322 



University of California. 



[Vol. 2. 



This gives a ratio of ferric oxide to water of approximately 

 I :4, corresponding to the formula Fe 2 3 4H2O. 



The mineral resembles limonite in the nature of its impurities 

 and color of its powder, but differs from it radically in the amount 

 of water, low specific gravity, hardness, and its clearly amorphous, 

 glassy nature. The analysis approaches more closely to those of 

 limnite. It is the same kind of deposit and probably resembles in 

 appearance melanosiderite, described by J. P. Cooke,* but there is 

 no question about the mineral being an oxide. 



The mineral has such distinctive characteristics that the name 

 esmeraldaite is proposed, to distinguish it from the other hydrous 

 ferric oxides. 



Portions of the deposits, while still retaining the structure and 

 general characteristics of the black mineral, have by some change 

 lost the bright vitreous luster and become dull and dark brown in 

 color. A partial analysis of this brown variety gave Fe 2 3 50.26% 

 and H 2 22.70%. This shows the relative proportion of ferric 

 oxide to water to be about the same as in the black mineral; but a 

 greater amount of impurities is indicated. 



IV. COOUIMBITE, FROM THE REDINGTON MINE, KNOXVILLE, CAL. 



A green ferric sulphate, which was found in the Redington 

 mercury mine in a wet, mushy condition, became quite compact 

 and crystalline after a natural drying for several months in the air, 

 and the compact material has been recently analyzed by Mr. 

 Schaller. 



The color of the sulphate is light yellowish-green with patches 

 of dark green, and the structure is distinctly fine granular. Under 

 the microscope the mass is seen to consist of minute, doubly- 

 refracting plates and grains which resemble rhombohedral plates in 

 form, but the small size of the individual grains prevented any 

 definite conclusions regarding the crystal system. An attempt to 

 recrystallize the mineral produced only a highly deliquescent salt. 



In its various properties the mineral is similar to coquimbite. 

 The hardness of the more compact portion is about 2-2.5. The 



*J. P. Cooke, Trans. Amer. Acad. Arts and Sciences, 1875, 10, 451. 



