90 
THE CONSTITUTION OF THE SILICATES. 
[bull. 125. 
those of the pyroxenes, for the reason that the atomic replacements 
seem to occur by fourths rather than by halves. This point is exempli- 
fied by a comparison between diopside and tremolite; which, reduced 
to their simplest empirical formulae, become 
Diopside 0aMgSi 2 O 6 
Tremolite CaMg 3 Si 4 O l2 
The pyroxenes, however, are somewhat heavier than the amphiboles, 
and from their greater density we may suppose them to have the larger 
molecules. Hence the formula of diopside should be a multiple of that 
just cited, and presumably greater than Ca 2 Mg 2 Si 4 I2 . Upon this point 
the phenomenon of uralitization has definite bearing. In this process 
pyroxene is converted into amphibole, with increase of volume and 
little or no change of composition. In other words, a complex molecule 
has been dissociated into simpler molecules — a phenomenon the direct 
opposite of polymerization. In the face of this evidence it is difficult 
to see how the current views as to the relative molecular magnitudes of 
pyroxene and amphibole can be maintained. The pyroxenes must form 
the more complex group, and the amphiboles the simpler. 
In the amphibole group the orthorhombic anthophyllite is the equiva- 
lent or isomer of enstatite and hypersthene. Then follows a mono- 
clinic series, containing tremolite, actinolite, cummingtonite, dannemo- 
rite,etc.,all being represented by the general empirical formula RSi0 3 , 
with calcium, magnesium, iron, or manganese as the bivalent metal. 
In griinerite the salt FeSi0 3 exists by itself, and in richterite and 
astochite alkaline silicates appear. If we regard the minerals as 
pseudometasilicates, having molecular weights lower than the pyrox- 
enes, and, with the bases replaceable by fourths, the typical amphiboles 
are most simply represented by formulae like the following : 
Anthophyllile. 
Mg 
Tremolite. 
Mg 
/Si0 4x 
Mg<( >Mg 
II 
Mg 
Richterite becomes a mixture of salts, 
E 
II 
/Si0 4x 
R< >R 
X Si 3 0/ 
II 
Na, 
+ R< 
/Si0 4x 
Mg< >Mg 
X Si 3 0/ 
II 
Ca 
R 
II 
SiQ 4 
R 
Si 3 O f 
R 
commingled in ratios near 1:1, and astochite is similar, but with ISaH 
