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PLAGIOCLASE
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PLAGIOCLASE , an important See also:group of See also:rock-forming minerals, constituting an isomorphous See also:series between See also:albite, or soda-See also:felspar and See also:anorthite, or See also:lime-felspar. Inter- mediate members are thus soda-lime-felspars, which in their crystallographical, See also:optical and other See also:physical characters vary progressively with the chemical See also:composition between the two extremes albite (Na.AlSisOs) and anorthite (CaAl2Si2Os). This variation is continuous in the series, but specific names are applied to members falling between showing the relations of the foetal villi to the placental sinuses, the See also:Oligoclase> AbsAni to AbsAni'> See also:Andesine, AbsAns to See also:fusion of the amnion with the inner See also:surface of the chorion, and the thinning of the fused deciduae (capsularis and See also:vera). AbsAni; See also:Labradorite, AbsAni to AbsAni; See also:Bytownite, phylum as Saipa, a See also:placenta is formed, and the embryo is nourished Ab1Ana to AbiAns; Anorthite, An (= CaAl2Si2Os). within the See also:body of its See also:parent. In some of the viviparous sharks, All the members of the series crystallize in the anorthic e.g. the See also:blue See also:shark (Carcharias), the yolk-See also:sac has ridges which See also:fit (triclinic) See also:system. They possess a perfect cleavage parallel into grooves in the See also:wall of the oviduct and allow an interchange of to the basal pinacoid P (oor) and a somewhat less pronounced materials between the maternal and foetal See also:blood. This is an example of an " umbilical placenta." In the viviparous blennies (Zoarces cleavage parallel to the pinacoid M (Or)). The See also:angle between viviparus), among the teleostean fishes, two or three See also:hundred See also:young these two cleavages varies from 86° 24' in albite to 85° 50' in are nourished in the hollow ovary, which develops villi secreting anorthite. It was on See also:account of the oblique angle between nutritive material. Among the See also:Amphibia the alpine See also:salamander the cleavages that A. Breithaupt in 1847 gave the name plagio-(Sa ra atra) nourishes its young in its oviducts until the gilled a See also:stage e o of f development is past, while in the Reptilia the young o. clase (Gr. 7rXa ytoc, oblique, and tcXav, to cleave) to these felspars, viviparous See also:lizard (Seps chalcides) establish a communication between to distinguish them from the See also:orthoclase felspar in which the the yolk-sac anteriorly and the allantois posteriorly, on the one corresponding cleavage angle is a right angle. It should be See also:hand, and the walls of the oviduct on the other. In this way both noted that the potash--and potash-soda-felspars, See also:microcline an umbilical and an allantoic placenta are formed. The mammals are divided into Placentalia and Aplacentalia; (q.v.) an anorthoclase, though also anorthic, are not included in the latter group, to which the monotremes and most marsupials in the plagioclase series of soda-lime-felspars. Crystals are Fused decidua capsularis and decidua vera. Fused mesoderm of amnion and chorion. From A. H. Young and A. See also:Robinson, in See also:Cunningham's See also:Text-See also:Book of See also:Anatomy. usually See also:tabular in See also:habit, parallel to the See also:plane M, as shown in the accompanying figure; sometimes, however, they are flattened parallel to P, this being a characteristic habit of the pericline variety of albite; microlitic crystals forming the ground-See also:mass of volcanic rocks are usually elongated in the direction of the edge between P and M. Twinning is an important See also:character, which is almost invariably See also:present and affords a ready means of distinguishing the plagioclases from other feispars. Most frequent is the twinning according to the "albite See also:law " with M as twin-plane. One See also:half of the twin is turned through 18o° about the normal to this plane and the two portions are See also:united along the same plane (for figures of twinned crystals see ALBITE). The basal planes of the two portions are inclined to each other at a salient or re-entrant angle of 7° 12' in albite and 8° 20' in anorthite. This twinning is usually polysynthetic, being many times repeated, and giving rise to numerous thin lamellae, which are the cause of the See also:fine striations on the cleavage planes P and parallel to the edge PM, so characteristic of the plagioclases as seen in hand specimens. Viewed in polarized See also:light, thin sections of twinned crystals show a very characteristic banded structure parallel to M. A second twin-law is known as the "pericline-law" because of its frequent occurrence in pericline. Here the See also:axis of rotation is the edge x P (the crystallographic axis b) and the plane of composition is the " rhombic See also:section ": the latter is a plane which intersects the See also:prism faces T and l in a rhomb; it is not a possible See also:face of the crystal, and its position