0. Andersen — Aventurine Feldspar. 369 



position to establish the mutual orientation between the crystal 

 axes of the feldspar and those of the lamellae. 



In view of these facts, it was of minor interest to undertake 

 extensive measurements of the orientation of the edges. 

 Measurements were only made, on sections after (001) and 

 (010) of the angles between the a-axis (cleavage lines) of the 

 feldspar and the projections of the edges of the lamellae on 

 the respective cleavage face. These measurements can be most 

 conveniently included in the special descriptions of the speci- 

 mens. 



The Properties op the Reflecting Lamella. 



The lamellae were too thin to be separated mechanically from 

 the feldspar. They were, therefore, chiefly studied under the 

 microscope in cleavage pieces or sections of the feldspar. 



The different aventurine feldspars showed considerable 

 variations as to shape and size of the lamellae, but no distinc- 

 tion between different varieties could be made on the basis of 

 other qualities. There is, therefore, no reason to consider that 

 the lamellae consist of more than one mineral species. In the 

 following descriptions we refer to the general qualities of 

 lamellae from all the specimens examined. 



Size; crystal outlines. 



The smallest lamellae were hardly visible under the micro- 

 scope ; the largest could be easily seen with unaided eye. 

 Between these there were all transitions. The lamellae of the 

 microcline were generally smaller than those of the plagioclase, 

 the former seldom measuring more than - 2 mtn in diameter, the 

 latter often measuring as much as 3 mm . 



As observed by Scheerer* and others who studied aven- 

 turine feldspars under the microscope, the lamellae sometimes 

 form nearly regular hexagons but more often they show 

 distorted, six-sided or rhomb-shaped outlines. Unsymmetrical 

 eight- or ten-sided outlines are also often seen, and narrow 

 rectilinear strips several times as long as wide are very com- 

 mon. Lamellae with rounded or irregularly curved outlines 

 are frequently observed. When parallel to one of the cleavage 

 faces the lamellae often showed more regular hexagonal or 

 rhombic outlines than in other positions (see Plates I— III). 



Interference colors / thickness. 



The intense colors displayed by most of the lamellae in 

 reflected light were explained by Sheererf as colors of thin 

 films. The correctness of this explanation could be readily 

 proved in the course of the present microscopic study. The 



*Loc. cit., p. 156. fLoc. cifc., p. 157. 



