212 Rillebrand and Penjield — Additions to the 



The crystals belong to the rhombohedral division of the 

 hexagonal system, and the angle c^7\ 51° 53^, which is prob- 

 ably very nearly correct, has been assumed as fundamental, 

 and from it the following axial ratio has been calculated : 



G = 1-104. 



That the axial ratio as given is very near the truth is shown 

 by the fact that the measurements of c^s and r^r do not 

 vary many minutes from the calculated values ; while on a 

 number of other crystals, measurement of the angle G/\r, 

 though varying considerably, was found to be not far from 52°. 

 The angles of c^r and 7'^r' of the ordinary potassium jarosite 

 are 55° 16^ and 90° 45^, respectively. 



Under the microscope the crystals exhibit normal optical 

 properties. Using a high power lens and convergent light, the 

 thicker crystals show the dark cross and the beginnings of the 

 first ring of the interference figure. The birefringence is 

 negative. The color of single crystals, when seen under the 

 microscope in transmitted light, is golden-yellow\ Many of 

 the crystals show numerous brown inclusions. The color 

 shown by a mass of the crystals is yellowish-brown, and the 

 material glistens, owing to reflections from the basal planes of 

 the minute crystals. 



The material used for the chemical analysis was the purest 

 that could be obtained, although crystals containing the brown- 

 ish inclusions just mentioned could not be avoided, and there 

 were occasional brown ferruginous particles mixed with the 

 crystals. The specific gravity of the material was found to be 

 3*18 at 30*5° C. The results of the analysis by Hillebrand are 

 as follows : 



Ratio. 



Feft^ 50-98 0-319 3-29 



Na^O* 6-03 0-094) 



K,0 0-35 0-004 f ^ ^^^ 



SO3 30-96 0-387 4*00 



HO below 105° 0-12 



H^O above 105° 11-03 0-613 6-33 



As,0, 0-20 



SiO,_.. 0-23 



CaO 0-04 



99-94 



The ratio of Fe,03 : Na^ : SO3 : H,0 is evidently 3:1:4:6 

 as in ordinary jarosite, where the alkali is potash instead of 



* Of the soda 22 per cent is not extracted by hot water after full ignition 

 of the mineral and hence may belong to a feldspar or some other foreign 

 mineral. Only 5 '81 per cent is assumed to belong to the jarosite and used 

 in deriving the molecular value. 



