124 Penfield and Foote — Composition of Tourmaline. 



this acid H 9 Al 3 (B.OH) 2 Si 4 19 the mass effect of the complex 

 radical [Al 3 (B.OH) 2 Si 4 19 ] is so great, or, the role played by 

 the replacement of the nine hydrogens is so subordinate, that 

 the bivalent elements, Fe, Mg, Mn and Ca, and to a slight 

 extent the alkalies, Li, Na and K, can replace aluminium as 

 isomorphous constituents. This conclusion in some respects is 

 analogous to that reached by Rammelsberg, who stated in 1870 

 that all tourmalines were derived from an acid H 6 SiO B , in 

 which the six hydrogens were replaced in varying propor- 

 tions by R',, R ;/ 3 , Al 2 and B a . Applying Rammelsberg's idea 

 to the acid H 18 (B.OH) 3 Si 4 19 , all varieties of tourmalines may 

 be regarded as mixtures of the molecules 



R' 18 (B.OH) 2 Si 4 19 , R' =Li, Na, K and H 

 R" 9 (B.OH) 2 Si 4 ]9 , R" =Fe, Mg, Mn and Ca 

 R'" 6 (B.OH) 2 Si 4 19 , R"'= Al; Cr and small amounts of Fe and Ti. 



It seems more logical and satisfactory, however, to consider 

 all varieties of tourmalines as salts of the acid H fl Al 3 (B.OH), 

 Si 4 19 , in which the complex aluminium-borosilicic acid radical 

 [AJ 3 (B.OH) a Si 4 19 ] exerts a mass effect by virtue of which the 

 remaining hydrogens may be replaced by metals of essentially 

 different character without bringing about any pronounced 

 change in crystalline form. 



Note concerning the detection of ferrous and ferric iron in Sili- 

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



In the preliminary work connected with the study of the 

 methods of the tourmaline analysis, considerable time was 

 devoted to ferrous and ferric iron determinations. The ordi- 

 nary method of dissolving in a mixture of hydrofluoric and sul- 

 phuric acids and titrating with potassium permanganate can 

 not be applied readily to tourmaline because it is extremely 

 difficult to dissolve the mineral. Riggs overcame this difficulty 

 by using very fine powder and heating under pressure in a 

 heavy platinum crucible provided with a lid which was held 

 down by clamps. Jannasch and Kalb succeeded in decompos- 

 ing the exceedingly fine, elutriated powder by heating in sealed 

 glass tubes with a mixture of sulphuric and hydrofluoric acids, 

 but the method was far from satisfactory. We have employed 

 fusion with borax. In order to test the method, silicates con- 

 taining varying proportions of ferrous and ferric iron were 

 first carefully standardized by the method described by Pratt.* 

 The powdered silicate and borax glass were then fused in a 

 platinum boat in a combustion tube, through which a current 

 of either nitrogen or carbon dioxide gas was conducted. These 



* This Journal, xlviii, p. 149, 1894. 



