38 CONTRIBUTIONS TO MINERALOGY. [bull. 262. 
site, except that they are a trifle smaller and white, or colorless, wher 
seen under the microscope. 
From a chemical standpoint the most interesting feature of the ne^ 
minerals is the light they throw upon the isomorphism of potassium, 
sodium, and lead. Ordinarily, even potassium and sodium are nol 
isomorphous, as shown by the fact that their simple salts seldon 
crystallize in the same form. Although KC1 and NaCl both crystal 
lize in cubes, it is not certain that both salts belong to the same gvo\i\ 
of the isometric system. It has been shown, for example, by etching 
that KC1 crystallizes like NH 4 C1 in the plagihedral group of the iso< 
metric system, while the etchings produced on halite seem to indicate 
that it crystallizes in the normal group. Again, at Stassfurt, Gen 
many, sylvite and halite both occur crystallized side b} T side upon th< 
same hand specimen, instead of mixing as isomorphous molecules 
Even in such complex molecular compounds as the feldspars, th< 
potassium and sodium salts crystallize as orthoclase and albite, rathe 
than as isomorphous mixtures. Lastly, potassium has a strong tend 
enc}^ to form alums, which is not shared b}^ sodium. In contrast U 
these differences in chemical nature, we have in the jarosite-alunit 
group of minerals not only the alkali-metals, potassium and sodium! 
but, what seems still more remarkable, lead, playing the same role i 
the compounds, and yielding crystals which are surprisingly alike i 
all their physical properties. The writers can at present offer nu 
other reason for the isomorphism in the group of minerals under cor 
sideration than that the alkalies and lead play so small a role, and th I 
remaining constituents so prominent a part in the complex enemies I! 
molecules, that the latter control or dominate the ^crystallization bl 
virtue of what may be called their mass effect. 
The alunite from Red Mountain, described by Hurlburt, was ami 
lyzed in the Sheffield mineralogical laboratory under the direction c (i 
one of the present writers, and it was found that water was fir,' I 
expelled from the compound at a rather high temperature, thus indtfj 
eating that the mineral contains hydroxyl and no water of crystallizi 
tion; accordingly it was shown that the seemingly complex formu 
of the mineral, expressed by the ration A1 2 3 : K 2 : S0 3 : H 2 = 3:1 
4:6, maybe much simplified to K[Al(OH) 2 ] 3 [SOJ 2 . In the light < 
the present investigation it now seems best to abandon the abo^ i 
simple formula and adopt one containing double the number of atom 
in order to make clear the isomorphism between K 2 , Na 2 , and PI J 
The formulas of the minerals of the group would then be express* 
as follows: 
Alunite K 2 [A1(0H) 2 ] 6 [S0 4 ] 4 or K 2 A1 6 [0H] 12 [S0 4 ] 4 
Natroalunite Na 2 [Al(OH) 2 ] 6 [S0 4 ] 4 or Na 2 Al 6 [OH] 12 [S0 4 ] 
Jarosite K 2 [Fe(OH) 2 ] 6 [SOJ 4 or K 2 Fe 6 [OH] 12 [SOJ 4 
Natrojarosite Na 2 [Fe(OH) 2 ] c [S0 4 ] 4 or Na 2 Fe 6 [OH] 12 [S0 4 ] 
Plumbojarosite Pb[Fe(OH) 2 ] f) [S0 4 l 4 or Pl)Fe,[OH] 12 [S0 4 ], 
