118 Penfield and Foote — Composition of Tourmaline. 



Constitution of Tourmaline. — The evidence is convinc- 

 ing that all tourmalines are derivatives of a complex boro- 

 silicic acid H 20 B 2 Si 4 O Q1 , and it is believed that further analyses 

 will not alter this result, although they may furnish important 

 data concerning the constitution of the acid. All of the hydro- 

 gen atoms of this acid in tourmaline are not replaced by 

 metals, for the different varieties have always been found to 

 contain water, which indicates the presence of hydro xyl. The 

 ratio of the silica to the hydrogen derived from water 

 (hydroxyl) plus the fluorine is not constant, but varies in Riggs' 

 and our analyses from 4:3*14 (pale green tourmaline from 

 Auburn, Maine) to 4 : 248 (colorless tourmaline from De 

 Kalb). In all of the analyses in .which water has been esti- 

 mated directly, a sufficient quantity has been obtained to yield 

 two hydroxyls in the formula ; in only a few cases has the 

 amount been sufficient to yield three hydroxyls. We are thus 

 led to believe in the existence of two hydroxyls in all tourma- 

 lines, and it seems natural to associate them with the two- 

 boron atoms. The acid consequently becomes H 18 (B.OH) a Si 4 19 . 

 The small amount of fluorine which is found in many 

 varieties of tourmaline presumably plays the same role as 

 hydroxyl, or is isomorphous with it, as in the case of topaz, of 

 chondrodite, and of other minerals containing fluorine and 

 hydroxyl. The slight excess of hydrogen over and above the 

 two hydroxyls may be regarded as basic hydrogen, which 

 plays the role of a metal. Such a relation is not unknown in 

 complex mineral compounds.* 



One of the peculiar features of tourmaline is that varying 

 proportions of metals of different valences and of essentially 

 different character replace the hydrogens of the acid 

 H 18 (B.OH) 2 Si 4 19 . In all cases thus far examined aluminium 

 predominates and is present in sufficient quantity to replace 

 more than half the hydrogens. From this it has been inferred 

 that an aluminium-borosilicic acidH B Al,(B.OH) 9 Si 4 19 is char- 

 acteristic for all varieties of tourmaline. The constitution of 

 this acid may be expressed graphically as follows : 



Al— 0-(B . OH)-0-(B . OH)-0-H 



O-H 



*This Journal, xl, p. 396, 1890. 



