398 0. Andersen — Aventurine Feldspar. 



was found to vary between 50^ and 500//^. The lamellae 

 were shown to possess no appreciable pleochroism. The ap- 

 parent pleochroism observed in lamellae forming large angles 

 with the section was explained as due to the effect of polariza- 

 tion by reflection and refraction at the surface of the lamellae. 

 The appearance of interference spectra in these lamellae was 

 explained as due to the action of the lamellae as polarizers or 

 analyzers for the wedge-shaped parts of the feldspar that lie 

 above or below them in the sections. 



Thermal experiments with one of the varieties showed that 

 the hematite lamellae persist up to about 1235°. At this 

 temperature they disappeared, presumably by melting together 

 with a small part of the surrounding feldspar to thin, invisible 

 glass films. The feldspar remained otherwise unchanged 

 (crystallized). By heating at lower temperatures some of the 

 lamellae (originally opaque ones) reappeared in the same places 

 and with the same outlines as before. By a long heating at 

 temperatures around 1050° (of cleavage pieces not previously 

 heated) the opaque lamellae generally became transparent and 

 the others became a little lighter. 



The origin of the hematite lamellae was explained as due to 

 unmixing of an originally homogeneous feldspar which con- 

 tained iron oxides in solid solution. Thin lamellae of hematite 

 then separated along certain structural planes of the feldspar. 



In the concluding section all the specimens examined are 

 described and the measurements tabulated. 



The Geophysical Lahoratory of the 



Carnegie Institution of Washington, 

 Washington, D. C, July 16, 1915. 



