THE MINERAWGRAPHY OF THE FELDSPARS 293 



of the true relationship between the two minerals is a very difficult 

 matter. Since dimorphism (isomerism) is a thermal property of 

 matter, it is reasonable to expect that the effect of ''heat treatment" 

 on natural feldspars might throw light upon this problem. 



Merian and Wahl^ through heating of fragments and thin pieces 

 endeavored to convert microcline into orthoclase. In spite of heat- 

 ing to the melting point, they observed no change in the optical 

 orientation of the specimens. Makinen made similar experiments 

 which, while they throw considerable light upon the nature of anor- 

 thoclase, leave some doubt regarding the relation between microcline 

 and orthoclase. He says^ (in translation) : "Although the expected 

 transition (inversion) of microcline into orthoclase is indicated by 

 these researches, still one does not dare to use this in support of the 

 poly symmetry theory [of Groth].^ Apparently the rate of transition 

 is so slow and requires so long a time that it (inversion) cannot be 

 demonstrated in the laboratory." He says that the transition from 

 orthoclase to microcline is a "secondary"'' process. "However, it 

 gives no reason to assume that the final modification should be enan- 

 tiotropic, i.e., that orthoclase can be transformed into microcline 

 but not microcline into orthoclase." Morey and Bowen^ found that 

 there is "no appreciable difference of behavior connected with the 

 difference in form of orthoclase and microcline," in breaking up into 

 leucite and liquid at about 1170° C. 



In Part I the results of heating the microcKne-microperthite 

 (hypoperthite) from San Diego County, California,^ were offered 

 on page 267, in support of inversion of orthoclase into microchne 

 by "annealing" at about 900° C. The writer found that micro- 

 clinic twinning was produced by such treatment; the amount of 

 twinning formed being in proportion to the time the material was 

 subjected to heat. Further study of the material has caused the 

 writer to question the evidence. An examination of the optical 



^ A. Merian and Wahl, Nenes Jahrb.f. Min., I (1884), 195. 



^Eero Makinen, "tjber die Alkalifeldspate," Geol. Foren. Fdrhandl., XXXIX, 

 H. 2 (February, 1917), 127. 



3 Paul Groth, "Chemical Crystallography" (Marshall) Wiley, 1906, 7. See 

 Harker, op. cit., p. 258; also Wahl, Ofversiki afFinska Vet. Soc. Fork., L (1906-7), No. 2. 



4 In the original the word is "sekularer," which the writer suspects is a typo- 

 graphical error for "sekundarer." 



s Morey and Bowen, Amer. Jour. Sci. (5), IV (1922), 1-21. ^ Specimen 958. 



