0. Andersen — Aventurine Feldspar. 397 



oriented after (112) or (112) and that on (010) was due to 

 lamellae after (150) and (150), that is, the same as in the other 

 specimens examined. The measurements were made with the 

 microscope. The localities are given below, together with 

 brief characteristics of the different specimens. 



Morefjer, Arendal, Norway. \ — Microcline, microperthite of 

 normal structure. Hematite lamellae few, transparent. 



Hosaas, Iveland, Norway.^ — Microcline perthite partly 

 with visible albite lamellae, partly a microperthite. Strongest 

 .aventurization on (010). Hematite lamellae sometimes very 

 regular, six-sided or rhomb-shaped, transparent. Numerous 

 lamella? of mica after faces of (110). 



Hiltveit, Tveland, Norway* — Microcline microperthite with 

 very fine microcline structure and thin rod-shaped albite inter- 

 growths. Strongest aventurization on (010). Hematite 

 lamellae sometimes elongated and sometimes rather regular, 

 six-sided ; transparent. 



Renfrew, Canada.f — A microcline perthite of very coarse 

 structure. Few opaque hematite lamellae. 



SUMMAEY. 



A number of varieties of aventurine feldspars were examined. 

 Orientation angles of the reflecting lamellae were measured, 

 chiefly with the goniometer, and the properties of the lamellae 

 were determined under the microscope. Brief discussions of 

 the optical problems are included in the record of these exami- 

 nations. 



The reflecting lamellae are always oriented after simple 

 crystal forms of which (112), (112), (150) and (150) occur as 

 planes of orientation in all varieties, the first two causing aven- 

 turization on (001), the last two on (010). The forms (001), 

 {010), (110) and (110) also frequently contain reflecting lamellae. 

 Exceptionally (021) and (113) are planes of orientation. The 

 orientation of the edges of the lamellae is evidently regular but 

 simple crystallographic relations could not, in general, be 

 found. 



The reflecting lamellae were determined as hematite. They 

 vary widely from one variety to another as to shape and size, 

 showing hexagonal, eight- or ten-sided, rhomb-shaped, strip- 

 shaped or irregular outlines. The largest measured 3*5 mm in 

 one direction, the smallest were of submicroscopic size. The 

 absorption colors are those characteristic of hematite. It was 

 shown that the colors in reflected light are interference colors 

 of thin films. By means of these colors the thickness of the 

 transparent lamellae could be approximately determined. It 



* Specimens from the Mineralogical Museum of Kristiania University. 

 f Specimens from U. S. National Museum ; No. 83218. 



