26 Chemical Examination of American Minerals. 



bisilicate, as is obvious from previous analyses, the protoxide 

 of manganese will be combined with 34*59 grains of silica. 

 The remaining 6 grains of silica will saturate 0*75 grains of 

 protoxide of iron ; there remain 5*75 grains of protoxide of 

 iron for the carbonate ; this will require for saturation 3*23 

 grains of carbonic acid. The mineral then consists of about, 



7 atoms bisilicate of manganese. 

 1 atom carbonate of iron. 



There is a remarkable peculiarity in this specimen of bisili- 

 cate of manganese. A portion of the protoxide of manganese 

 is replaced by an equal quantity of protoxide of iron. 



Note. This mineral has been announced as a carbonate of 

 manganese : but its property of effervescing with acids, is now 

 ascertained to depend on the presence of a small quantity of 

 the carbonate of iron. 



By long exposure to the air its surface becomes of a black 

 color. Sometimes it is covered with a dark crust an inch or 

 more in thickness. This is owing to the protoxide of manga- 

 nese absorbing more oxygen from the atmosphere ; so that a 

 persilicate of that metal is formed. 



3. SILICATE OF MANGANESE. 



This mineral, so far as my reading extends, has not been 

 hitherto noticed, though it constitutes a well marked species. 

 I got it about two years ago from Dr. Torrey, under the 

 name ofrhomhoidal silicate of zinc. It was found at Franklin, 

 in New-Jersey. 



The color of this mineral is light brownish-red. The frac- 

 ture is foliated. Two distinct cleavages are perceptible, con- 

 stituting a prism slightly oblique. But I am not certain as 

 to the exact measurement. My first measurements gave me 

 80", and 100° for the angles of the prism. But the mea- 



