Zl CALIFORNIA ACADEMY OF SCIENCES. 



0.2 mm. diameter down to the smallest dimensions, often 

 Touglily arranged in rosette form; also of smaller fibrous 

 masses of the same mineral. Under the microscope these 

 latter are mixed with minute grains of uwarowite and 

 <chromite. The mineral in its massive state is grown together 

 •with chromite, shows a fine parallel fibrous structure and 

 ^ pale purplish color. The crystals are hexagonal tables 

 with perfect cleavage parallel to the base, apparently bor- 

 dered by oR, and small faces of a striated R, the sign of 

 %vhich cannot be ascertained; it is too small to give sig- 

 nals in the goniometer, and the very slight thickness of the 

 tables prevents accurate measurements with micrometer 

 and micrometer screw. H.--=2. G. (massive variety)^2.69. 

 Streak white. Lustre of cleavage face somewhat pearl}'. 



Under the microscope the tables, when resting on their 

 base, are transparent and of a pale purplish color. Between 

 crossed nicols the crystals prove to be double-refracting 

 and biaxial. The apparently strictly hexagonal tables are 

 twinned monoclinic crystals; the most regular forms are 

 xisually divided in six sectors having a common apex in 

 the center and their axes of elasticity in different position. 

 ^ee Fig. 1, in Avhich the shading indicates the position of 

 the plane of the optical axes in each sector. Tiie colors of 

 interference are low on the base, not exceeding the greys 

 of the first order; those of the fibrous aggregates are 

 more vivid, more so in fact than is usual with the chlorites. 



In convergent light the plane of the optical axes is found 

 in each sector to be parallel to its base; angle of the opticid 

 axes quite large, probably about 30^. Double refraction 

 positive. The character of the dispersion cannot be well 

 observed on account of the small size of the crystals. 



The acute bisectrix stands nearly normal to the base, 

 •oP, and the extinction of the fibres is consequently quite 

 small. Pleochroismus distinct: a & b purplish, c (vibrat- 

 ing perpendicularly to oP) yellowish red. 



