0. Andersen — Aventurine Feldspar. 379 



by the melting and recrystallization. We have no means of 

 deciding of what nature this change may have been. It is not 

 unlikely, however, that at least a part of the hematite in 

 melting has been reduced to magnetite. 



Origin of the Hematite Lamellae. 



The investigations above described show that the aventurine 

 feldspars must be considered oriented intergrowths between 

 feldspar and hematite. The essential features of the inter- 

 growths are these : The hematite crystals form exceedingly 

 thin plates after the base and the plates are oriented parallel 

 to some simple crystal faces of the feldspar with the edges of 

 the hematite crystals also, in general, definitely oriented. 

 These facts should be borne in mind when we seek an adequate 

 explanation of the origin of the hematite lamellae. 



Of the different possible theories there are two that seem to 

 account well for the oriented intergrowth, viz.: The theory of 

 simultaneous crystallization suggested by Scheerer and the 

 theory of unmixing in the solid state intimated by Johnsen 

 (see review of literature). In discussing the origin of the 

 hematite lamellae in carnellite and cancrinite Johnsen points 

 out the reasons for preferring the theory of unmixing to the 

 theory of simultaneous crystallization in the cases of the two 

 minerals mentioned. Similar reasons are evidently valid also 

 for the aventurine feldspars. 



It is obvious that the planes of growth of the feldspar have 

 not, in general, coincided with the principal planes of orienta- 

 tion of the hematite lamellae, as the latter planes represent 

 extremely rare forms with the feldspars. It is highly probable, 

 for example, that the faces 112 and 150 have never existed as 

 crystal faces (faces of growth) in any of the specimens con- 

 sidered, and still they are the most important planes of orien- 

 tation of the lamellae in all aventurine feldspars. If the 

 aventurine feldspars were considered products of simultaneous 

 crystallization of feldspar and hematite, therefore, we would 

 have to assume that the majority of the extremely thin hema- 

 tite lamellae during their growth formed angles with the prin- 

 cipal faces of growth of the feldspars. This is improbable 

 according to the common experience with crystal intergrowths, 

 and the orientation of the hematite lamellae thus forms a 

 strong objection against the theory of simultaneous crystalliza- 

 tion ; a theory which otherwise would seem very reasonable. 



The formation of aventurine feldspars by unmixing in the 

 solid state may be conceived as follows: The feldspar crystals 

 were, at the time of their separation, wholly or in part homo- 

 geneous and contained small amounts of Fe 2 3 in solid solution, 



