144 WARREN, 



the marginal parts of the crystals. Instead we fine that a striking 

 feature of such intergrowths is the uniformity of the albite distri- 

 bution taken as a whole throughout the entire crystal. We should 

 also hardly expect that a wholesale replacement such as would be 

 represented by the case of feldspar No. 1 (47.7 per cent Ab.) could be 

 effected without causing a profound dislocation, or even a complete 

 breaking up, of the original microcline crystal structure. It seems 

 necessary to abandon this theory, particularly as the one given below 

 offers an entirely reasonable explanation of the observed facts and is 

 in keeping with the physico-chemical theory relative to such systems. 

 J. H. L. Vogt ^* appears to have been the first to definitely formulate 

 a physico-chemical theory to account for the origin of the perthitic 

 intergrowths. As a result of an extensive statistical study of the 

 chemical data regarding the compositions of the feldspars which were 

 first to crystallize from rock magmas of different composition, he was 

 led to conclude that the alkalic feldspars, orthoclase and albite (plus 

 some anorthite), formed a broken series of mixed-crystals with a 

 eutectic point between them. He estimated that the two mixed- 

 crystal phases had the approximate composition. Or, 72 per cent; 

 Ab + An, 28 per cent, and Ab -f x\n, 88 per cent; Or, 12 per cent 

 (Points i and q on the diagram, Fig. IV). He was also led to conclude, 

 judging, among other things, from their rough constancy of chemical 

 composition and their structure, that the so-called " cryptoperthites " 

 described by Brogger and others,^^ represented a eutectic mixture ^^ — 

 a simultaneous crystallization of the two mixed-crystals — and that 

 the eutectic composition lay in the region Or, 40^4 per cent; Ab + 

 An, 60-56 per cent or approximately Or, 42 per cent; Ab -}- An 58 

 per cent (Point E, Fig. IV) . He furthermore pointed out that, in all 

 probability with lowering temperatures (and perhaps also only under 

 considerable pressures) a change would occur in the chemical composi- 

 tions of the mixed-crystals, due to diminished solubility of the com- 

 ponents and also, probably, to the occurrence of an inversion point 

 in the potassic feldspar from an a to a |S modification, viz. — from 

 orthoclase to microcline. He estimated the chemical compositions 

 of the finally resulting crystals of microcline to be in the neighborhood 



14 Loc. cit. 



15 Zoit. fur. Kryst. n. Min., 16, 537. 



16 It of course does not follow that the cryptoperthitic feldspars represent 

 the only form which the feldspar eutectic can assume. Separate crystalliza- 

 tions of the two members might form under certain favorable conditions. A 

 fine grained aggregate or an intergrowth would perhaps be the most probable 

 form to result from such a mixture. See later regarding this point. 



