THE CONSTITUTION OF TOURMALINE. 
By F. W. Clarke. 
Some years ago, in an extended paper upon the constitution of the 
silicates, 1 I proposed a formula for tourmaline which seemed to satisfy 
all known conditions. Recently Penneld and Foote 2 have offered still 
another interpretation of the analyses, and support their views with a 
considerable weight of argument. The appearance of their paper has 
led me to rescrutinize the evidence more in detail than previously, and 
the result has been to modify my formulas in some particulars while 
retaining them in their general form. 
According to Penneld and Foote all tourmaline may be represented 
as salts of the alumino-borosilicic acid HuA^BjSi^i, in which two 
hydroxyls are permanently linked to boron, leaving an available valency 
or basicity of nine. In my formulae all tourmalines are derived from 
the similar acid HuAIsBsSIgOm, with all of the hydrogen atoms replace- 
able by bases. Upon bringing the two acids to the common basis of 
six silicon atoms, they compare as follows: 
Penfield and Foote Hie^Al^BsSieO^ % 
Clarke H 14 Al 5 B ;! ~Si 6 :!1 
Replacing aluminum by hydrogen, in order to show the ultimate acids, 
these expressions become 
Penfield and Foote H3oB 3 Siii03i y„ 
Clarke H. 2 ,B,Si,;0 :? , 
The small difference between the empirical formulas is thus made evi- 
dent, and it hardly amounts to more than the uncertainties in the 
analyses. In fact, the trustworthy analyses of tourmaline give ratios 
lying between and beyond both extremes, as the following formulas, 
computed from Riggs's data, show: In these expressions liuorine has 
been assumed equivalent to hydroxyl. 
Pierrepont, black H28.87B2.9rjSieO30.83 
Paris, black Hno.TsB.j.tr.SisO:?) .35 
Stony Point, black : H30.23B3.02SieO31.65 
Auburn, colorless H-29.-25B2.77SieO30.78 
Brazil, red H30.79B2.78SieO31.fl6 
Gouverneur, brown H28.3eB2.95SieO30.6i 
Hamburg, brown H30.93B3.05Si6O32.14 
1 Bull. TJ. S. Geol. Survey No. 125, 1895. 2 Am. Jour. Sci., 4th series, Vol. VII, 1899, p. 97. 
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