0. Andersen — Aventurine Feldspar. 



373 



In thick sections or cleavage pieces the lamellae of large 

 angles p show in polarized light (with only one nicol) dis- 

 tinct interference spectra with bands parallel to the section 

 lines. These lamellae show the same apparent pleochroism as 

 those in thin sections. The thicker the part of the feldspar 

 penetrated by the lamellae the larger is the number of bands 

 in the spectra. In lamellae penetrating only a thin part of the 

 section we see, as in ordinary thin sections, no extended spectra 

 but sometimes single, colored stripes. By rotating the micro- 

 scope stage all spectra disappear in two positions at right 

 angles with each other coinciding with the extinction direc- 

 tions of the feldspar between crossed nicols. This phenomenon 

 is explained by the fact that the lamellae (of large angles p) are 

 included in the doubly refracting feldspar. In fig. 13 E F 



Fig. 13. 



f 



C 



D 



indicates a lamella included in a feldspar section A B C D at 

 an angle p large enough to make the rays transmitted through 

 it become noticeably polarized. If we use the lower nicol of 

 the microscope the lamella will form a somewhat imperfect 

 analyzer for the wedge-shaped part of E Ff e of the feldspar 

 that lies below it. If we use the upper nicol the lamella will 

 be polarizer for the wedge-shaped \>&vtEFf e' of the feldspar 

 that lies above it. In either case we shall obtain an interfer- 

 ence spectrum corresponding to the part of the feldspar wedge 

 that lies between the lamella and the surface of the section, 

 (the lower surface in one case, the upper in the other). It is 

 obvious that the bands of the spectra must always be parallel 

 to the section lines of the lamellae. In such a stauroscopic 

 system, where one of the nicols is replaced by lamellae included 

 in the section to be observed, we can not change the angles 

 between the "nicols" and at the same time the relative posi- 

 tion between the vibration directions of the " nicols " and those 

 of the section at will. If the "nicols" are crossed the feldspar 



