306 Clarke and Schneider — Experiments upon the 



Hence olivine, at the temperature of 383°— 412°, is hardly 

 attacked by dry hydrochloric acid ; and the slight action here 

 recorded may possibly be due to incomplete drying of the gas. 

 The aqueous acid, on the other hand, decomposes olivine with 

 great ease. On this mineral no other experiments were neces- 

 sary for our purposes. 



2. Talc. 



The mineral studied was a typical, apple-green, foliated talc 

 from Hunter's Mill, Fairfax County, Virginia. Analysis as 

 follows : 



SiO„ 62-27 



A1 2 3 -15 



Fe; 2 3 -95 



MwO 30-95 



FeO -85 



MnO trace 



HO 4-91 



100-08 

 In detail, the water determination was as follows : 



Loss at 105° -07 



Loss at 250°-300° -06 



Loss at red heat 4-43 



Loss at white heat. . -35 



Hence the water is practically all constitutional. 



Heated in dry HCl gas to 383°-412° for fifteen hours, the 

 talc underwent inappreciable change of weight. Upon leach- 

 ing, only - 23 per cent of magnesia went into solution. By 

 rapid evaporation with 75 cc. of fuming aqueous HCl, 1*05 

 per cent of MgO and OL6 of (FeAl) 2 3 were dissolved. Upon 

 eight days' digestion on the water bath with acid of 1*12 sp. 

 gr., 1*94 MgO and - 23 of sesquioxides were removed. By 

 digesting in like manner for thirty-two days, 3*94: MgO and 

 0*41 of sesquioxides were taken out. Talc, therefore, is, as 

 should have been expected, remarkably stable in presence of 

 hydrochloric acid, both aqueous and dry. This fact has a very 

 important bearing upon the constitutional formula of the min- 

 eral. 



In empirical composition the talc analyzed agrees quite 

 sharply with the accepted formula Mg 3 H 2 Si 4 12 . This is com- 

 monly interpreted as an acid metasilicate, Mg 3 H 2 (Si0 3 ) 4 ; but 

 Groth has lately proposed to consider talc as a basic salt of 

 pyrosilicic acid, H 2 Si 2 6 . On this supposition its rational for- 

 mula becomes Mg(Si 2 6 ) 2 (MgOH) 2 . Between these two for- 

 mulae, experiment is able to decide. 



