and Properties of the Manganese Ores. 4>5 



is important to attend to this difference in the perfection of 

 cleavage ; the more so, because the cleavage parallel to the 

 short diagonal of P + oo = 99° 40', is at the same time paral- 

 lei to the long diagonal of another prism, (Pr+ao ) 5 = 7^° 36' 

 (the supplement of which is 103° 24'), which occurs very fre- 

 quently in the same mineral, and might be, or has actually 

 been, mistaken for it, in a more superficial examination of the 

 crystalline forms of the species. 



Descriptions of single varieties are particularly desirable, 

 when the characters of the whole species are yet so imper- 

 fectly ascertained as in the present case. The variety to 

 which the preceding description refers, was brought by Dr 

 Turner from Ilefeld in the Hartz, and to him I have been 

 indebted for the crystals described above. The most re- 

 markable peculiarity in the series of crystallization of this 

 species, is its hemi-prismatic character, the faces of those 

 forms which assume it being inclined to each other. Those 

 marked c, if sufficiently enlarged, would give rise to a form 

 resembling a tetrahedron, like Fig. 8, the planes of which are 

 equal and similar scalene triangles. Among the remaining spe- 

 cies, whose forms belong to the prismatic system, only the sul- 

 phates of zinc, of magnesia, and of nickel, are known to pos- 

 sess an analogous formation. This was first placed beyond a 

 doubt by Professor Mitscherlich, who observed the fact, that 

 the faces s and t, Fig. 9, appear only contiguous to the 

 alternating faces of I; although the alternating enlargement 

 of these same faces, represented in Fig. 10, had been previ- 

 ously noticed in the sulphate of magnesia, by mineralogists, 

 so far back as Rome de LTsle and Linnaeus. Large crystals 

 of this salt generally show the hemi-prismatic character much 

 more distinctly than small ones. 



In the description given above, the streak of the crystals is 

 stated to be reddish-brown, contrary to most indications in 

 works on mineralogy. It is very often the case, however, 

 that we meet with crystals, and still more frequently with 

 compound varieties, consisting of the columnar individuals, 

 which actually afford a black streak. The hardness of these 

 varieties is much inferior to the hardness of the crystals that 

 present a brown streak, being generally between 2.5 and 3.0, 



