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NEPHELINITES

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Originally appearing in Volume V19, Page 384 of the 1911 Encyclopedia Britannica.
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NEPHELINITES . The See also:

group of effusive rocks which contains See also:nepheline with See also:plagioclase See also:felspar is subdivided into nephelinetephrites and nepheline-basanites, while those which contain nepheline but not felspar are nephelinites and nepheline-basalts. The tephrites differ from the basanites in the See also:absence of See also:olivine, and the same distinction subsists between the nephelinites and nepheline-basalts. Lavas with nepheline, plagioclase and See also:augite = nephelinetephrites. Lavas with nepheline, plagioclase, augite and olivine= nephelinebasanites. Lavas with nepheline and augite = nephelinites. Lavas with nepheline, augite and olivine = nepheline-basalts. In their essential and See also:accessory minerals, See also:appearance and structure, these rocks have much in See also:common, and they tend to occur in a natural association as basic rocks comparatively See also:rich in alkalis and alumina. The nephelinites and tephrites are ratherclosely linked to the phonolites and pass into them by various gradations. They are usually richer in alkalis and See also:silica and contain less See also:iron, See also:lime and See also:magnesia than the basanites and nepheline-basalts, a difference which finds expression in the presence of olivine and the smaller amount of felspars and felspathoids in the latter. The nepheline is colourless and transparent when fresh, often in six-sided prisms, but also as irregular interstitial masses filling the spaces between the other minerals, and hard to identify owing to its See also:low See also:double See also:refraction and frequent decomposition. See also:Leucite appears in some tephrites; hauyne is more frequent as small dodecahedra often filled with See also:black inclusions.

The augite varies a See also:

good See also:deal, being See also:bright See also:green or dark green (aegirine) and rich in soda in some tephrites and nephelinites, while in basanites and basalts it is often See also:brown " basaltic " augite or See also:purple titaniferous " augite. It has often good crystalline See also:form, and occurs as eight-sided See also:monoclinic prisms, but the soda augites may be of See also:late See also:crystallization and form mossy or irregular growths in the See also:matrix. Brown See also:hornblende is much less common, and a red See also:biotite is very characteristic of certain nephelinites. Of the felspars, See also:labradorite is probably the most common, with more See also:acid varieties of plagioclase. Sanidine is by no means absent, but may be considered as an accessory. The olivine presents no peculiarities. Melilite, perofskite, psetidobrookite, melanite See also:garnet, iron oxides, See also:apatite and See also:chromite are occasionally met with, All these rocks are practically confined to lavas of See also:Tertiary and See also:recent See also:age, though some occur as dikes or small intrusive masses. The plutonic facies of these rocks are found among the theralites, shonkinites, essexites and 'ijolites. In the See also:British Isles they are exceedingly scarce, though nepheline-basanite occurs in a See also:dike which is presumably Tertiary, cutting the Triassic rocks at Butterton in See also:Staffordshire, and nepheline-See also:basalt has been found in a single See also:neck at See also:John o' See also:Groat's in See also:Caithness and at one or two places near See also:North See also:Berwick in See also:Haddingtonshire. They attain a See also:great development in the See also:Canary Islands (See also:Teneriffe, See also:Grand Canary, &c.) and in the See also:Azores, Cape Verde Islands and Fernando Noronha. In See also:Germany they are represented among the Tertiary eruptive rocks of the See also:Rhine See also:district and Thuringia, at the See also:extinct craters of the Eiffel and at the Kaiserstuhl. In central Bohemia there are many occurrences of nepheline-tephrites, basanites and basalts which though See also:fine grained contain all their minerals in excellent preservation.

The nephelinite of Katzenbuckel in the See also:

Odenwald is well known. Contrasted with the phonolites and leucitophyres these rocks are scarce in See also:Italy and the Mediterranean See also:province, but leucite-bearing nepheline-tephrites occur at See also:Monte See also:Vulture and nepheline-basalts in See also:Tripoli. In See also:America these rocks occur in See also:Texas, in the Bearpaw Mountains of See also:Montana and at Cripple See also:Creek, See also:Colorado. From See also:Argentina some members have been described : thay have a great See also:extension in See also:East See also:Africa (See also:Somaliland and See also:Masai-See also:land) and occur also in North See also:Nigeria. A few also have been described from New See also:South See also:Wales, New See also:Zealand (See also:Dunedin) and See also:Tasmania. (J. S.

End of Article: NEPHELINITES

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