60 
THE CONSTITUTION OF THE SILICATES. 
[BULL. 125. 
Thus, if we compute the atomic ratios from the analyses cited by 
Brogger,* the following relation appears: 
Cappelinite . 
Melanocerite 
Karyocerite . 
Tritomite ... 
Si. 
236 
218 
216 
226 
B. 
F. 
488 
92 
134 
210 
304 
296 
226 
B + F. 
488 
396 
430 
436 
That is, Si:B-}-F::l:2 nearly, variations being clue to the fact that 
in the hrst three minerals the boric acid was determined by difference, 
and also, probably, to the occasional replacement of fluorine by 
hydroxyl. Another source of variation is found in the presence of 
tetrad bases, as will be seen later; but for the moment the relation 
indicated seems to be reasonably clear. 
The first member of the group, cappelinite, is a borosilicate of yttrium 
and barium, and approximates in composition to 
B0 2 
-B0 2 
Si0 4 ; 
+ 4Y- 
BaH 
,B0 2 
-B0 2 
Si0 4 =Y 
With the earths of uncertain molecular weight designated as "yttria," 
are a little lanthanum oxide and trifling quantities of Th0 2 and 0eO 2 , 
and with the barium are some calcium and alkalies. 
The other three members of the group are all more complicated than 
cappelinite, and vary from it in type by containing tetrad oxides, such 
as OeU 2 , Th0 2 , and Zr0 2 . In eudialyte and catapleiite we have two 
rhombohedral silicates of zirconia, which help to explain these com- 
pounds. Catapleiite probably has the constitution (OH) 3 Zr.Si 3 0( ] .R / 3. 
If we regard the tetrad bases in the cappelinite group as forming 
orthosilicates of this same type, the remainder of each mineral may 
be written as a mixture of molecules like those already designated, 
but with cerium earths predominating over yttrium, and fluorine 
replacing some boric radicles. Thus, melanocerite is not far from 
/OH 
+2B'"— BQ 3 = 
=Ca 
=Ca 
+ 7R///—F 
^SiG 4 E 
ECaH 
\Si0 4 z 
EB'" 
^Si 
Karyocerite may be written similarly, and tritomite becomes 
/OH /B0 2 
/OH / 
RlV <OH + R'"-B0 2 
\SiO=CaH \si6 4 =H 2 .B"'F 2 
Zeit. Kryst. Mm., XVI, pp. 462-469. 
