318 PROCEEDINGS OF THE AMERICAN ACADEMY. 



that mixtures of potasli-fcldspar and plagioclase belong to type V of 

 Roosebooms' ^^ classification of mixed crystals; viz. — they have a 

 limited miscibility in the crystalline condition with a eutectic point 

 between the two resulting types of mixed crystals. His figures are 

 admittedly only approximations and future investigation will doubtless 

 modify them in some particulars, l)ut nevertheless his views are to the 

 highest degree suggestive and important. At temperatures lower than 

 that at which the anorthoclase or cryptoperthite crystallizes, some 

 diminution of the solubility of the one feldspar in the other in the 

 mixed crystals, would probably result connected, probably, with a 

 transition to another crystalline modification in the case of at least 

 one component, and a transformation, an unmixing {cnimischung), 

 in the solid condition would then take place. The perthite or micro- 

 perthite which might in such case result, would consist of a potash 

 feldspar containing less Ab or Ab + An than it would at higher tem- 

 peratures, and a plagioclase member, likewise poorer in Or. Vogt 

 estimates that the potash member would contain in such cases 10- 

 15% of Ab or Ab + An. That the perthitic intergrowth is due in 

 general to an unmixing of an earlier existing homogeneous mixture, 

 has also been entertained by several other petrographers,^° although 

 none have put their ideas into such definite form as has Vogt. 



It appears to be pretty well established that there are all gradations 

 between microperthite and cryptoperthite; that there is a truly 

 gradual gradation into a perfectly homogeneous soda-potash feldspar 

 is not so certain. It may be entirely correct to look upon the homo- 

 geneous anorthoclase as being a crystallized eutectic mixture, but 

 the writer is led to go somewhat further and suggest that the appar- 

 ently homogeneous mixtures may be a true mixed crystal belonging 

 to Type 1 of Rooseboom, viz. — the two feldspars are miscible in all 

 proportions in the solid crystalline state. ^^ It may be noted that 

 anorthoclase presents certain peculiarities that perhaps suggest a 

 mixed crystal. It is a familiar fact that its crystals frequently show 

 a division into fields when examined under crossed nicols, in fact, these 

 may be visible megascopically. This phenomenon is common in 

 mixed crystals such as the garnets and alums. Again the crystalline 



79 Zeitschr. f. Phys. Chem., 30 (1897). 



80 Rosenbusch, F. Becke and others. 



81 Since this was written a paper by Dr. E. Dittler (T. M. P. M., Nos. IV- 

 V, 31, pp. 511-22, 1912) has come to the author's attention, in which ex- 

 perimental evidence is given to the effect that the alkahe feklspars form 

 mixtures belonging to type 111 of Rooseboom's classification; viz. — a con- 

 tinuous Hne of mixed crystals tending toward a minimum freezing point. 



