36 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



passed through a "ioo-mesh" sieve, and then melted again. This 

 process probably gives a fairly homogeneous mixture, though a third 

 fusion in the resistance furnace was generally made before determining 

 the constants. 



We prepared in this way albite (Ab), anorthite (An), and the follow- 

 ing mixtures of the two: AbiAn 5 , AbiAn 2 , AbiAn lt Ab 2 Ani, Ab 3 Ani, 

 Ab 4 Ani. All of these were obtained in wholly or partially crystalline 

 form, by crystallization from the melt, except albite. The syntheses 

 were controlled by analyses of a number of the products, the results 

 of which are shown in the table on p. 35. 



Anorthite (Plate I). 



Of the whole series of feldspars, anorthite is in many respects 

 the simplest to deal with. It is of relatively 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 

 crystallization from few nuclei, the crystals always being 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 fre- 

 quent. 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 to 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 heat- 

 ing 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 determined by three different 

 thermoelements upon two different mineral preparations. It will be 

 seen from the table on p. 37 that the determinations agree remarkably 

 well. This is of considerable significance with reference to the method 

 of temperature measurement employed. It will be remembered that 

 the established temperature scale ends at 1 1 50 and that temperatures 

 beyond that point are extrapolated with the help of some trust- 

 worthy phenomenon which varies with the temperature. We chose 

 for this purpose the thermo-electric force developed between pure 



