40 
CONTRIBUTIONS TO MINERALOGY. 
[bull. 263 
The relations of the minerals of the alunite-jarosite group are as 
follows: 
Alunite 
Jarosite 
Natrojarosite . . 
Plumbojarosite 
Axial 
length. 
r ^ '■' 
C yy r 
o / 
O 1 
1.252 
90 50 
55 19| 
1. 245 
90 45 
55 16 
1.104 
85 54 
51 53 
1.216 
89 42 
54 32 
Birefringence. 
Positive. 
Negative. 
Do. 
Do. 
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 ciystals than the substitution of the bivalent metal lead 
for potassium. 
The three minerals, natrojarosite, plumbojarosite, and the Na-K- 
alunite from Red Mountain, are very interesting when studied togothei 
as microscopic mounts, the crystals being practically alike in size and 
development, and illustrating very beautifully on the one hand the 
isomorphism of aluminum and iron, on the other the isomorphism oj 
potassium, sodium, and lead: The three substances must have formec 
under like conditions, and it is believed that they are solfataric prod 
ucts, formed under the combined action of heat and pressure. Beici 
difficultly soluble, the\^ have formed, like many precipitates, as find 
crystalline powders. 
The three products just mentioned, when heated in closed tubes 
behave alike; the} 7 suffer no change on gentle heating, but when th 
temperature is sufficiently high to decompose the chemical molecules 
the crystals break up into fine powder or dust, which is carried alonji' 
by the escaping vapors and deposited for a considerable distance alon] 
the sides of the tubes. In addition to water, SO a and S0 3 are copi 
ously given off during decomposition. In the case of natrojarosite < ! 
and the same would doubtless hold true for the Na-K-alunite, it i ) 
found that after ignition one-fourth of the sulphate radicle has bee | 
retained by the alkali metal, and may be extracted by water. In th I 
case of plumbojarosite, however, all of the sulphate radicle is expelle lj 
by ignition, doubtless because the ferric-oxide present serves to decon j 
pose any lead sulphate which might have a tendency to form. Angle | 
site, PbS0 4 , when heated alone in a closed tube suffers no decompos 
tion, but when finely triturated with limonite and heated, acid watc 
is given off. Finely powdered natrojarosite and plumbojarosite ai 
slowly but completely soluble in boiling hydrochloric acid. Plumbc 
jarosite when fused with sodium carbonate on charcoal yields globuk-j 
of lead and a coating" of lead oxide. 
