Properties of the Feldspars. 



109 



In one instance, while cooling the molten mass at a rapid 

 rate, we obtained a result which has a most important bearing 

 on the relation of the feldspars to one 

 another, which will be referred to again 

 in the concluding discussion of the experi- 

 mental data. When the charge had cooled, 

 it was found to consist of a compact mass 

 of rather large crystals, radial in struc- 

 ture, at the bottom of the crucible (fig. 6), 

 and a beautiful, transparent glass above. 

 It was easy to separate the crystalline por- 

 tion from the glass and to analyze the two 

 separately. The composition of the two 

 portions is practically identical, save for 

 a slightly higher percentage of iron in the 

 crystals.* In harmony with this latter 

 circumstance the color of the crystals was 

 a decided amethyst brown, while the glass 

 was but slightly tinted. The analyses fol- 

 low : 



Fig. 6. 



SiO 



AB 



Gh 



iAN 5 



iss Eesidue 

 Found. 



47-46 



33-56 



•29 



16-99 



1-87 



Cr> 



r stalline Cake 

 Found. 



47-34 



AI.O.-- 



Fe 3 8 ------ 



CaO 



Na 2 





33-51 



•47 



16-84 



1-89 



100 17 



100-05 



It is at once clear from these determinations that the solid 

 phase has the same composition as the liquid phase, so far 

 as it is within the power of chemical analysis to establish it. 



Ab^An v — In this feldspar we observe the same characteris- 

 tics as in the two preceding, but they are still less sharply 

 marked. The viscosity is greater, both solidification and melt- 

 ing take place more slowly, and the undercooling is so per- 

 sistent that the furnace must be cooled slowly or the charge 

 will come out wholly or partly vitreous. 



* A small quantity of iron was contained in the quartz used in preparing 

 the feldspars. 



Am. Jour. 



Sci. 

 8 



Fourth Series, Vol. XIX, No. 110.— February, 1905. 



