CHAPTEE V. 
THE ORTHOSILICATES OF TETRAD BASES. 
On account of their relatively small number and general scarcity, the 
silicates of the tetrad metals, titanium, zirconium, and thorium, are 
difficult to interpret in any satisfactory maimer. Evidence exists, 
however, which seems to show that they are explainable by the same 
principles which apply to aluminum and the dyads, and that the theory 
of substitution from normal salts is a good working hypothesis to start 
from. 
One definite normal salt is known in this series, the mineral zircon, 
ZrSi0 4 . As with the other inorganic silicates, the true molecular weight 
of this compound is unknown, and it can be inferred only from a study 
of its derivatives. If we assume it to be represented by the polymeric 
expression Zr 4 (Si0 4 ) 4 , it contains replaceable basic atoms, and a number 
of other zirconium silicates fall naturally into series derivable from this 
as tli e fundamental member. In this connection the mineral auerbachite 
is peculiarly suggestive, for its composition is best indicated by the 
formula Zr 4 (Si 3 8 )(Si0 4 ) 3 ; which goes to show an important analogy 
between this group of silicates and those which have been previously 
considered. This formula, compared with Hermann's analysis of auer- 
bachite, gives the following results : 
Hermann. 
Calculated. 
SiO, 
42.91 
55.18 
.93 
.95 
42.45 
57. 55 
ZrO. 
FeO 
HO 
99.97 
100. 00 
Although zircon is a very stable and definite mineral, it alters by 
hydration into malacone, cyrtolite, and a variety of other indefinite 
substances which can not as yet be interpreted with any clearness. At 
the same time other bases, such as lime and the rare earths, are taken 
up, producing mixtures of great complexity. Malacone is probably the 
first hydration derivative, and its relations to zircon, regarding the 
latter as Zr 4 (Si0 4 ) 4 , may possibly be as follows: 
Zircon. 
Si0 4 =Zr 
/Si0 4 / 
\^Si0 4 ^ 
SiO.=Zr 
Malacone. 
OH 
/sio 4 - H 
\Si0 4 v 
Si0 4 =Zr 
75 
