28 



Siderite 73° О' 



Smithsonite 72° 20'. 



The earthy alteration products of these crystals give with 

 soda and saltpetre on a platinum plate a strong reaction for 

 manganese. 



The freshest material that could be obtained was subjected 

 to a partial analysis in order to ascertain the quantitative 

 proportion betw'cen the iron and the manganese entering into 

 the mineral. The material consisted of tolerably firm cleavage 

 fragments, which, however, were quite black and soiled the 

 fingers. The specific gravity was found to be = 3,666. The 

 mineral was dissolved in hydrochloric acid with slight evolu- 

 tion of carbonic acid and a strong smell of chlorine. The 

 values obtained are : 



MnO 



57,06 



FeO 



15,76 



CaO 



2,68 



1 





CO, . 



(24,50) 



H.,0 





100,00 



In the fresh state these crystals have therefore probably 

 consisted of carbonate of manganese with a large percentage 

 of iron carbonate and some calcium carbonate. The process 

 of alteration has been as follows: the mineral has given off 

 carbon acid, the liberated manganous or ferrous oxide at the 

 same time taking up oxygen and passing into higher degrees 

 of oxidation, and by the absorption of water, hydrates have 

 resulted. In most cases the alteration has proceeded so far 

 that no trace of carbonate remains. The alteration products, 

 chiefly «black manganese», probably have no constant com- 

 position; I have not, therefore, found it worth while to try 

 to determine the degree of oxidation of the metals. The black 



