CLARKE.] 
TOURMALINE GROUP. 
57 
Takiug now the smallest amount of aluminum found in the formula 
of any tourmaline, all varieties of the species may be regarded as con- 
taining the nucleus Al 5 (Si0 4 ) 6 (B0 3 )(B0 2 ) 2 , in which 14 units of affinity 
are still unsatisfied, and among which all of the essential variations 
in composition take place. The orthoboric group, B0 3 , is obviously 
trivalent, B0 2 being univalent, and the radicle of metaboric acid. 
From this nucleus four types of tourmaline are derivable, thus : 
1. 
Al— Si0 4 =Al 
\si0 4 =Al— B0 2 
2. 
.Si0 4 =MgH 
Al-Si0 4 =MgH 
\Si0 4 = Al— B0 2 
Al— BG 3 =NaH 
I 
SiG 4 =Al— B0 2 
/ 
Al— Si0 4 =Al 
\siQ 4 =Al 
Al— Bl) 3 =NaH 
I 
.Si0 4 =Al— B0 2 
Al-Si0 4 ZIAl 
\si0 4 =Al 
,Si0 4 =MgH 
Al— Si0 4 =MgH 
\si0 4 =Al— B0 5 
.Si0 4 =MgH 
Al-Si0 4 ~MgH 
\si0 4 =Al— B0 2 
Al— B0 3 =NaH 
Al— B0 3 =NaH 
.Si0 4 =Al— BO a 
Al— SiO^MgH 
^SiQ 4 =Al 
.Si0 4 =Al— B0 2 
Al-Si0 4 =MgH 
\si0 4 =MgH 
and to all of these, known varieties of the mineral correspond. The 
only notable variations are due to Or or Fe replacing Al, Fe or Mn 
replacing Mg, small quantities of F replacing B0 2 , and. in No. 4, Ca 
sometimes replacing NaH. If in No. 1 R' 3 =LiH 2 , and in No. 2 Fe 2 
replaces Mg 2 , these formulae give the subjoined percentage compositions : 
1 
2 
Si0 2 
38.05 
11.10 
43.13 
35.16 
10. 25 
34.86 
14.06 
3.03 
'"2. 64" 
B 2 3 
Al , 
FeO 
Na,0 
Li/O 
3.28 
1.58 
2.86 
H ,0 
100. 00 
100. 00 
