MINERALOGY. 59 



the state. It forms beds in the gneissoid rocks. It is found sometimes 

 of the rose color characteristic of the pure, unaltered mineral, but usually 

 of various shades of brown, the color being dependent upon the degree 

 of oxidation or decomposition. A bed of it is situated on the top of 

 Stony mountain near Winchester, and another on a hill a mile south-east 

 of Hinsdale. The latter bed is seven feet thick, and quite extensive. 

 Smaller deposits are found at other points in these same neighborhoods. 



This mineral is subject to easy alteration, for the lower oxide of man- 

 ganese is unstable, and has a constant tendency to oxidation. This 

 change results in the production of a brown or black silicate of man- 

 ganese sesquioxide, which is called marceline. On the exterior of the 

 mineral the silica is sometimes removed, leaving a coating of manganese 

 oxide, or pyrolusite, and when water containing carbonic acid acts upon 

 the mineral, a carbonate of manganese is formed which is rhodochrosite. 

 Products resulting from one or all of these methods of decomposition are 

 common at the localities mentioned, as at all other localities of this ore. 

 Such products have been often analyzed, and names have been given to 

 them, but the decomposition is not usually so complete as to produce 

 a perfectly homogeneous product, and hence they are usually mixtures 

 of minerals. 



The following is an analysis, by Dr. Jackson, of this mineral from 

 Winchester : 



Silica, 26.4 



Iron oxide, 4. 



Manganese oxide, 68. 



Loss, 1.6 



100.0 



Pure rhodonite is composed of 45.9 of sihca, and 54.1 of manganese pro- 

 toxide. Comparing this with Jackson's analysis, we see that he analyzed 

 a decomposition product. His analysis is just like many others that have 

 been made upon such products, and proves it to be the variety marceline, 

 in which the manganese exists for the most part in the state of sesqui- 

 oxide. 



Rhodonite, when crystallized, is isomorphous with pyroxene; but our 

 mineral is massive. It fuses easily, and imparts a deep violet color to a 

 borax bead, the color becoming red-brown when the bead is cold. 



