Mesotype and nepheline constitute, according to Girard, an important part in the composition and internal texture of basalt.
Even with the help of the microscopenepheline basalts are not always easy to determine, as the crystals may be exceedingly small and imperfect, and they readily decompose into analcite and zeolites.
Minerals of the felspathoid group occur in a large number of basaltic rocks; nepheline and leucite are the most common, but haüyne is occasionally present.
Most nepheline basalts are fine grained, very dark coloured rocks, and belong to the Tertiary period.
The nephelineis in small six-sized prisms, and usually cannot be detected with the unaided eye.
If nepheline entirely replaces felspar, the rock is known as nepheline-basalt; if the replacement is only partial the term nepheline-basanite is used.
The minerals haüyne, nosean, sodalite and melilite tend to occur with some frequency in nepheline and leucite-basalts, though rare in ordinary basalts.
In most cases, though not in all, they have a close association with nepheline or leucite syenites and similar rocks rich in alkalies.
The other is found in association with nepheline syenites, essexites and teschenites, and is exemplified by camptonites, monchiquites and alnoites.
Vesuvian lava, glass paste with numerous crystals of leucite; others of augite and nepheline porphyritically developed; also small grains of magnetite.
The facial development of crystals of nepheline give no indication of this type of symmetry, and the mineral has been referred to this class solely on the evidence afforded by the etched figures.
The mineral nephelineis placed in this class because of the absence of symmetry in the etched figures on the prism faces (fig.
Among the accessory minerals biotite, brown hornblende, hauyne, iron oxides and apatite are the commonest; melanite and nepheline may also occur.
It is most common in lavas of recent and Tertiary age, which have a fair amount of potash, or at any rate have potash equal to or greater than soda; if soda preponderates nepheline occurs rather than leucite.
Sodalite or hauyne may also be present, but nepheline is typically absent.
Nepheline appears to decrease in amount as leucite increases.
They are rich in leucite, but contain also some sanidine and often much nepheline with hauyne or nosean.
In Finland, melanite-bearing nepheline rocks have been found and described as Ijolite, but the only other locality for melanite-leucite-syenite is Magnet Cove in Arkansas.
These last consist of an intermixture of nepheline or sodalite and alkali-felspar.
They are associated principally with basalts, nepheline and leucite basalts and monchiquites.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "nepheline" in a variety of sentences. We hope that you will now be able to make sentences using this word.
