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VIIVI

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Originally appearing in Volume V22, Page 105 of the 1911 Encyclopedia Britannica.
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VIIVI .. 7554.20 54.46 1219.96.65 1.53 2.34 3.33 0.61 2.12 8.68 2.76 5.20 0.28 0.26 0.60 4.14 5.67 0.12 I., Elvan or See also:

granite See also:porphyry (with pinite after cordierite)-Prah sands, See also:Cornwall. II., Granophyre-Armboth, See also:Cumberland. III., Granophyre-Carrock See also:Fell, Cumberland. IV., Rhomben-porphyry -Tonsterg, See also:Norway. V., Elaeolite porphyry-Beemerville, New See also:Jersey. VI., Tinguaite-See also:Kola. VII., Grorudite-Assynt, See also:Scotland. Porphyrites.-The porphyrites as above mentioned are intrusive or hypabyssal rocks of porphyritic texture, with phenocryste of See also:plagioclase See also:felspar and See also:hornblende, See also:biotite or See also:augite (sometimes also. See also:quartz) in a See also:fine ground-See also:mass. The name has not always been used in this sense, but formerly signified rather decomposed andesitic and basaltic lavas of Carboniferous See also:age and older. Both the red porphyry and the See also:green porphyry of the ancients are more properly classified in this See also:group than with the granite-porphyries, as their dominant felspar is plagioclase and they contain little or no See also:primary quartz. Porphyrites occur as dikes which accompany masses of See also:diorite, and are often called diorite-porphyrites; they differ from diorites in few respects except their porphyritic structure.

The phenocrysts are plagioclase, often much zoned with central kernels of See also:

bytownite or See also:labradorite and margins of See also:oligoclase or even See also:orthoclase. In a See also:special group there are corroded blebs or porphyritic quartz: these rocks are called quartz-porphyrites, and are distinguished from the granite-porphyries by the scarcity or See also:absence of orthoclase. The hornblende of the porphyrites is often green but sometimes See also:brown, resembling that of the See also:lamprophyres, a group from which the porphyrites are separated by their containing phenocrysts of felspar, which do not occur in normal lamprophyres. Augite, when See also:present, is nearly always See also:pale green; it is not so abundant as hornblende. Dark brown biotite is very See also:common in large hexagonal plates. See also:Muscovite and See also:olivine are not represented in these rocks. The ground-mass is usually a crystalline aggregate of granular felspar in which plagioclase dominates, though orthoclase is rarely absent. The Alpine See also:dike rocks known as ortlerites and suldenites are porphyrites containing much green or brown hornblende and augite; these, however, hardly require a distinctive designation. Diorite-porphyrites have almost as wide a See also:distribution as granite-porphyries, and occur in all parts of the See also:world where intrusions of granite and diorite have been injected; they are in fact among the commonest hypabyssal rocks. To gabbros and norites certain types of porphyrite correspond which have the same See also:mineral and chemical, See also:composition as the See also:parent rocks but with a porphyritic instead of granitic structure. See also:Gabbro-porphyrites are not numerous; or rather most of these rocks are described as porphyritic basalts and dolcrites. The beerbachites are finely granular, dike rocks resembling gabbros SiO2 Al20a Fe20a FeO CaO MgO K2O Na2O See also:H2O I.

64.94 1 17.50 0.69 3.94 2.59 2.83 3.11 3.44 I.36 II. 61.58 18.84 4.68 - 6.59 2.04 1.49 4.27 I.61 I , Quartz-porphyrite-Lippenhof, Schwarzwald. II., Porphyrite-Esterel, See also:

France. III., Norite-porohyrite-Klausen, See also:Tirol. (J. S.

End of Article: VIIVI

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