Penfield and Foote — Composition of Tourmaline. 97 



Art. XII. — On the Chemical Composition of Tourmaline; 

 by S. L. Penfield and H. W. Foote. 



Introduction and Historical. — There is probably no 

 common mineral whose chemical composition has proved more 

 perplexing and been so little understood as tourmaline. Some 

 reasons for this are, first, that the mineral presents certain 

 peculiarities in chemical composition of an unusual nature ; 

 second, the analysis of tourmaline has been one of the difficult 

 problems of analytical chemistry, hence reliable data for the 

 calculation of the formula have not been easily obtained ; and, 

 lastly, although good analyses have been made, the results have 

 not been thoroughly relied upon, nor have they been inter- 

 preted to the best advantage. The present investigation was 

 undertaken, therefore, with the hope that by making a few 

 analyses with the utmost possible care on tourmalines of excep- 

 tional purity, it would be possible to find a satisfactory explana- 

 tion of the chemical composition of this interesting mineral. 



In order to appreciate the problem in hand, it will be neces- 

 sary to review briefly the work and the results of previous 

 investigators. 



The analyses of Vauquelin and Klaproth, made in the early 

 part of this century, were naturally defective, because at the 

 time they were made, lithium was unknown, it had not been 

 discovered that tourmaline contained boron, and analytical 

 methods were not perfected. 



In 1818 the presence of boron was detected by Lampardius,* 

 and in the same year Arfvedsonf discovered the new alkali 

 metal lithium, and showed its presence in spodumene, petalite 

 and tourmaline. 



In 1827 GmelinJ published analyses of ten varieties of tour- 

 maline, but his results led to no satisfactory formula, although 

 the essential constituents of the mineral, with the exception of 

 the boric oxide, were determined with a considerable degree 

 of accuracy. 



In 1845 Hermann§ published the results of four analyses of 

 tourmaline from Russian localities. He proved conclusively 

 that the iron was ferrous, and not ferric as considered by pre- 

 vious investigators. Boric oxide was not directly determined, 

 but estimated by difference, and the results compare favorably 

 with the direct determinations made by our present methods. 



* Ann. d. Phys. u. Chem., xxx, p. 107. 



f Schweiger's Jour. d. Chem. u. Phys., xxii, p. 111. 



% Pogg. Ann., ix, p. 127. 



§ Journal fur prakt. Chem., xxxv, p. 232. 



Am. Jour. Sci. — Fourth Series, Yol. VII, No. 38. — February, 1899. 



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