ndpenfield.] ADDITIONS TO THE ALUNITE-JAKOSITE GROUP. 
HII 
39 
In the case of the lead compound one atom of lead, and in the others 
two atoms of either potassium or sodium, are combined in complex 
molecules containing fifty other atoms, hence that the complex of 
fifty atoms, to the right of the K 2 , Na 2 and Pb in the foregoing formu- 
las, should control or dominate crystallization by virtue of mass effect, 
and condition an isomorphism between such unlike elements as sodium, 
potassium, and lead is not so surprising as would at first appear. 
Having adopted the double formulas, as given above, there are 
numerous ways of writing developed formulas, of which the following 
are perhaps the simplest and most satisfactory: 
™>Fe-0 
h8> f *-° 
ig>Fe-0 
O 
o 
s 
o 
o 
S— O-Pb— O— s 
\ / 
o /0 
o = s = o 
,°- Fe <OH 
-0-Fe<OH 
0-Fe<8^ 
Hg >Fe _ ? _ Fe< OH 
HO >Fe-0 
o o 
() 
o^ 
S— OK K-O— S 
^0-Fe<§g 
o o 
H0 ^Fe OS 
HO >ie-0-S 
O 
O 

Fe< 
OH 
OH 
It is interesting to note that although K 2 , Na 2 and Pb play so small 
a role in the al unite- jarosite molecules, the substitution of Na 2 for K 2 
is attended by a marked variation in the angles of the crystals, 
greater, in fact, than is generally observed in isomorphous replace- 
ments. That alunite and jarosite containing potash would be nearly 
alike in their angles is expected, since crystals of corundum and 
hematite are surprisingly alike, as shown by the following comparison: 
Corundum, A1 2 3 
Hematite, Fe 2 3 .. 
Axial 
length. 
1. 3630 
1. 3656 
93 56 
94 00 
57 34 
57 37 
