A PYRRHOTITIC PERIDOTITE 



131 



plagioclase feldspar and olivine or pyrrhotite. Most of these zones 

 vary in width from 0.02"^"^ to i"^"^, and show a more or less radiate 

 structure, amphibole fibers or brushes being directed about at right 

 angles to the borders of the feldspar crystals. The mineral forming 

 the rims is identified as amphibole by the fact that it is in some places 

 in crystallographic continuity with the massive hornblende already 



Fig. 3. — Reaction rims of amphibole bordering feldspar. i^ = Feldspar (andesite- 

 labradorite), P = Pyrrhotite, = Olivine. Note the double character of the rims 

 between feldspar and olivine, their single character between feldspar and pyrrhotite, 

 and their entire absence between olivine and pyrrhotite. (Magnification about thirty 

 diameters, polarized light.) 



described, by its double refraction (about o . 024), and its index of refrac- 

 tion, which is intermediate between those of olivine and serpentine. 



Some of these fringe-like borders show two layers of fibers, usually 

 in close contact with each other, but in some cases separated by a 

 narrow band of a non-fibrous hornblende, which usually shows a 

 faint tint of brown. In the double borders developed between 



