14 STRUCTURAL AND FIELD GEOLOGY 
NVosean) are akin to the felspars in chemical composition, and 
play much the same part as rock-formers. They are not 
nearly so important, however, the rocks of which they are 
characteristic constituents being much less widely distributed. 
They are restricted, indeed, to a few igneous rocks mostly — 
belonging to a late geological period. They never occur as 
ingredients of the crystalline schists. 
Leucite (silicate of potassium and aluminium) generally appears in 
the form of more or less well-defined single crystals, having the shape of 
icositetrahedra (24-faced trapezohedra). In cross-sections the larger 
crystals often yield six-sided or eight-sided contours, while the smaller 
crystals are rounded. The mineral has a hardness of 5-5 to 6, and a 
specific gravity of 2-45 to 2-50. If pure, it is transparent and colourless, 
but most frequently, owing to the presence of impurities, it appears ash- 
grey or greyish-yellow, and then it is only translucent on thin edges. 
It is almost infusible before the blowpipe ; when reduced to a powder it 
readily dissolves in hydrochloric acid, with separation of pulverulent 
silica. Under the microscope, leucite usually shows abundant 
symmetrically arranged inclusions of glass, gas pores, and minute 
microlites, grains, etc., of such minerals as felspar, augite, and magnetite. 
Between crossed nicol-prisms it exhibits weak, anomalous double 
refraction—yielding dark grey colours—the crystals being traversed by 
intersecting alternately light and dark twin lamellz. This structure, 
however, is not seen in the smaller crystals, which are usually isotropic. 
The mineral is readily altered in nature, becoming white and opaque as 
it is changed into zeolites or kaolin. Probably its proneness to altera- 
tion is the reason why it seldom occurs in very old igneous rocks. It 
is a Macroscopic and microscopic constituent of certain basic Vesuvian 
lavas. Similar rocks occur elsewhere in Italy, and in a few other 
countries. 
Nepheline is essentially a sodium-aluminium silicate, but contains 
some potassium. Hardness, 5:5 to 6; specific gravity, 2:58 to 2-64. 
As a rock-constituent it appears in the form of somewhat stout hexagonal 
prisms with a glassy lustre, and is either water-clear or white. Its 
hardness is similar to that of leucite. It is fusible before the blowpipe 
with some difficulty ; and gelatinises with acids. It is an essential 
constituent of phonolite and nepheline-basalt, and very commonly occurs 
in rocks which contain leucite as an essential ingredient. Like leucite, 
the mineral is unstable and thus frequently altered into fibrous 
zeolites or muscovite. A dull grey variety of nepheline, with a greasy 
lustre, and known as £/colite, is a conspicuous component of certain 
syenites. 
Sodalite is another sodium-aluminium silicate, but it contains 
chlorine. It crystallises in isometric forms (dodecahedra), and has a 
hardness of 5:5, and a specific gravity of 2-2 to 2-4. It fuses to a 


