THE MINERALOGRAPHY OF THE FELDSPARS 357 



Zonal feldspars are not exclusively plagioclase, for orthoclase, 

 microcline, and anorthoclase occur with this structure. "In certain 

 porphyritic granites (Rapakiwi) oligoclase forms a shell about pheno- 

 crysts of orthoclase."^ 



THE TWINNING OF FELDSPAR 



It seems a pity that while the twinning phenomena of feldspars 

 are listed and illustrated in every textbook of any pretensions, no 

 adequate theory is offered in explanation. Marker,^ following Vogt,^ 

 has stated that microcline structure seems "to .be sufficiently 

 accounted for by spontaneous changes consequent upon fall of tem- 

 perature, " a view maintained in Part I. Still the setting up of 

 microclinic structure is also a function of stress."* The two ideas can 

 be united by saying that the inversion of orthoclase to microcline 

 (if we grant dimorphism) involves a change in volume^ and this 

 introduces stress into the system. Now it is conceivable that 

 inversion may take place and a change in volume ensue, but at such 

 a slow rate that twinning does not occur. The writer has repeatedly 

 observed in cutting and grinding feldspars for thin sections that 

 microcline twinning is produced by the process. Hence he reasoned 

 that gentle and slow grinding might result in a section free from 

 twinning. This proved to be correct after repeated failures. The 

 microcline from Lincoln County, Nevada, thin sectioned in this 

 careful manner, showed the following extinction angles, (010) 5.6°, 

 and (001) 16.1° although exhibiting no microcline twinning, even when 

 magnified to 2,400 diameters. An ordinary slide of the same speci- 

 men showed "scotch plaid" structure and the same extinction 

 angles. Iddings^ says " Microcline .... may be free from lamel- 

 lar twinning and resemble orthoclase except in its optical orienta- 

 tion." The conclusion is clear: that microcline twinning is not 

 always characteristic of microcline but merely of a strained condition 



' J. P. Iddings, Igneous Rocks, II, 47. 

 ^ Op. cit., p. 260. 



3 Vogt, Tsch. Min. Petr. Mitt. (2), XXIV, 537-41- 



4 See Part I, pp. 275-76. 



5 Note the two sets of specific gravities for potash-soda feldspars. 

 ^ Op. cit., p. 46. 



