Properties of the Feldspars. 107 



Anorthite. — Of the whole series of feldspars, this member 

 is in many respects the simplest to deal with. It is of rela- 

 tively low viscosity when melted and crystallizes easily, very 

 rapidly, and always in large, well developed crystals. A 100- 

 gram charge crystallized completely in ten minutes. Sudden 

 chilling gave a beautiful clear glass entirely free from bubbles, 

 somewhat slower cooling usually resulted in a partial crystal- 

 lization from few nuclei, the crystals being always large. In 

 appearance it resembles the natural mineral in every respect. 

 Its hardness is also equal to that of natural anorthite. Thin 

 sections show good cleavage, and twinning according to the 

 albite law is frequent. The extinction and other microscopic 

 characteristics are as well marked as in natural specimens. 



The heating curve of crystalline anorthite is perfectly 

 smooth except for the single break which marks the melting 

 point. No trace of a second crystalline form (Umwandlung) 

 appeared in this or any other of the feldspars within the 

 temperature range of the observations (300°-1600°). Some 

 undercooling always occurs in solidification even if the rate of 

 cooling is slow, but it is less, under like conditions, with 

 anorthite, than with any other member of the series. The 

 heating curve of the glass shows a strong evolution of heat 

 which may occur as low as 700°, when crystallization takes 

 place. The melting point of crystalline anorthite was deter- 

 mined by three different thermoelements upon two different 

 mineral preparations. It will be seen from the appended 

 table that the determinations agree remarkably well. 









First Preparation. 









E.M. F. 



Tem- 





Date 





Element 



inMV. 



perature. 



Eemarks. 



Oct. 7, 



L903 



A 



15939 



1534° 



solid charge, open crucible. 



u a 



u 



A 



15914 



1532 



a a 



" 10 



a 



A 



15878 



1530 



covered crucible. 



a tt 



a 



No. 3 



16074 



1533 



U It 



a a 



a 



3 



16058 



1532 



a a 



a a 



a 



3 



16068 



1532 



l. li 



u a 



a 



2 



16095 



1532 



a a 









Mean 1532° 











Second Preparation. 



Jan. 16, 



1904 



A 



15860 



1532° 



covered crucible. 



a it 



a 



A 



15864 



1532 





" 20 



a 



No. 3 



15960 



1533 





a a 



a 



2 



16102 



1532 





a tt 



a 



2 



16092 



1532 





Mch. 31 



a 



3 



15932 



1531 1st & 2d preparations together. 



Mean 1532° 



Melting temperature 1532°. 



