368 0. Andersen — Aventurine Feldspar. 



of the plagioclases), but could only be referred to the appar- 

 ently monoclinic elements of the entire twinned cleavage 

 pieces. Moreover, the difference in angles between orthoclase 

 and microcline is insignificant. 



The faces representing the planes of orientation of the re- 

 flecting lamellae are plotted in stereographic projections in 

 figs. 8-11. Figs. 8 and 9 show the faces observed in rnicro- 

 clines and figs. 10 and 11 those observed in plagioclases. In 

 figs. 8 and 10 the plane of projection is (001), in figs. 9 and 11 

 it is (010). The most important planes of orientation are 

 marked with large dots ; the less important with smaller dots. 



The most numerous lamellae are those which produce the 

 aventurization on the cleavage faces. In the plagioclases the 

 aventurization on (001) is chiefly caused by lamellae after 

 (112) and to a less extent by lamellae after (11*2). In the 

 microclines these two forms are equal. The aventurization 

 on (010) is always caused by lamellae after (150)r and (150), 

 both of about equal development, in plagioclases as well as in 

 microclines. Sometimes we observe numerous lamellae parallel 

 to one or both of the cleavage faces (001) and (010), but as a 

 rule these faces contain very few lamellae and often none. 

 The same holds true for the faces (110) and (110). Parallel 

 to (021) there were seen a few lamellae in two of the plagio- 

 clase varieties examined. Lamellae parallel to (113) were only 

 observed once, also on a plagioclase. 



Orientation of the Edges of the Lamellce. 



In cases where the lamellae showed definite crystal outlines 

 it was plainly seen that there were certain directions along 

 which the edges were more frequently oriented than along 

 others. This orientation was, however, not so regular as the 

 orientation along the planes. Simple crystallographic relations 

 of the directions of the edges were only found exceptionally. 

 The angles varied considerably even on the same specimen, 

 and on different specimens the orientation was often quite dif- 

 ferent. 



It should be emphasized that we do not know the crystallo- 

 graphic relation between any of the edges and the crystal axes 

 of the lamellae, even if we take it for granted that the lamellae 

 are hematite crystals. In the six-sided lamellae, for instance, 

 we have no means of deciding whether the edges are formed by 

 faces of the first or the second order ; and in the distorted lamel- 

 lae with eight- or ten-sided outlines the identification of the 

 edges is still more uncertain. Moreover, many of the lamellae 

 have no regular outlines. We are, therefore, in general not in a 



