CLARKE. 1 
CONSTITUTION OF TOURMALINE. 
35 
committing myself to any definite formulation in this instance. As 
for the analyses published by Jannasch and Kalb, they fit in well with 
those of Riggs, and are amenable to the same treatment. 
At first glance some of the formulas which 1 have proposed may 
seem to be complex; but they arc all of the same type, and can be 
reduced to a few general expressions, as follows: 
/ 
Si0 4 =R' 
Al— SiO.,==Al 
\si0 4 =A!B0 3 
Al— Si0 4 =Al 
\si0 4 =AlBO a 
I 
Al — BO: 3 = R / ->(R // ) 
Al — BO:, = R / 2 (R") 
i 
.Si0 4 =AlBO* ,Si0 4 = AlB0 2 
Al— Si0 4 =:Al Al— Si0 4 =Al 
^SiO.^Al \si0 4 =R / :! 
1. 2. 
ySi0 4 SR"H 
Al— Si0 4 =R"H 
\3i0 4 =AlB0 3 
Al — BO 1=11" 
| 
.Si0 4 =AlB0 3 
Al— Si0 4 =R"B 
\si0 4 =R"H 
3. 
These formuhc cover all of the established variations in the compo- 
sition of tourmaline; they render the various replacements or isomor- 
phous admixtures intelligible, and they indicate the directions into 
which the species commonly alters. There is one objection to them, 
namely, that one of the end products contains no alkali metal, and no 
alkali-free tourmaline is known. The same objection applies to the 
Penfield-Foote formula, as will be seen by anyone who attempts to 
apply it in the discussion of the iron tourmalines. Under either system 
of formulation the existence in tourmaline of alkali-free salts must be 
assumed. 
One further possible advantage in the proposed formulae remains to 
be pointed out. All of the chemists who ot late years have discussed 
the composition of tourmaline agree in adopting the ratio between 
silicon and boron of 2:1, or 4SiG 2 : B 2 ;i . And yet many of the 
analyses vary from this ratio to an extent which may not be due to 
experimental errors. For example, from among Riggs' s analyses the 
following cases show large variations, the boron being too low. I 
give the silica and boric oxide as determined, the boric oxide as calcu- 
lated from the silica by the accepted ratio, and the amount of variation 
between the two. 
Si0 2 . 
B,0, B 2 3 
found. calculated. 
Difference. 
Rumford, red 
38.07 
35.03 
36. <*1 
36.91 
38.14 
9.99 11.10 
9. 02 10. 22 
9. 65 10. 62 
9. 87 10. 76 
10.25 11.12 
-1.11 
—1.20 
—0. 97 
—0. 89 
—0.87 
Paris, black 
Monroe, brown 
Brazil, green 
Auburn, colorless 
