Dana and Penfield — Crystalline form of Polianite. 248 



Aet. XXI. — On the crystalline form of Polianite • by 

 Edwaed S. Dana and Samuel L. Penfield. 



The true- crystalline form of manganese dioxide, Mn0 2 , and 

 the relation to each other of the two minerals having this com- 

 position — the common soft variety and the rarer hard form — 

 have long been uncertain points in Mineralogy. The name 

 pyrolusite was given by Haidinger* in 1827 to the mineral 

 which had been earlier called Grey Manganese Ore (Grau 

 Braunstein in part of Werner, 1789, and Hausmann, 1813). 

 Haidinger also gave a partial description of crystals, with a 

 figure which is reproduced in Dana's System (1868, fig. 171, 

 p. 165). The only angle given is that of the jjrism 86° 20', to 

 which Hausmann (1847) added that of a brachydome d = 40° 

 (over the base). 



In 1844 Breithauptf gave the name Polianite to the anhy- 

 drous Mn0 2 from Platten, Bohemia, which, as he showed, had 

 a hardness almost equal to that of quartz. He insisted, further, 

 that the common soft Mn0 2 ( Weichmangan, Germ.) was not 

 an independent species but only an alteration product from 

 other minerals, chiefly manganite, but also in part from poli- 

 anite. ' Breithaupt's description of the form of polianite is very 

 imperfect ; he gives the angle of the prism as 87° 8', and men- 

 tions the occurrence of the three pinacoids, a brachydome in- 

 clined to the brachypinacoid 62°, and two macrodomes. 



The subject has been repeatedly discussed since this time by 

 different authors but very little light has been thrown upon it, 

 although the general correctness of Breithaupt's opinion has 

 been usually accepted. The most recent contribution is that of 

 Kochhn^: who, while concluding that Breithaupt was right in 

 his general position, failed to obtain material suitable for an 

 accurate determination of the form. He was able to show a 

 certain relation between the crystals examined by himself and 

 the axial ratio deduced from Breithaupt's angles, but his meas- 

 urements varied widely and only an appeal to a twinning 

 hypothesis sufficed to bring them into partial agreement. 



The paper of Kochlin has led us to carry on to completion 

 some work on these minerals undertaken a year or more ago. 

 Our results serve to establish the independent position of polia- 

 nite beyond all question, and to show that in form it is tetragonal 

 and isomorphous with cassiterite, and the allied species of the 

 B0 2 group. 



* Trans. Eoy. Soc. Edinburgh, xi, IIP, 1827; Pogg. Ann., xiv. 204, 1828. 

 f Lichtes Graumanganerz, Charakt. Mineralsystems. 103, 241, 1823; Pogg., lxi, 

 191, 1844. 

 % Min. petr. Mitth., ix, 22, 1887. 



