28 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



group. Investigation from the thermal point of view has been even 

 less satisfactory by reason of the subjective methods employed, to 

 which reference has already been made, though the recorded results 

 indicate with reasonable unanimity that the melting point of anorthite 

 is above that of albite and that the intermediate feldspars will prob- 

 ablv fall between the two.* Beyond this conclusion, the great body 

 of evidence is more or less contradictory and sometimes contro- 

 versial in character. 



Orthoclase (Preliminary). 



Somewhat unluckily, our measurements began with natural ortho- 

 clase (microcline) from Mitchell County, North Carolina, a quantity 

 of which was placed at our disposal by the U. S. National Museum. 

 The material was powdered so as to pass readily through a ioo-mesh 

 sieve, and placed in ioo cc. or 125 cc. platinum crucibles, sometimes 

 open and sometimes covered, in charges of from 100 to 150 grams. 

 These charges were heated slowly in the electric furnace from 600 to 

 above 1400 C, but, although the thermal apparatus was sufficiently 

 sensitive to detect an unsteadiness of a tenth of a degree with certainty 

 not the slightest trace of an absorption or release of heat was found. 

 The charge at the beginning of the heating was a dry crystalline 

 powder which was prodded from time to time with a stout platinum 

 wire to ascertain its condition as the heating progressed. At about 

 1000 traces of sintering were evident; at 1075 it had formed a solid 

 cake which resisted the wire, at 1150 this cake had softened suffi- 

 ciently to yield to continued pressure, and at 1300 it had become a 

 viscous liquid which could be drawn out in glassy.- almost opaque 

 threads by the wire. Under the microscope the opacity was seen to 

 be due to fine included bubbles, the material being entirely vitreous. 

 The cooling was equally uninstructive ; the vitreous mass solidified 

 graduallv without recrystallization or the appearance of any thermal 

 phenomenon. Frequent repetitions with fresh charges and varied 

 conditions added nothing to our knowledge of the melting tempera- 

 ture, and the matter began to look very unpromising. 



We also reheated charges of the resulting glass, which was some- 

 times repowdered and sometimes in the cake as it had cooled. But 

 except to observe that the glass powder began to sinter earlier (8oo), 

 no new facts appeared. f 



* J. H. L. Vogt, loc. cit., p. 154, expresses the opinion that the soda-lime feld- 

 spars fall under Type III of Roozeboom's types of isomorphous series with a 

 minimum between anorthite and albite. 



f These sintering temperatures varied within considerable limits with the fine- 

 ness of the material and, therefore serve only in a very rough way to define the 

 state of the charges. 



