142 Day and Allen — Isomorphism of the Feldspars, etc. 



considerably varied "by merely changing the rate of heating. 

 In moderate charges of albite or orthoclase at atmospheric pres- 

 sure this melting began so slowly that it was not possible to 

 locate even approximately a lowest temperature for the begin- 

 ning of the change of state. As a matter of definition, this 

 minimum temperature above which melting will continue (for 

 a given pressure) more or less rapidly according to the con- 

 ditions, is the u melting point," whether it can be located or 

 not, so far as the equilibrium of the system is concerned, and 

 the crystals which continue to exist unmelted at higher tem- 

 peratures appear to form a metastable phase, perhaps compar- 

 able with that of a crystalline solid when heated above the 

 " Umwandlungstemperatur " without immediate change of crys- 

 tal 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 metastable stage can easily 

 extend over 150° in albite and orthoclase and would persist for 

 days in the lower position 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 whatever to the melting point unless equi- 

 librium is reestablished before solidification is complete — a con- 

 dition whieh rarely obtains and often can not be produced in 

 viscous mineral melts. Especial attention is directed to this 

 because of the importance of the lowering of the solidifying 

 point in the study of solutions, and the possibility of its appli- 

 cation to mineral solutions recently suggested by Vogt.* 



(5) Incidental to the experimental work upon the feldspars 

 we were able to establish the fact that there are no differences 

 of density in the feldspar glasses due to the rate of cooling, 

 which are greater than our errors of observation (db"001). Also 

 that powdered feldspar glasses sinter tightly at temperatures 

 as low as 700°, — a phenomenon which we ascribe to flow in the 

 undercooled liquid. Pure, dry, crystalline feldspars also sinter 

 at least 150° below their melting temperature, but very slowly. 

 This may be due to certain crystalline nuclei growing at the 

 expense of others; perhaps through exceedingly slow subli- 

 mation. 



Also that powdered feldspars, when exposed to the atmos- 

 phere, adsorb moisture, in quantities of an order of magnitude 

 equal to those usually quoted in analyses (Dana's System of 

 Mineralogy, p. 314). It is therefore altogether possible that 

 the significance of this moisture has been mistaken. It cannot 

 be completely driven off below a red heat. 



Geophysical Laboratory, Carnegie Institution, 

 Washington, D. C, November, 1904. 



*J. H. L. Vogt, loc. cit. 



