^T. 18 MINERALS OP ITALIAIT MOUNTAIN" — CROSS AND SHANNON 25 



W^ and varies from positive with dispersion r < v to negative with 

 r > V. The ^ index of refraction similarly varies from about 1.530 

 to 1.540. The crystals are zoned in composition and show poly- 

 synthetic twin lamellae. The material is evidently feldspar vary- 

 ing from nearly pure albite to oligoclase. Many of the crystals are 

 partly covered with drusy crusts of minute crystals of heulandite. 

 The feldspar is j^ounger than the garnet and older than the 

 lieulandite. 



Small and rather perfectly formed lutrous white albite crystals, 

 not exceeding 2 mm. in diameter, occur sparsely scattered over the 

 interior of a cavity in a single greatly altered specimen of garnet 

 diopside rock (No. 84583). 



The largest group of albite specimens is numbered 84583 and con- 

 sists of pale pink crusts of drusy crystals averaging about 2 mm. 

 Taroad by % mm. in thickness. The druses occur along open cracks 

 in the centers of feldspar-filled seams in fine-grained altered diorite 

 and the specimens show a layer of granular massive pink feldspar 

 lip to 1 em. thick between the druse and the diorite. Under the 

 microscope this albite is somewhat muddy from incipient kaoliniza- 

 tion. It is biaxial positive (-f ) with 2V large, dispersion r < v 

 weak, refractive indices a= 1.525, ^=1.530. 



The crystals have a peculiar flat habit as is illustrated in the 

 drawing, Figure 10, in which the notation follows the table of 

 Brezina's elements in Goldschmidt's Winkeltabellen. The measure- 

 ments are as follows : 



Crystal measurements of albite, Figure 10 



Associated with or overlying the pink crusts of albite are chaba- 

 zite in simple rhombohedrons or penetration twins up to II/2 mm. 

 in diameter, minute rare quartz crystals of normal habit, and tiny 

 honey-yellow crystals of titanite. 



White albite ocurs in opaque crystals up to 1 mm. in size coating 

 a banded rock which is probably a metamorphosed shale (84584). 



