4 Morey & Bowen — Melting Potash Feldspar. 



inversion point in the neighborhood of 700°, where they 

 experience a volume change of more than 2 per cent. 5 

 This may seem a small change, but it is twice as great as 

 the whole change between 585° and room temperature. 



Detailed Results with Artificial and Natural Feldspars. 



The above observations showed very clearly that potash 

 feldspar has no true melting-point, that the point in the 

 neighborhood of 1200°, which has hitherto been regarded 

 as its melting-point, is really the temperature at which it 

 breaks up into liquid and leucite or, as it is commonly 

 stated, it melts incongruently. It is perhaps not sur- 

 prising that the existence of leucite in the material 

 obtained near the decomposition-point has not been 

 detected hitherto. Megascopically the "glass" has all 

 the appearance of an ordinary glass ; it is perfectly trans- 

 parent, with only a faint suggestion of a bluish opales- 

 cence, the leucite crystallites being too minute and too 

 closely matched in refraction by the medium in which 

 they are embedded to cause a scattering of light. Even 

 under the microscope the leucite appears, as we have seen, 

 merely as an indefinite crosslining of the "glass" and 

 only when the temperature was raised, to see if this 

 structure would disappear, did the structure assume more 

 definite form and finally become identifiable leucite 

 crystals. 



After obtaining these results we then proceeded to 

 determine the temperature of incongruent melting more 

 accurately and also to fix the temperature at which the 

 leucite crystals disappear and the mass is entirely liquid. 

 While carrying out these determinations we studied the 

 similar changes as displayed by natural potash feldspars 

 in as pure a state as we could obtain them. For this 

 purpose we made use of three analyzed feldspars, micro- 

 cline from Mitchell County, North Carolina, sanidine from 

 Laacher See and adularia from St. Gotthard, the last 

 being the nearest to pure potash feldspar, though even it 



5 So far as we are aware the volume change has not been measured, 

 but it may be estimated from the change of refractive index between 585° 

 and 750° as observed by Einne and Kolb, Neues Jahrb. 2, 157. 1910. 



