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Nephelinites

1911 Encyclopedia Britannica

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The group of effusive rocks which contains nepheline with plagioclase 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 absence of olivine, and the same distinction subsists between the nephelinites and nepheline-basalts.

Lavas with nepheline, plagioclase and 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 accessory minerals, appearance and structure, these rocks have much in common, and they tend to occur in a natural association as basic rocks comparatively rich in alkalis and alumina. The nephelinites and tephrites are rather closely linked to the phonolites and pass into them by various gradations. They are usually richer in alkalis and silica and contain less iron, lime and 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 low double refraction and frequent decomposition. Leucite appears in some tephrites; haiiyne is more frequent as small dodecahedra often filled with black inclusions. The augite varies a good deal, being bright green or dark green (aegirine) and rich in soda in some tephrites and nephelinites, while in basanites and basalts it is often brown "basaltic" augite or purple "titaniferous" augite. It has often good crystalline form, and occurs as eight-sided monoclinic prisms, but the soda augites may be of late crystallization and form mossy or irregular growths in the matrix. Brown hornblende is much less common, and a red biotite is very characteristic of certain nephelinites. Of the felspars, labradorite is probably the most common, with more acid varieties of plagioclase. Sanidine is by no means absent, but may be considered as an accessory. The olivine presents no peculiarities. Melilite, perofskite, pseudobrookite, melanite garnet, iron oxides, apatite and chromite are occasionally met with.

All these rocks are practically confined to lavas of Tertiary and recent 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 British Isles they are exceedingly scarce, though nepheline-basanite occurs in a dike which is presumably Tertiary, cutting the Triassic rocks at Butterton in Staffordshire, and nepheline-basalt has been found in a single neck at John o' Groat's in Caithness and at one or two places near North Berwick in Haddingtonshire. They attain a great development in the Canary Islands (Teneriffe, Grand Canary, &c.) and in the Azores, Cape Verde Islands and Fernando Noronha. In Germany they are represented among the Tertiary eruptive rocks of the Rhine district and Thuringia, at the extinct craters of the Eiffel and at the Kaiserstuhl. In central Bohemia there are many occurrences of nepheline-tephrites, basanites and basalts which though fine grained contain all their minerals in excellent preservation. The nephelinite of Katzenbuckel in the Odenwald is well known. Contrasted with the phonolites and leucitophyres these rocks are scarce in Italy and the Mediterranean province, but leucite-bearing nepheline-tephrites occur at Monte Vulture and nepheline-basalts in Tripoli. In America these rocks occur in Texas, in the Bearpaw Mountains of Montana and at Cripple Creek, Colorado. From Argentina some members have been described: thay have a great extension in East Africa (Somaliland and Masai-land) and occur also in North Nigeria. A few also have been described from New South Wales, New Zealand (Dunedin) and Tasmania. (J. S. F.)