AET. 18 MINERALS OF ITALIAN MOUNTAIIT — CROSS AND SHANNON 21 



The better crystals of vesuvianite are all about alike optically. 

 Under the microscope the mineral exhibits low birefringence and the 

 larger crystals are homogeneous, have uniform extinction and show 

 faint zoning in birefringence in basal sections. Different zones vary 

 very slightly in refractive indices and from uniaxial to biaxial with 

 2V very small. The average refractive indices are 



£=1.713, ^=1.715. 



The thicker grains show pleochroism with OT=pale yellow-green, c= 

 colorless. No cleavages were noted. 



The crystals vary in size from a few which are minute to an ob- 

 served maximum of 2.5 cm. thick by 5 cm. long. The most frequent 

 and best developed ones are aroimd 1 cm. in size. 



Figs. 6-7. — Crystals of Vesuvianite 



Some 60 specimens of this yellow-green vesuvianite, all from 

 Italian Mountain, proper, are included in the collection. The matrix 

 is in all cases a dense hornfels composed sometimes almost entirely 

 of massive pale brown garnet, more frequently of a mixture of garnet 

 and diopside, usually with a little interstitial calcite and a fibrous 

 zeolite referred to scolecite. The vesuvianite does not appear as a 

 constituent of the massive matrix but appears always to be a late 

 introduction into the cavities. This mineral rests upon and is clearly 

 later than crusts of pale buff grossularite garnet. Later minerals 

 associated with and resting upon the vesuvianite are epidote, chaba- 

 zite, and stilbite. 



48180—27 4 



