JY. L. Bowen — Plagioclase Feldspars. 577 



Art. XLIX.— The Melting Phenomena of the Plagioclase 

 Feldspars ; by N. L. Bowen. 



Introduction. 



The exact relation which the feldspars, albite .and anorthite, 

 bear to each other has for a good many years been a question 

 of much interest, to mineralogists and penologists. The con- 

 tinuous passage from the one to the other in chemical compo- 

 sition, found in the plagioclase feldspars, and the like passage 

 in optical and other physical properties, has been regarded by 

 the great majority as satisfactory evidence of complete solid 

 solution. Since Day and Allen* added thermal properties to 

 this list the evidence has been generally conceded to be con- 

 clusive, f 



These investigators, working with chemically pure, artificial 

 mixtures, studied the temperature of change of state from solid 

 to liquid of various compositions under precisely similar con- 

 ditions and were able to establish a perfectly continuous relation 

 between them. The temperatures at which maximum heat 

 absorption was found to occur, when plotted against compo- 

 sition, gave a smooth curve. The method used was not suitable 

 for the determination of the magnitude of the melting interval 

 or, more technically stated, the position of solidus and liquidus. 

 The curve obtained by Day and Allen might, therefore, lie 

 anywhere between the solidus and liquidus, and is not necessa- 

 rily the liquidus itself, as some appear to have assumed. 

 The attempt of Day and Allen to decide the question of a 

 melting interval by trying to establish a difference of compo- 

 sition between the first crystals and the residue gave only neg- 

 ative results, but this fact, as they recognized,^: might have been 

 due to the extreme viscosity of the melts. Since the comple- 

 tion of this pioneer work of its kind, the facilities of the Geo- 

 physical Laboratory have been greatly increased, and one of 

 the methods introduced (sudden cooling after long exposure to 

 a fixed temperature) is especially useful for the study of melt- 

 ing phenomena in substances which form viscous liquids on 

 melting. It was, therefore, deemed advisable to undertake the 

 study of the plagioclase feldspars by this method of sudden 

 quenching. The time consumed in the w T ork was greatly les- 



* The Isomorphism and Thermal Properties of the Feldspars, Carnegie 

 Institution of Washington, Pub. No. 31. 



fCf., however, Weinschenk-Clark, Petrographic Methods, New York, 1912, 

 p. 331-332. 



\L. c.,p. 72. 



Am. Jour. Sci.— Fourth Series, Vol. XXXV, No. 210.— June, 1913. 

 41 



