50 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS 



Surmising that we had accidentally hit upon the approximate melt- 

 ing temperature, a fresh charge of like material was prepared and the 

 same experiment carefully repeated, except that the temperature was 

 carried up to 1206 and maintained there for 30 minutes. Instead 

 of showing the melting to be complete, the slides (Plate XXI) looked 

 precisely like the first, save that the lanes of glass were somewhat wider 

 and the crystal fragments relatively smaller than before. Further 

 trials under precisely the same conditions, with the temperature in- 

 creased to 1225 (Plate XXII) and 1250 (Plate XXIII), respectively, 

 for like periods of time, showed only more advanced stages in the same 

 process. In the latter case the remaining crystal fragments were rela- 

 tively very small compared with the separating lanes of glass, but the 

 orientation of the tiny particles still remained perfectly undisturbed. 



The evidence contained in this series of slides shows plainly that we 

 have here an unfamiliar condition a case of a crystalline compound 

 persisting for a long time above its melting temperature for a given 

 pressure. Albite or orthoclase glass sinters tightly at 8oo. At the 

 temperature where melting began, therefore (below 1200 ), the charge 

 consisted of crystal fragments of microscopic size embedded in a large 

 vitreous mass of the same composition and known temperature. 

 These fragments melted so slowly over the 50 included between the 

 first slide and the last, with the rate of heating slow (i in 2 minutes) 

 and the upper temperature continued for 30 minutes, as to leave con- 

 siderable portions unmelted at the close. Furthermore, the extreme 

 viscosity, of which further evidence will be given directly, and the 

 absence of any disturbance in the orientation of the particles indicat- 

 ing flow, assured us that the lanes of glass represented actual melting 

 and not an inflow of glass from without. Finally, the perfectly homo- 

 geneous character of the glass and the unchanged appearance of the 

 crystals as heating progressed gave no hint of any chemical decom- 

 position. 



In the hope of obtaining a point of value for comparison with the 

 melting points of the other feldspars, some time and patience were 

 expended in trying to locate the lowest temperature at which certain 

 evidence of melting appeared. We did not extend any single trial 

 beyond a single day, so that our results can not pretend to establish 

 the lowest point at which albite melts. Such an effort with a natural 

 specimen known to contain impurities would yield nothing of value. 

 Mitchell County albite showed signs of melting after four hours at 

 1 ioo. Under a high power the crystal edges appeared weathered or 

 toothed strongly resembling the incipient melting of the ice on a 

 frosted window pane. These extremely fine teeth could be followed 



