160 



Ford and Bradley — Margarosanite, 



was observed which was doe, apparently, to the presence of a 

 crystal face, as no interior cleavage cracks parallel to it were 

 observed. The trace of this face upon the surfaces of the 

 plates made angles of about 120° and 60° with one of the 

 cleavage directions and 42° and 13S° with the other. This 

 crystal form is not perpendicular to the chief cleavage but 

 cuts across the edge of the cleavage plate at some 'oblique angle. 

 These various relations are shown diagrammatically in the figure. 



The two extinction directions in the sections make angles of 

 about 44° and 46 c with one of the cleavage directions and of 

 34° and 56° with the other. The faster of the two rajs nearly 

 bisects the smaller angle (78°) of the rhomb, formed by the 

 two cleavages which are nearly perpendicular to the surface of 

 the sections. In convergent light the sections show a biaxial 

 figure with one optical axis revolving just outside the field of 

 the microscope. The axis lies along the vibration direction a, 

 so that the cleavage plates are at least nearly perpendicular to 

 the optical axial plane. These optical facts are also summa- 

 rized in the figure. The indices of refraction of the two rays 

 vibrating in the section were determined by immersion in high 

 refracting oils and low fusing solids with the following results : 

 1-730 ±'002 and 1'795 ± -005. Of these the value 1-795 

 must be close to that of the intermediate index of refraction, 

 /3. From a consideration of the above facts, it is probable that 

 the mineral belongs in the triclinic crystal system. 



Upon the specimens, on which the margarosanite was found, 

 the following species were also observed : light and dark-brown 

 garnet (almandite), hancockite, roeblingite, nasonite, franklin- 

 ite, willemite, yellow axinite, datolite and a biotite-like mica 