varies in the different See also:species. In addition to being twinned according to these two See also:laws, plagioclase may also be twinned on the See also:Carlsbad-, See also:Baveno- and Manebach-laws, as in orthoclase (q.v.). a specific infectious See also:fever, one variety being characterized by buboes (glandular swellings) and carbuncles. This See also:definition excludes many of the celebrated pestilences recorded in See also:history —such as the See also:plague of See also:Athens, described by See also:Thucydides; that not less celebrated one which occurred in the reign of See also:Marcus Aurelius and spread over nearly the whole of the See also:Roman See also:world (A.D. 164-180),1 which is referred to, though not fully described, by the contemporary See also:pen of See also:Galen; and that of the 3rd See also:century (about 2S3), the symptoms of which are known from the allusions of St See also:Cyprian (Sermo de mortalitate). There is a certain resemblance between all these, but they were very different from See also:Oriental plague. " Plague " was formerly divided into two See also:chief varieties: (I) mild plague, pestis See also:minor, larval plague (See also:Radcliffe), peste fruste, in which the See also:special symptoms are accompanied by little fever or See also:general disturbance; and (2) See also:ordinary epidemic or severe plague, pestis See also:major, in which the general disturbance is very severe. Cases which are rapidly fatal from the general disturbance without marked See also:local symptoms have been distinguished as fulminant plague (pestis siderans, peste foudroyante). History up to 880.—The first See also:historical See also:notice of the plague is contained in a fragment of the physician See also:Rufus of See also:Ephesus, who lived in the See also:time of See also:Trajan, preserved in the Collections of Oribasius.2 Rufus speaks of the buboes called pestilential as being specially fatal, and as being found chiefly in See also:Libya, See also:Egypt and See also:Syria. He refers to the testimony of a physician See also:Dionysius, who lived probably in the 3rd century B.c. or earlier; Constants of Plagioclase Felspars. Composi- SiO2. Al20,. Na2O. CaO. Sp. Melting- Cleavage Angle of Mean Optical Extinction. Rhombic Refractive On P.* On M.* In sections non. gr. point Angle Section. * See also:Index (Centigrade). PM. Ab 68.7 19.5 11.8 0 2.624 — 86° 24' + 27° 1.534 + 4° 30' +19° -16° Ab3An, 62•o 24.0 8-7 5.3 2.659 1340° 86° 8' + 3° 1.542 + 1° 4' + 4° 36' + 7° Ab,Anl 55.6 28.3 5.7 10.4 2.694 1419° 86° 14' — 1° 1.558 — 5° 10' -16° +27° Ab,Ab, 49.3 32.6 2.8 15.3 2.728 1477° 86° 4' — 9° 1.570 -17° 40' -29° 28' +48° An 43.2 36-7 0 20.1 2.758 1532° 85°50' — 16° 1.582 -37° -36° +53° * Angles measured to the edge PM. The optical characters of the plagioclases have been the subject of much study, since they are of See also:great value in determining the constituents of rocks in thin sections under the See also:microscope. The mean indices of See also:refraction and the angles of extinction on the cleavages P and M are given in the accompanying table. (The meaning of the + and — directions will be seen from the figure, where the face P slopes from See also:left to right, i.e. the angle between the normals to the faces lettered P and M is less than 90°). The extinction angles on other faces, or in sections of known See also:orientation in the crystal, also give constants of determinative value: for example, in sections perpendicular to the plane M the extinctions, which in crystals twinned according to the albite-law are symmetrical with respect to this See also:place, reach the maximum values given in the table. Not only do the directions of extinction (axes of light-See also:elasticity) vary in the different species, but also the optic axial angle, so that while albite is optically See also:positive, anorthite is negative, and a member near andesine has an axial angle of 90°. The figures seen in convergent polarized light through the P and M cleavages are characteristic of the different species. A detailed See also:summary of the optical characters and their employment in discriminating the several members of the plagioclase series is given by H. Rosenbuch, Mikroskopische Physiographie der Mineralien and Gesteine (4th ed. See also:Stuttgart, 1905). The plagioclases occur as See also:primary constituents of igneous rocks of almost every See also:kind, and are also frequent as secondary minerals in metamorphic rocks. Albite and oligoclase are more characteristic of acidic rocks, whilst the basic members at the anorthite end of the series are characteristic of rocks containing less See also:silica. The composition may, however, vary even in the same crystal, zoned crystals with a basic See also:nucleus and with shells successively more and more See also:acid towards the exterior being See also:common. For further particulars respecting individual species and their modes of occurrence see ALBITE ; ANDESINE ; ANORTIIITE ; BYTOWNITE LABRADORITE; OLIGOCLASE. (L. J. Additional information and CommentsThere are no comments yet for this article.
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