THE MINEEALS OF IDAHO 



465 



JAROSITE (801) 



Basic sulphate of potash and ferric iron, Rhornbohedral . 



K 2 0.3Fe 2 3 .4S03.6H 2 0. 



Jarosite is a sulphate which has been found to occur abundantly in 

 certain Utah and Nevada ore bodies in the oxidized ore, associated 

 with other members of the jarosite group in which the potash is 

 replaced b}^ soda, lead, or silver. The mineral was long overlooked 



because it closely resembles limonite and 

 it may have been generally overlooked in 

 Idaho also. It has been identified from 

 two localities in this State. 



CUSTER COUNTY 



Unusually well-crystallized jarosite oc- 

 curs in a specimen labeled "Quartzite 

 from just above limestone, Cherry Creek, 

 Mackay district," and the accompanying- 

 notes read "went up Cherry Creek until 

 I came to iron-stained quartzite outcrop, 

 then went to top of ridge and found 

 limestone-quartzite contact." It is not 

 clear whether the specimen came from the 

 creek or the top of the ridge, but pre- 

 sumably it is from the creek and can be 

 found again by the same procedure. To 

 judge from this single chance specimen, a 

 little work would result in securing ex- 

 ceptionally fine specimens of the mineral. 

 The rock is cherty and probably is from a 

 brecciated and silicified zone in limestone. 

 The jarosite forms thick tabular crystals up to 2 mm. in diameter, 

 of a golden brown color. They are simple in development as illus- 

 trated in Figure 161. The form -% (TT22) is apparently new for 

 the species. The crystals are not well adapted for measurement and 

 give poor reflections. The measurements obtained are as follows: 



Measurements of jarosite from Custer County 



Fig. 161.— Jarosite. Mackay dis- 

 trict, Custer County 



