed to serpentine, in which process water is absorbed and enters into 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-peridotite, 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, &c.
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 Thal, 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 Picrite).
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, &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 rule very well shown.
The lherzolites 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. (J. S. F.) Perier, Casimir Pierre (1777-1832), French statesman, was born at Grenoble on the I ith of October 1777, the fourth son of a rich banker and manufacturer, Claude Perier (1742-1801), in whose house the estates of Dauphiny met in 1788. Claude Perier was one of the first directors of the Bank of France; of his eight sons, Augustin (1773-1833), Antoine Scipion (1776-1821), Casimir Pierre and Camille (1781-1844), all distinguished themselves in industry and in politics. The family removed to Paris after the revolution of Thermidor, and Casimir joined the army of Italy in 1798. On his father's death he left the army and with his brother Scipion founded a bank in Paris, the speculations of which he directed while Scipion occupied himself with its administration. He opposed the ruinous methods by which the duc de Richelieu sought to raise the war indemnity demanded by the Allies, in a pamphlet Reflexions sur le projet d'emprunt (1817), followed in the same year by Dernieres reflexions.. . in answer to an inspired article in the Moniteur. In the same year he entered the chamber of deputies for Paris, taking his seat in the Left Centre with the moderate opposition, and making his first speech in defence of the freedom of the press. Re-elected for Paris in 1822 and 1824, and in 1827 for Paris and for Troyes, he elected to represent Troyes, and sat for that constituency until his death. Perier's violence in debate was not associated with any disloyalty to the monarchy, and he held resolutely aloof from the republican conspiracies and intrigues which prepared the way for the revolution of 1830. Under the Martignac ministry there was some prospect of a reconciliation with the court, and in January 1829 he was nominated a candidate for the presidency of the chamber; but in August with the elevation to power of Polignac the truce ceased, and on the 15th of March 1830 he was one of the 221 deputies who repudiated the pretensions put forward by Charles X. Averse by instinct and by interest to popular revolution he nevertheless sat on the provisory commission of five at the hotel-de-ville during the days of July, but he refused to sign the declaration of Charles X.'s dethronement. Perier reluctantly recognized in the government of Louis Philippe the only alternative to the continuance of the Revolution; but he was no favourite with the new king, whom he scorned for his truckling to the mob. He became president of the chamber of deputies, and sat for a few months in the cabinet, though without a portfolio. On the fall of the weak and discredited ministry of Laffitte, Casimir Perier, who had drifted more and more to the Right, was summoned to power (March 13, 1831), and in the short space of a year he restored civic order in France and re-established her credit in Europe. Paris was in a constant state of disturbance from March to September, and was only held in check by the premier's determination; the workmen's revolt at Lyons was suppressed after hard fighting; and at Grenoble, in face of the quarrels between the military and the inhabitants, Perier declined to make any concession to the townsfolk. The minister refused to be dragged into armed intervention in favour of the revolutionary government of Warsaw, but his policy of peace did not exclude energetic demonstrations in support of French interests. He constituted France the protector of Belgium by the prompt expedition of the army of the north against the Dutch in August 1831; French influence in Italy was asserted by the audacious occupation of Ancona (Feb. 23, 1832); and the refusal of compensation for injuries to French residents by the Portuguese government was followed by a naval demonstration at Lisbon. Perier had undertaken the premiership with many forebodings, and overwork and anxiety prepared the way for disease. In the spring of 1832 during the cholera outbreak in Paris, he visited the hospitals in company with the duke of Orleans. He fell ill the next day of a violent fever, and died six weeks later, on the 16th of May 183 2.
His Opinions et discours were edited by A. Lesieur (2 vols., 1838); C. Nicoullaud published in 1894 the first part (Casimir-Perier, depute de l'opposition, 1817-1830) of a study of his life and policy; and his ministry is exhaustively treated by Thureau-Dangin in vols. i. and ii. (1884) of his Histoire de la monarchie de juillet. His elder Son, Auguste Victor Laurent Casimir Perier (1811-1876), the father of President Casimir-Perier (see Casimir Pfrier), entered the diplomatic service, being attached successively to the London, Brussels and St Petersburg embassies, and in 1843 became minister plenipotentiary at Hanover. In 1846 he resigned from the service to enter the legislature as deputy for the department of Seine, a constituency which he exchanged for Aube after the Revolution of 1848. On the establishment of the Second Empire he retired temporarily from public life, and devoted himself to economic questions on which he published a series of works, notably Les Finances et la politique (1863), dealing with the interaction of political institutions and finance. He contested Grenoble unsuccessfully in 1863 against the imperial candidate, Casimir Royer; and failed again for Aube in 1869. In 1871 he was returned by three departments to the National Assembly, and elected to sit for Aube. He was minister of the interior for a few months in 1871-1872, and his retirement deprived Thiers of one of the strongest elements in his cabinet. He also joined the shortlived ministry of May 1873. He consistently opposed all efforts in the direction of a monarchical restoration, but on the definite constitution of the republic became a life senator, declining MacMahon's invitation to form the first cabinet under the new constitution. He died in Paris on the 6th of June 1876.
For the family in general see E. Choulet, La Famille CasimirPerier (Grenoble, 1894).
These files are public domain.
Chisholm, Hugh, General Editor. Entry for 'Peridotite'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/p/peridotite.html. 1910.
