0. C. Farrington — Chemical Composition of Iolite. 13 



Art. III. — The Chemical Composition of Iolite; by O. C. 



Farrington. 



As is well known the formula of iolite has never been satis- 

 factorily established. This is chiefly for the reason that the 

 state of oxidation of the iron, in the analyses hitherto pub- 

 lished, has not been determined. Stromeyer,* Gmelin-f- and 

 Schiitz,^: who made the earlier analyses, regarded the iron as 

 protoxide. Scheerer,§ however, in 1846, in connection with 

 analyses of iolite from Kragero, urged that it was more proba- 

 bly present as sesquioxide, his reason being stated as follows : 



" Das Verhaltniss des Sauerstoffs der Kieselerde zu dem der 

 Thonerde und zu dem der 1 und 1 atomigen Basen ergiebt sich 

 hiernach wie : Si 26*20 : M 15*26 : R 5*48 wenn man namlich an- 

 nimmt, dass die geringe Menge Eisen als Oxydul in Mineral 

 vorhanden sei. Diess diirfte aber schwerlich der Fall sein, da der 

 analysirte Cordierit fast vollig farblos war und auch nicht den 

 geringsten Stich ins Grunliche zeigte, wahrend es bekannt ist, 

 dass verhaltissmassig sehr kleine Quantitaten Eisenoxydul hin- 

 reichend sind, urn einen (nicht pulverformigen) Silicate eine deut- 

 lich grime Farbe zu ertheilen, sobald diess natiirlich nicht durch 

 andere farbende Substanzen verhindert wird. Nimmt man daher 

 gewiss mit mehr Recht das Eisen in Zustande des Oxyds an, so 

 wird das Sauerstoffverhaltniss Si26*20 : £ 15*64 : R 5*26." 



This conclusion of Scheerer has been accepted by most later 

 writers. RammelsbergJ regarding the iron as sesquioxide, 

 deduces the generally accepted formula, 2MgO, 2R 2 3 , 5Si0 2 , 

 although the ratios are not very satisfactory. He also suggests 

 Mg 3 R 6 Si 8 28 as a possible formula. Water seems to have been 

 disregarded. 



At the locality in Guilford, Conn., recently described by 

 Dr. E. O. Hovey,^ iolite occurs as stated by him, as a constitu- 

 ent of the rock mass. This locality was visited by the writer, 

 and it was also found that veins of more coarsely crystalline 

 material, running through the gneiss, contained the mineral in 

 grains as large as a walnut and even in pieces of sufficient size 

 for hand specimens. These large grains are very clear and 

 transparent, and show none of the tendency to alteration so 

 characteristic of the iolite from other localities. 



The exceptional purity of this material led the writer to 

 make a chemical analysis of it, and care was taken to use only 

 those grains which were perfectly clear and showed the charac- 



* Unters., Rg. Min. Ch. f Schwgg J., xiv, 316. % Pogg. Ann., liv, p. 565. 

 § Pogg. Ann , lxviii, p. 319. || Mineralckeniie, 1875, p. 652. 



1[ This Journal, III, xxxvi, 57. 



