A. F. Rogers — Manganese Minerals. 445 



Interfacial angles of hausmannite. 



No. of meas. Meas. Record. 



(001) A (HI) 4 58° 321/2' 58° 32' 



(001) A (113) 1 28 25 28 34% 



(111) A (111) 4 74 5 74 10 



(001) A (221) 1 71 6 72 59 



In thin sections most of the hausmannite is opaque, but 

 on the edges of the sections some of it is translucent and 

 dark red. A few of the deep red crystals are dark 

 between crossed nicols but most of them are doubly- 

 refracting with definite extinction four times in a revolu- 

 tion. Polysynthetic twinning was noted in some sections. 



Crushed fragments of the hausmannite examined with 

 the polarizing microscope under ordinary conditions 

 appear opaque, but in direct sunlight between crossed 

 nicols many of them are translucent dark red. (The 

 crossing of the nicols shuts off the brilliant reflection of 

 the sun and so the fragments are visible.) They are 

 doubly-refracting but the color of the mineral masks the 

 interference colors. This method may be of service in 

 determining some of the submetallic minerals which are 

 opaque in ordinary light, provided of course they are 

 doubly refracting. 



The hausmannite is readily and completely soluble in 

 HC1 with the evolution of chlorine. 



Pyrochroite. — The pyrochroite occurs in euhedral 

 crystals (up to 5 mm. in size) in cavities and in cleavable 

 plates in masses. The euhedral crystals are thin lentic- 

 ular without well-defined faces. A few are hexagonal 

 in outline with a low vicinal rhombohedron. In most 

 specimens the pyrochroite is a bronze-red but soon 

 changes to black on exposure. Freshly broken speci- 

 mens often show the deep red reflections that give the 

 name to the mineral. In a few cases it is colorless or 

 pale green. It has a perfect cleavage in one direction. 



The index of refraction of cleavage flakes was found 

 to be ny= 1-733 ± -003 (sodium light) which is a little 

 higher than the recorded value (1-723). One of the 

 cleavage flakes gave a negative uniaxial interference 

 figure in convergent light. 



In thin sections the pyrochroite is colorless to salmon- 

 colored. The areas without cleavage are non-pleochroic 

 and are either dark between crossed nicols or have very 



