22 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
Although garnet, as a rule, is unattacked by acids, and epidote is 
only in part decomposable, both species are so broken up by strong 
ignition as to be readily acted upon by hydrochloric acid, with separa- 
tion of gelatinous silica. According to Doelter and Hussak,* garnet 
yields upon fusion sometimes anorthite and an olivine; or meionite, 
augite, and olivine; or melilite and anorthite; and occasionally spinel. 
Epidote, says Doelter, f yields lime-augite and anorthite, and prehnite 
behaves like garnet. These facts indicate analogies between the several 
species named, but are not easy of complete interpretation. The dif- 
ferent products observed may be due in part to a direct splitting up of 
the original insoluble mineral into soluble species like anorthite and a 
lime-olivine, and in part to secondary reactions taking place during the 
prolonged fusion and cooling of the resultant magma. In other words, 
two sets of phenomena are probably involved in the observations, and it 
is impossible at present to distinguish between them. The first set alone 
is immediately relevant to the constitutional question now before us. 
In the four species sodalite, hauynite, nosite, and lazurite we have a 
group of minerals which Brogger has classified as alkali garnets. J 
Like garnet, they are all isometric, and they are characterized by the 
presence of the bivalent groups =A1 — CI, =A1 — S0 4 — Na, and 
=A1— S — S — S — Na. There are al so artificial products, ultramarines, 
in which the groups =A1— S— S— Na and =A1 — S— Na appear. 
Adopting Brogger's formulae, which are preferable to those formerly 
proposed by myself, § these species may be written as follows : 
Sodalite Al 2 (Si0 4 ) 3 Na 4 (AlCl) 
Haiiynite Al 2 (Si0 4 ) 3 Na 2 Ca( AlS0 4 Na) 
Nosite Al 2 (Si0 4 ) 3 Na 4 (AlS0 4 Na) 
Lazurite Al 2 (SiQ 4 ) 3 Na 4 (AlS 3 ]Sa) 
They fall, therefore, properly under the second subtype, but are con- 
sidered at this point on account of their analogies with garnet. 
Through these species, and especially through sodalite, a connection 
with several other minerals is established. Sodalite occurs in eheolite 
syenytes, and is evidently derived from nephelite, and like the latter 
it yields natrolite, hydroneph elite, and muscovite by alteration. 
Furthermore. G. and G. Fried el, || on heating powdered muscovite with 
soda solution and sodium chloride at a temperature of 500°, obtained 
sodalite artificially, although nephelite was probably first formed as 
an intermediary, and the two species were commingled in the product. 
The two hexagonal species, cancrinite and microsommite, are also, 
like sodalite, undoubtedly derivatives of nephelite, but their formula 
are rather uncertain. Cancrinite approximates to the composition 
* Allgem. Chem. Mineralogie, p. 182. 
t Allgem. Chem. Mineralogie, p. 183. 
J Brogger and Backstrbm, Zeit. Kryst. Min., XVIII, p. 209. 
§ Bull. U. S. Geol. Survey No. 42, p. 38. 
|| Bull. Soc. Min., XIII, p. 183. 
