246 Dana and Penfield — Crystalline form of Polianite. 











Calc. 



hh' 



210 ~ 120 



36° ,12', 36° 3', 



35° 56' 



36° 52' 



hh vii 



210^210 



54° 10', 54° 43', 



54° 50' 



53° 8' 



ee' 



101 ^011 



46° approx. 





46° 5' 



zz' 



321^231 



19° 58', 20° 53', 



20° 38' 



20° 51' 



zz vii 



321*321 



62° 11', 60° 46', 





61° 35' 



ZZ Tiii 



321*321 



45° 28', 45° 50', 





45° 18' 



zz iK 



321*231 



50° 10', 50° 34', 





50° 22' 



All the above angles were measured with a Fuess goniometer 

 with two telescopes. In addition, two subordinate planes n and 

 g were determined by approximate measurements as follows : 

 ^=19°, calc. 19° 26 ; ; m=18°-20°, calc. 18° 46'. 



As shown above the measured angles vary somewhat 

 widely, but when carefully discussed it is found that no better 

 agreement is obtained when an assumption is made that the 

 crystals belong to a system of lower symmetry. On the con- 

 trary the best angles lead to the tetragonal system, to which 

 the symmetry in the development and character of the indi- 

 vidual planes emphatically conforms. An explanation of the 

 variation of angle is doubtless to be found in the fact stated 

 that the crystals are all composite, and the individuals of 

 which they have been built up are not absolutely in parallel 

 position. A comparison of the angles given with those of 

 Kochlin shows that he must have had crystals resembling 

 figure 1 in hand, in fact he says that for a time he was inclined 

 to consider his crystals as tetragonal. He was unfortunate in 

 his material, for he adds that he was rarely able to use the 

 telescopes of the goniometer. Kochlin's planes referred to our 

 fundamental form are as follows: 310=A, 110=100,334=6. 

 If, however, the measurements left any doubt as to the sj^stem 

 to which polianite should be referred, this is removed by the 

 relation brought out by the above measurements, namely, that 

 polianite is isomorphous with cassiterite and the allied species 

 rutile and zircon. The relations between them are as follows . 



Cassiterite, Sn0 2 



0-6732 



46° 28' 



58° 19' 



Polianite, Mn0 2 



0-6647 



46° 5' 



57° 56' 



Rutile, Ti0 2 



0-6442 



45° 2' 



56° 52-J' 



ry- f Zr0 2 



Zircon, } Si02 



0-6404 



44° 50' 



56° 40^' 



The interesting group of oxides having the general formula 

 E.O a thus receives an important addition, a result which was 

 not anticipated when our work was begun but which can 

 occasion no surprise. 



The hardness of both specimens of the polianite is 6 to 6'5. 

 The specific gravity of A was found to be 4-£>92, the mean of 

 three determinations, 4-971 on 0-833 gr., 4-965 on 0-813 gr., 



