58 THE CONSTITUTION OF THE SILICATES. [buix.125. 
These figures agree well with the best analyses of the lithia tourma- 
lines and the ordinary black tourmalines which are found in the same 
localities ; divergencies being due to trifling admixtures of one with 
the other, and to the presence of fluorine as previously noted. 
Formula No. 3 corresponds to 
Si ° 2 37.58 
BoO: 5 
10. 96 
A1 *°> 31.94 
M S° 12.52 
Na *° 3.24 
H 2° 3. 76 
100. 00 
which is very near the composition of the dark-brown tourmalines from 
Orford, N. H., and Monroe, Conn., except that in them a little Fe 
replaces Mg, and a little Ca takes the place of NaH. 
The brown tourmalines of Hamburg, N. J., and Gouverneur, N. Y. 
with the white tourmaline from DeKalb, all approximate toward formula 
No. 4, but with admixtures of No. 3, and with Ca in place of NaH. The 
mixture 
/ Si0 4 =MgH /Si0 4 =MgH 
Al— Si() 4 =Al Al— Si0 4 =MgH 
^Si0 4 =Al— B0 2 \si0 4 =Al— B0 2 
I I 
Al— B0 3 =NaH + 2 Al— B0 3 =Ca 
i I 
/ ,Si0 4 =Al— B0 2 .Si0 4 =Al— B0 2 
Al— Si0 4 =Al Al— Si0 4 =MgH 
\Si0 4 =MgH ^SiO.EZMgH 
gives the following percentage composition: 
Si0 - : - 37.19 
B -°" 10. 85 
M i°> 29. 86 
M S° 13.77 
Ca0 3. 86 
Na *° 1.06 
H 2 U 
3 41 
100. 00 
which represents the DeKalb tourmaline fairly well. In the Ham- 
burg mineral there is more of the calcium compound, and in that from 
Gouverneur rather less. The black tourmaline from Pierrepont gives 
formula No. 4 very closely, except that about three-tenths of the Mg 
has been replaced by Fe, and one-half of the NaH by Ca. Calculating 
for the following mixture of molecules : 
3Al 5 (Si0 4 ) 6 (B0 2 ) 2 B0 3 NaFe 4 H 5 
2Al 5 (Si0 4 ) 6 (B0 2 ) 2 B0 3 NaMg 4 H 5 
5Al 5 (Si0 4 ) 6 (B0 2 ) 2 B0 3 CaMg 4 H 4 
