Dana and Penfield — Crystalline form of Polianite. 247 



5 - 040 on 6'092 gr. The first two determinations were made 

 on a chemical balance the last in a pycnometer. These results, 

 which are somewhat higher than those of Breithaupt, were 

 obtained with great care, the material being first boiled in 

 water for some time to drive off the air. The crystals show 

 perfect cleavage parallel to m. 



The chemical examination (Penfield) showed that the material 

 was pure Mn0 2 , free from all but a trace of water. The ana- 

 lytical results are given below together with the analysis of 

 Plattner of the original polianite : 



A. Ratio. B. Plattner. Eatio. 



MnO 80-81 1-138 81-17 1-143 



18-16 1-135 [18-21] 1-132 



Fe 2 3 '--- -16 -17 



Si0 2 -36 



Insol __ -16 trace -13 



H 2 0. -28 trace -32 



9993 100-00 



Loss by ignition ... 12 -44 12*42 12-43 



The ratio of MnO : O is almost exactly 1 : 1 and the mineral 

 is therefore a very pure Mn0 2 . The material which was 

 analyzed was almost wholly soluble in HC1, leaving a very 

 slight residue 0*16 per cent, the remaining 0*36 per cent Si0 2 

 separated from the solution by evaporation to dryness. 



The method of analysis was as follows: a weighed quantity 

 of material was ignited over the blast lamp till a constant 

 weight was obtained, the MnO„ being converted into Mri 3 4 . 

 After dissolving the oxide in HC1 the Fe 2 3 and Si0 2 were 

 determined and the solution tested for Ba, Ca and Mg. From 

 the weight of the Mn 3 4 after deducting Fe 2 3 and Si0 2 the 

 MnO was calculated. The excess of O over MnO was deter- 

 mined by converting oxalic acid into C0 2 and collecting and 

 weighing the same in potash-bulbs. The H 2 was determined 

 by igniting the mineral in a hard glass tube and collecting the 

 water in a chloride of calcium tube. The chemical identity of 

 B was proved by the fact that it gave only traces of water in a 

 closed tube and lost 1242 per cent by strong ignition. The 

 ignited oxide was soluble in HC1, and gave traces of SiO s after 

 evaporation to dryness. 



We purpose in a later article to present some observations 

 upon pyrolusite and the related minerals braunite and haus- 

 mannite, especially with reference to the relation of pyrolusite 

 to polianite. 



