354 0. Andersen — Aventurine Feldspar. 



embracing aventurine feldspars, came to the following conclu- 

 sion concerning the origin of the inclusions :* " These enclos- 

 ures are of the nature of negative crystals which are more or 

 less completely filled with products of decomposition of the 

 mineral." Judd considered these products of decomposition 

 as chiefly consisting of amorphous hydrates of silica and ferric 

 oxide. 



H. Tertschf examined the sunstone from Tvedestrand and 

 found the reflecting lamellae oriented parallel to (538) and (417), 

 which forms were considered boundary positions (" Grenzla- 

 gen ") of the simpler form (213). 



A. Johnsen determined the inclusions in carnallite^: and can- 

 crinite§-as hematite, and found them identical with those con- 

 tained in aventurine feldspars. The presence of the inclusions 

 in carnallite and cancrinite was explained as due to secondary 

 reactions and unmixing in the solid state. It was intimated 

 that the inclusions in aventurine feldspars might be explained 

 in the same way. 



I. METHODS OF EXAMINATION AND GENERAL RESULTS. 



The study of the specimens embraced three groups of exam- 

 inations : 



(1) Microscopic examinations with the object of determining 

 the feldspars. 



(2) Determination of the orientation of the reflecting lamellae. 



(3) Examination of the properties of the reflecting lamellae. 

 The results of the first group will be given in the section in 



which the specimens are described and details of all measure- 

 ments are given. The present section contains a brief review 

 of the general results of the last two groups in connection 

 with descriptions of the methods applied and discussions of 

 the simple optical problems involved. 



Determination of the Orientation of the Lamellae. 



For a complete determination of the orientation of the 

 lamellae we must know both the coordinates of the different 

 planes parallel to which the lamellae are oriented and the direc- 

 tions of certain edges of the lamellae. All orientation must be 

 referred to some crystallographic planes or axes of the feld- 

 spar. 



* Quart. Journ. Geol. Soc. xli, 384, 1885. 



•fTseh. Min. Petr. Mitt., xxi, 248, 1902. 



\ Centralbl. Min., 1909, 168. § Centralbl. Min., 1911, 369. 



