74 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



The melting points and specific gravities plotted above are brought 

 together in a convenient table here. 



Feldspar. 



An ... . 

 At^Aris 

 Ab,An 3 

 AbjAri! 

 Ab.Aiii 

 AbaArii 

 Ab . . . . 



Melting 

 temperature 



(degrees). 



1532 

 15OO 



H63 

 1419 



1367 



J 340 



Specific gravity. 



Crystals. 



2.765 



2-733 

 2.710 

 2.679 

 2 .660 

 2.649 

 2.605 



Glass. 



1 . 700 

 :.648 

 '59 1 



:-533 



483 

 1.458 



!.382 



(3) In the melting of albite and microcline we appear to have sub- 

 stantial evidence of a phenomenon which is unfamiliar both to physics 

 and to mineralogy. Microscopic crystals of a homogeneous com- 

 pound, when slowly heated, were shown to persist for 150 or more 

 above where melting began, the amorphous melt remaining of the 

 same order of viscosity as the rigidity of the crystals. By careful 

 observation, curves were also obtained showing that the absorbed 

 heat of fusion was distributed over this interval. 



From the experimental standpoint a substance of this kind can 

 hardly be said to have a melting point, but passes gradually from 

 crystalline to amorphous at temperatures which can be considerably 

 varied by merely changing the rate of heating. In moderate charges 

 of albite or orthoclase at atmospheric pressure this melting began so 

 slowly that it was not possible to locate even approximately a lowest 

 temperature for the beginning of the change of state. As a matter of 

 definition, this minimum temperature above which melting will con- 

 tinue (for a given pressure) more or less rapidly, according to the 

 conditions, is the "melting point," whether it can be located or not, 

 so far as the equilibrium of the system is concerned; and crystals 

 which continue to exist unmelted at higher temperatures appear 

 to form a metastable phase, perhaps comparable to that of a crys- 

 talline solid when heated above the "Umwandlungstemperatur" 

 without immediate change of crystal form. It is also possible that 

 the mass is fluid when heated above the melting point, but that 

 deorientation of the molecules is delayed by viscosity. This meta- 

 stable stage can easily extend over 1 50 in albite and orthoclase and 

 would persist for days in the lower portion of this range. 



(4) We also found that viscous and poorly-conducting melts which 

 solidify only after considerable undercooling do not give constant 

 solidifying points. The solidifying point must not be used, therefore, 

 without great caution as a physical constant; it bears no relation 



