ZOISITE. 79 



phur Bank, which will be described on a future page, consists mainly of 

 glaucopliane and zoisite; but fine needles of the former penetrate the latter. 

 The purest lot of zoisite had a specific gravity of 3.21, which is less than 

 that of unmixed zoisite, but greater than that of glaucophane. Its compo- 

 sition was found to be as follows: 



Water at above 100- , H-O T). 25 



Silica, SiO^ Iffl. HO 



Titanic acid, TiO- Traco 



Alumiua, Al-0^ 22. 72 



Ferric oxide, Fe-0 ■ 4. 85 



Ferrous oxide, FeO 1. 41) 



Manganous oxide, MuO 0.26 



Lime, CaO 17. .55 



Magnesia, MgO ;l. 89 



Soda, Na=0 4.09 



Potassa, K^O 0. 1 2 



Total 100.02 



The atomic ratio ff : R" : it" : Si is represented by 2.62 : 4.54 : 6.82 

 : 12. This clearly does not coi-respond to pure zoisite, of which the ratio 

 is 1 : 4 : 9 : 12, and the question arises whether it may represent any other 

 lime-alumina silicate. A glance at the minerals of similar composition, the 

 density of Avhich lies between 2.90 and 3.50, shows that the choice is small, 

 and in fact, among known minerals, is limited to zoisite and prehnite. The 

 specimeA analyzed was more acid than zoisite, but more basic than prehnite. 

 Considering that the maximum density of prehnite is 2.95 and that the 

 known impurity of the specimen is acid, the tendency is all to the suppo- 

 sition that the mineral is zoisite. If one supposes the admixture to be 

 simply a bisilicate of a protoxide base and that this impurity contained 

 about one-fourth of the silica, the above atomic ratio reduces almost ex- 

 actly to 3 : 4 : 9 : 12. It is true that glaucophane is an aluminous amphi- 

 bole and that if sesquioxides are subtracted from the analysis the ratio 

 4:9:12 cannot be exactly preserved; but the rock also contains quartz 

 and the atomic ratio of zoisite is known to vary to some extent. Tlie 

 figures discussed, in connection with the known impurities, are thus suffi- 

 cient to show that the mineral is not prelinite and is far more closely allied 

 to zoisite than to any other known mineral. There is an excess of water 



