Dana and Penfield — Crystalline form of Polianite. 245 



of a rather open aggregate of manganese dioxide in part devel- 

 oped into distinct prismatic crystals. The mass proved upon 

 examination to be anhydrous Mn0 2 with a hardness of 6 to 6 - 5. 

 This was throughout distinctly crystalline in structure, consist- 

 ing of minute tetragonal pyramidal and prismatic crystals built 

 up together into parallel position. 



The prismatic crystals were in part hollow shells with the 

 termination often gone, but the prism sufficiently preserved to 

 show that the original mineral was without question manganite 

 with its usual prismatic angle of 80°. In others of the crystals 

 the rebuilding process had gone farther with the result of giv- 

 ing a complete tetragonal crystal with the habit shown in figs. 

 3 and 4. 



The planes here present, being referred to the same funda- 

 mental form as variety A, are : 



m (110, 7), fe (210, i-2), s(lll, 1), w(221, 2), e(101, 1-*), 0(201, 2-i). 



In consequence of the fact that the crystals are all composite 

 growths the planes were none of them perfectly smooth and 

 simple. The best faces for measurement were those of the 

 pyramid s, though here there is commonly an oscillation with e 

 and sometimes the crystals are much elongated in this zone and 

 rounded over from s to the adjacent si. 



As the fundamental angle for the species the following was 

 accepted from the variety B : 



s*.s' 111 ^ 11] = 51° 56/ whence c = 0-66467. 



Two of the other pyramidal angles on the same crystal gave 

 58° 2' and 58° 5'. The following angles, measured on crystals 

 from the first specimen, will serve to show the variation men- 

 tioned : 



Am. Jour. Scl— Third Series.— Vol. XXXY, No. 207— March, 1888. 

 15 



