82 Penfield and Pratt — Chemical 



quartz, while garnet, cyanite, magnetite and mica were also 

 observed. These inclusions -of qtt'&jrtz, amounting sometimes 

 to 30-40 per cent of the total weight of the crystal, account 

 for the great variation of the silica percentages in the older 

 analyses. 



In 1873 Eammelsberg- reexamined the exceptionally pure 

 staurolite from St. Gothard and also the impure material from 

 Pitkaranta and Brittany, in which he had previously found 

 over 50 per cent of silica. After purifying these latter by 

 treatment with hydrofluoric acid, only from 29 to 30 per cent 

 of silica was found and the analyses agreed with that of the 

 St. Gothard mineral. From these analyses he deduced the 

 formula H 2 Fe 3 Al, 2 Si 6 S4 , the iron being regarded as ferrous 

 and replaced in part by magnesia. 



In 1885 Friedlf investigated carefully selected material from 

 St. Gothard and Tramnitzberg in Mahren, which by examina- 

 tion with the microscope had been found to be free from 

 foreign inclusions. From the results of his analyses he deduced 

 the formula H 4 Fe 6 Al 24 Si n 66 . In the same year Coloranio;}; anal- 

 yzed the St. Gothard staurolite, which had been carefully 

 selected and digested with hydrofluoric acid, the formula 

 deduced by him being H 2 Fe„Al 12 Si 5 31 . 



It is interesting to note the variations in the proposed 

 formulae, each investigator in turn finding a smaller amount of 

 silica as shown below, where the formulee of Bammelsberg and 

 Coloranio have been doubled for more ready comparison. 



Rammelsbero- H FeAl.SLC) 



O 4 6 24 12 68 



Friedl ... H 4 Fe 6 Al 24 Si n 66 



Coloranio H 4 Fe Al., d Si in O„ 



4 4 2 4 10 62 



From a consideration of the analyses of Friedl and Coloranio 

 Groth§ concludes that staurolite has a still simple formula and 

 suggests a basic orthosilicate (A10) 4 (A10H)Fe(Si0 4 ) 2 . 



Selection and preparation of material for analysis. — In 

 the present investigation, material of exceptional purity was 

 selected from the four following localities : St. Gothard, 

 Switzerland ; Windham, Maine ; Lisbon, New Hampshire and 

 near Burnsville, North Carolina. The material from the first 

 of these is too well known to need special description. Some 

 crystals from the Brush collection were available. At Wind- 

 ham, Maine, it occurs in crystals measuring up to 25 mm in 

 diameter, imbedded in mica schist, as represented by an excel- 

 lent suite of specimens in the Brush collection. This has 



*Zeitschr. Deutsch. Geo]. Gesell., xxv, p. 53. 



\ Zeitschr. Kryst., x, p. 366. 



j Bull. Soc Chemique, xliv, p. 427. 



§ Tabellarische TJebersieht der Miceralien, 1889, p. 104. 



