146 
sodalite and felspar, and the alteration-process attacks both 
minerals simultaneously. It is therefore probable that the for- 
mation of spreustein in this case begins according to the 
equation 
sodalite albite natrolite 
_Na,Al,CISi,0,, + NaAlSi,0, + 4H,0 — 2(Na,Al,Si,0,, . 2H,0) + NaCl. 
Next to the sodalite nepheline is the most idiomorpic mi- 
neral of the naujaite. It occurs in hexagonal, short prisms, one 
or two centimeters across. The crystals as a rule are fresh, 
white or slightly grayish, and translucent. Under the micro- 
scope many of them are quite clear while others contain 
fluid-pores and minute prisms of arfvedsonite (rarely egirine); 
these prisms are of considerably larger dimensions than the 
microlites contained in the sodalite, but they are not very 
numerous. 
The mutual relations of nepheline and sodalite are some- 
what. intricate. Very often the larger nepheline crystals en- 
close crystals of sodalite in a poikilitic manner, but within the 
same slices also micrographic intergrowths of both minerals 
may be observed as well as instances where the nepheline has 
been partially converted into sodalite. In the latter case sodalite 
has formed not only round the margin of the nepheline but 
also in irregular patches in the interior, and at a more ad- 
vanced stage of conversion the nepheline is reduced to isolated 
lapped areas enclosed in the sodalite. The sodalite replacing 
nepheline in this manner is of the same microstructure as the 
marginal zone without microlites that surrounds some of the 
idiomorphic sodalite-crystals as mentioned above. This seems 
to indicate that the transformation of the nepheline has oc- 
curred before the final consolidation of the rock. There are, 
however, found indications that a conversion of nepheline into 
sodalite has also taken place after the consolidation, for in 
some cases the outer zone of the nepheline-crystals has been 
changed into a sodalite showing convex or mammillary surfaces 
