PERTHITIC FELDSPARS. 143 



7.0 per cent and 11.5 per cent. The albite mixed-crystal shows about 

 the same order of variation in the amounts of NaAlSisOg. The anor- 

 thite variations are proportionally larger, the amounts of this mole- 

 cule varying from about 3.0 per cent to 8.0 per cent. The writer, 

 therefore, concludes that the microcline mixed-crystals of the granite 

 pegmatite perthites will in general contain around 9.0 per cent of the 

 albite phase in solid solution, and that the albite mixed-crystal contains 

 about 8.0 per cent of the KAlSisOg molecule plus a small, but rather 

 variable amount, 3.0 per cent to 8.0 per cent, of anorthite. The 

 ^elati^^e amounts of the orthoclase molecule and of the albite plus 

 anorthite will in general be found to range from about 70-30 per 

 cent to 80-20 per cent, though in exceptional cases much greater 

 variations may occur, as No. 1 with 51.9 per cent of the orthoclase 

 molecule present. 



Discussion of the origin of Perthitic inter growths. — It was formerly 

 held by several mineralogists ^^ that the perthitic intergrowths were 

 formed by the introduction of albite from without into previously 

 formed microcline crystals. This view is probably now entertained 

 by few. While there are undoubted instances where albite has been 

 introduced from without by later mineralizing action along cleavage 

 or fracture lines, or by more irregular replacement about the margins, 

 there is usually in such cases, independent evidence in the surround- 

 ing rock that such action has been going on. In the case of the great 

 majority of perthitic intergrowths, at any rate, such evidence is lack- 

 ing. It is difficult to see how albite could be introduced into the 

 microcline so uniformly and extensively, with the perfect preservation 

 of the microcline's structure, if the openings, along which the former 

 was introduced, were prepared by a breaking of the original micro- 

 cline. Furthermore, such breaking could hardly be effected by any 

 form of crushing without a general deorientation of the fragments, 

 nor is it likely that the microcline crystal could be ruptured in such 

 fashion by any sudden change of volume incident to a possible 

 passage through an inversion point. Anyway fracturing of the micro- 

 cline would most likely follow along the more important directions 

 of weakness, the principle cleavages, and these directions are not 

 those followed by the albite growths. It also seems improbable 

 that the albite could have worked its way into the microcline along 

 lines of easy solubility by a replacement process. If such were the 

 case we should expect to find the replacement more marked about 



13 In this connection see particularly O. Wenglein, Inaug. Diss. Kiel., (1903). 



