100 Penfield and Foote — Composition of Tourmaline. 



/BO, 

 R. = SiO. 



R. 



^s,o>. 



He supposes R to include H, Li, Na, K, Ca, Mg, Fe, Al and 

 small amounts of the A1=0 or possibly Al-OH radicals. It 

 may here be stated that the foregoing formula is identical in 

 type with the special formula R' 3 A.] a BSi 3 O 10 (H 9 BSi 2 O 10 ) of 

 Rammelsberg, and considering boron as replacing hydrogen 

 like a metal the silicic acid from which the formula is derived 

 becomes H 12 Si 2 O 10 or H 6 Si0 5 . Riggs further states that, owing 

 to slight variations, the ratios give nearly the " equally simple 

 general formula R 10 BO 2 2SiO 4 ," stating that " between these 

 two views there are at present no means at hand of deciding." 

 It would seem, however, that the last formula is impossible, 

 for, considering hydrogen atoms as replacing R 10 , the acid can 

 not be split up like other oxygen acids into silicic and boracic 

 anhydrides and water. There are also given the following 

 special formulas for three pronounced types of tourmaline : 



I. Lithia tour. 12Si0 2 , 3B 2 3 , 4H 2 0, 8A1 2 3 , 2(NaLi) 2 0. 

 II. Iron tour. 12Si0 2 , 3B 2 3 , 4H 2 0, 7A1 2 3 , 4FeO, Na 2 0. 



III. Mg tour. 12Si0 2 , 3B 2 3 , 4H 2 0, 5A1 2 3 , ^MgO, fNa 2 0. 



By substituting hydrogen atoms for the metals in these spe- 

 cial formulas we obtain : 



I. and II. HJB,Si 12 6a or B^B^O,,. 

 III. H s8 B,Si 1 ,0, J orH 19i B 2 Si 4 24 . 



Soon after the appearance of Riggs' article, W tilting* recal- 

 culated the results of these twenty analyses and concluded that 

 all tourmalines may be regarded as isomorphous mixtures of 

 two aluminium silicates, " ' Alumosilicate" of the following 

 composition. 



I. Alkali tourmaline 12Si0 2 . 3B 2 3 . 8A1 2 3 . 2Na 2 . 4H 2 0, 

 II. Magnesia tourmaline 12Si0 2 . 3B 2 3 . 5A1 3 3 . 12MgO . 3H 2 0. 



In these formulas it is assumed that the isomorphous ele- 

 ments K and Li replace the Na ; Fe ;// the Al ; and Fe", Mn 

 and Ca the Mg. By substituting hydrogen atoms for the 

 metals in the foregoing formulas it is found that they both are 

 derivatives of the same acid, H 60 B 6 Si 12 O 63 or H 20 B 2 Si 4 O 21 . The 

 conclusions derived by Wulfing are that, although in most 

 cases the results of the analyses agree with the percentage 

 values calculated from his formulas, the agreement is not 

 always satisfactory. This he ascribes to the possible need of a 

 * Mineralogische und petrographische Mittheiluogen, x, p. 161. 



