PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 74 



After standing a year in the laboratory the mineral is still clean, 

 fresh, and glassy and exhibits no tendency to alteration or dehydra- 

 tion. Under the microscope the analyzed material was pure and uni- 

 formly isotropic with an index of refraction of 1.460, The specimens 

 exhibit no crystal faces, the free ends of the fibers being in most 

 cases somewhat dissolved or corroded. For comparison several previ- 

 ous analyses of tschermigite from other localities, including Erickson's 

 analysis of the Wj'^oming material are given in the following table : 



Previous analyses of tschermigite 



AhOj 



(NH4)20 



NasO 



KaO 



MgO 



SOg--- 



H2O 



Insoluble 



FejOa, CaO, 01 



11.57 



5.23 



.21 



Trace. 



.13 



35.11 



47.82 



.06 



Trace. 



11.28 

 5.74 



35.33 

 47.65 



11.40 

 5.80 



34.99 



47. e9 



11.39 

 5.02 



35.14 

 .2.47. 59 

 . .08 



r.oi 



100. 13 



100.00 



1. Average analysis of tschermigite from Wyoming. E. T. Erickson, analyst. Journ. Wash. Acad. Sci., 

 Tol. 12, p. 50, 1922. 



2. Composition for formula .Al2(S04)3.(NH4)S04.24H20. 



3. Tschermigite from Dux, Bohemia. J. V. Duchmuller, analyst, Centr. Min. Oeol., 1907, pp. 465-467. 



4. Tschermigite, from Brux, Bohemia. A. Sachs, analyst, Centr. Min. Geol., 1907, pp. 465-467. 



AM MONIO J AROSITB 



A preliminary paper announcing ammoniojarosite as a new mineral 

 has already been published by the writer.'^ Jarosite is a hydrous 

 ferric-iron-potash sulphate which has been known since 1838. It is 

 widespread in occurrence and lias been identified at a large number 

 of localities. The large content of ferric iron gives it a brown color, 

 and, although occasionally lustrous tran.sparent crystals a millimeter 

 or two in diameter are found, the usual form of the mineral is a 

 friable earthy-apj^earing mass of fine grained silky lustered material. 

 It greatly resembles earthy or ocherous limonite but, when examined 

 microscopically, it is found to consist of loose aggregates of micro- 

 scopic transparent crystals which are tabular rhombohedral in form. 

 Its resemblances to limonite have often caused it to be overlooked and 

 many occurrences have doubtless failed of recognition for this reason. 



Sixty-four years after the description of the original potash jarosite 

 Hillebrand and Penfield " described two new members of the group. 

 In the first of these potash is replaced by soda, a very ordinary 

 instance of the substitution of one alkali for another, and the mineral 

 was named natro jarosite. In the other, however, the somewhat sur- 

 prising situation was found wherein the univalent potassium oxide 



^ Earl V. Shanuon. Ammoniojarosite, a new mineral of the jarosite group from Utah. 

 Amer. Mineralogist, vol. 12, No. 12, pp. 424-426. 1927. 

 •W. F. Hillebrand and S. L. Penfleld. Amer. Journ. Sci., vol. 14, p. 211, 1902. 



