r ^ r' 



c -^ r 



Birefringence. 



90° 50' 



55° 19|' 



positive 



90 45 



55 16 



negative 



85 54 



51 53 



negative 



89 42 



54 32 



negative 



Alunite-Jarosite Gi'oujp of Minerals, 219 



The relations of the minerals of the alunite-jarosite group 

 are as follows : 



Axial length. 



Alunite -.. 1-252 



Jarosite . _ 1*245 



Xatrojarosite 1*104 



Plurabojarosite.- 1*216 



From the foregoing table it is seen that the substitution of 

 sodium for potassium in jarosite has brought about greater 

 variation in the angles of the crystals than the substitution of 

 the bivalent metal lead for potassium. 



The three minerals, natrojarosite, plumbojarosite and the 

 ]N"a-K-alunite from Red Mountain, are very interesting when 

 studied together as microscopic mounts, the crystals being 

 practically alike in size and development, aud illustrating very 

 beautifully on the one hand the isomorphism of aluminum 

 and iron, on the other the isomorphism of potassium, sodium 

 and lead. The three substances must have formed under like 

 conditions, and it is believed that they are solfataric products, 

 formed under the combined action of heat and pressure. 

 Being difficultly soluble, they have formed, like many precip- 

 itates, as fine crystalline powders. 



The three products just mentioned, when heated in closed 

 tubes behave alike ; they suffer no change on gentle heating, 

 but when the temperature is sufficiently high to decompose 

 the chemical molecules, the crystals break up into fine powder 

 or dust, which is carried along by the escaping vapors and 

 deposited for a considerable distance along the sides of- the 

 tubes. In addition to water, SO^ and SO3 are copiously given 

 off during decomposition. In the case of natrojarosite, and 

 the same would doubtless hold true for the JS'a-K-al unite, it is 

 found that after ignition, one-fourth of the sulphate radical has 

 been retained by the alkali metal, and may be extracted by 

 water. In the case of plumbojarosite, however, all of the sul- 

 phate radical is expelled by ignition, doubtless because the 

 ferric-oxide present serves to decompose any lead sulphate 

 which might have a tendency to form. Anglesite, PbSO^, 

 when heated alone in a closed tube suffers no decomposition, 

 but when finely triturated with limonite and heated, acid water 

 is given off. Finely powdered natrojarosite and plumbojaro- 

 site are slowly but completely soluble in boiling hydrochloric 

 acid. Plumbojarosite when fused with sodium carbonate on 

 charcoal yields globules of lead and a coating of lead oxide. 



It has seemed to the writers best to designate the new com- 

 pounds described in this article as natrojarosite and plumbo- 

 jarosite, the names signifying their relation to a well known 



