The angle between these two cleavages is 90°, hence the name orthoclase (from the Gr. ὀρθός, right, and κλᾶν, to break), given by A. Breithaupt in 1823, who was the first to distinguish orthoclase from the other felspars. There are also imperfect cleavages parallel to the faces of the prism m (110).
The hardness is 6, and the sp. gr. 2.56. Crystals are sometimes colourless and transparent with a glassy aspect, as in the varieties adularia, sanidine and the rhyacolite of Monte Somma, Vesuvius.
 Fig. 4. |
|  Fig. 5. |
|  Fig. 6. |
Twinned Crystals of Orthoclase.
The optical characters are somewhat variable, the plane of the optic axes being perpendicular to the plane of symmetry in some crystals and parallel to it in others: further, when some crystals are heated, the optic axes gradually change from one position to the other. In all cases, however, the acute negative bisectrix of the optic axes lies in the plane of symmetry and is inclined to the edge b/c at 3.7°, or, in varieties rich in soda, at 10.12°. The mean refractive index is 1.524, and the double refraction is weak (0.006).
Analyses of orthoclase usually prove the presence of small amounts of soda and lime in addition to potash. These constituents are, however, probably present as plagioclase (albite and oligoclase) intergrown with the orthoclase. The two minerals are interlaminated parallel to the ortho-pinacoid (100) or the pinacoid (801), and they may readily be distinguished in the flesh-red aventurine-felspar, known as perthite, from Perth in Lanark county, Ontario. Frequently, however, as in microperthite and cryptoperthite, this is on a microscopic scale or so minute as to be no longer recognizable. These directions (100) and (801) are planes of parting in orthoclase, and along them alteration frequently takes place, giving rise to schiller effects. Moon-stone (q.v.) shows a pearly opalescent reflection on these planes; and brilliant coloured reflections in the same directions are exhibited by the labradorescent orthoclase from the augite-syenite of Fredriksvärn and Laurvik in southern Norway, which is much used as an ornamental stone. The same effect is shown to a lesser degree by murchisonite, named in honour of Sir R. I. Murchison, from the Triassic conglomerate of Heavitree near Exeter.
Orthoclase forms an essential constituent of many acidic igneous rocks (granite, syenite, porphyry, trachyte, phonolite, &c.) and of crystalline schists and gneisses. In porphyries and in some granites (e.g. those of Shap in Westmorland, Cornwall, &c.) it occurs as embedded crystals with well-defined outlines, but usually it presents no crystalline form. In the trachyte of the Drachenfels and the Laacher See in Rhenish Prussia there are large porphyritic crystals of glassy sanidine. The best crystals are those found in the crystal-lined cavities and veins of granites, pegmatites and gneisses, for example, at Baveno and Elba in Italy, Alabashka near Mursinka n the Urals, Hirschberg in Silesia, Tanokami-yama in the province Omi, Japan, and the Mourne Mountains in Ireland. As a mineral of secondary origin orthoclase is sometimes found in cavities in basaltic rocks, and its occurrence in metalliferous mineral-veins has been observed. It has been formed artificially in the laboratory and is sometimes met with in furnace products.
The commonest alteration product of orthoclase is kaolin (q.v.) the frequent cloudiness or opacity of crystals is often due to partial alteration to kaolin. Mica and epidote also result by the alteration of orthoclase.
- (L. J. S.)
ORTHODOX EASTERN CHURCH (frequently spoken of as “the Greek Church,” and described officially as “The Holy Orthodox[1] Catholic Apostolic Eastern Church”), the historical representative of the churches of the ancient East. It consists Origins of the Greek or Eastern Church. of (a) those churches which have accepted all the decrees of the first seven general councils, and have remained in full communion with one another, (b) such churches as have derived their origin from these by missionary activity, or by abscission without loss of communion. The Eastern Church is both the source and background of the Western. Christianity arose in the East, and Greek was the language of the Scriptures and early services of the church, but when Latin Christianity established itself in Europe and Africa, and when the old Roman empire fell in two, and the eastern half became separate in government, interests and ideas from the western, the term Greek or Eastern Church acquired gradually a fixed meaning. It denoted the church which included the patriarchates of Antioch, Alexandria, Jerusalem and Constantinople, and their dependencies. The ecclesiastical division of the early church, at least within the empire, was based upon the civil. Constantine introduced a new partition of the empire into dioceses, and the church adopted a similar division. The bishop of the chief city in each diocese naturally rose to a pre-eminence, and was commonly called exarch—a title borrowed from the civil jurisdiction. In process of time the common title patriarch was restricted to the most eminent of these exarchs, and councils decided who were worthy of the dignity. The council of Nicaea recognized three patriarchs—the bishops of Rome, Alexandria and Antioch. To these were afterwards added the bishops of Constantinople and Jerusalem. When the empire was divided, there was one patriarch in the West, the bishop of Rome, while in the East there were at first two, then four and latterly five. This geographical fact has had a great deal to do in determining the character of the Eastern Church. It is not a despotic monarchy governed from one centre and by a monarch in whom plenitude of power resides. It is an oligarchy of patriarchs. It is based, of course, on the great body of bishops; but episcopal rule, through the various grades of metropolitan, primate, exarch, attains to sovereignty only in the five patriarchal thrones. Each patriarch is, within his diocese, what the Galilean theory makes the pope in the universal church. He is supreme, and not amenable to any of his brother patriarchs, but is within the jurisdiction of an ecumenical synod. This makes the Eastern Church quite distinct in government and traditions of polity from the Western. It has ever been the policy of Rome to efface national distinctions, but under the shadow of the Eastern Church national churches have grown and flourished. Revolts against Rome have always implied a repudiation of the ruling principles of the papal system; but the schismatic churches of the East have always reproduced the ecclesiastical polity of the church from which they seceded.
The Greek Church, like the Roman, soon spread far beyond the imperial dioceses which at first fixed its boundaries, but it The barbarian invasions in West and East. was far less successful than the Roman in preserving its conquests for Christianity. This was due in the main to the differing quality of the forces by which the area covered by the two churches was respectively invaded. The northern barbarians by whom the Western empire was overrun had long stood in awe of the power and the civilization of Rome, which they recognized as superior; the conquerors were thus predisposed to enter into the heritage of the law and the religion of the conquered empire and, whether they were pagans or Arian heretics, became in the end Catholic Christians. In the East it was otherwise. The empire maintained itself long, and died hard; but its decline and fall meant not only the overthrow of the emperors of the East, but largely that of the civilization and Christianity which they represented. The Arabs, and after them the Turks, attacked the empire as the armed missionaries of what they regarded as a superior religion; Christianity survived in the vast territories they
- ↑ The Orthodox Eastern Church has always laid especial stress upon the unchanging tradition of the faith, and has claimed orthodoxy as its especial characteristic. The “Feast of Orthodoxy” (ἡ κυριακὴ τῆς ὀρθοδοξίας), celebrated annually on the first Sunday of the Greek Lent, was founded in honour of the restoration of the Holy Images to the churches after the downfall of Iconoclasm (February 19, 842); but it has gradually assumed a wider significance as the celebration of victory over all heresies, and is now one of the most characteristic festivals of the Eastern Church.
