Hydrous Silicate of Manganese and Calcium. 505 





Kentrolite 



Melanotekite 



Orientite 



Form 



orthorhombic 



orthorhombic 



orthorhombic 



Habit 



short prismatic 



short prismatic 



short prismatic 



a 



.6334 



.6338 



.6720 



c 



.8830 



.9127 



.7916 



Orientite differs from this group chemically by havmg a 

 higher silica ratio as well as in its content of water. The 

 degree of chemical similarity to the members of the ken- 

 trolite group may be shown by comparing the formulas 

 thus: 



4Pb0.2Mn,03.4SiOo Kentrolite 



4Pb0.2Fe20..4SiOo. Melanotekite 



4Ca0.2Mno0..5SiO,.4HoO. Orientite 



Setting aside the excess of silica and writing all the water 

 as basic these minerals may be compared as follows : 



Ca2(Mn(0H)2).Si.0,.y2Si02 Orientite. 



Pb. (MnO) oSi.O,. Kentrolite. 



The excess of silica precludes the inclusion of the mineral 

 in this group, however, there being no sufficient evidence 

 for regarding such an amount (5%%) as other than 

 essential. The formulas of the minerals of the kentrolite 

 group have, however, been derived from analyses made 

 upon small amounts of material of doubtful purity and 

 their accuracy has been questioned. It is possible that 

 new analyses of these minerals might show them to be 

 more closely related to orientite. 



The amorphous material. — The amorphous material 

 is all very impure from the presence of various included 

 substances, chiefly black opaque grains of manganese 

 oxide. It was so obviously impossible to learn much from 

 analyses made upon this impure material that it was not 

 examined in any great detail. The mineral varies from 

 light to dark brown in the hand specimen, the variation 

 being caused by varying amounts of included impurities. 

 The purest grains are clear dark broAvn in color, having 

 much the color of the crystalline-granular orientite. The 

 luster is resinous and the mineral greatly resembles 

 common brown opal. The powder and streak are pale 

 brown. The material is very brittle and has a conchoidal 

 fracture. The specific gravity is about 2.5 and the hard- 



