Al unite- Jar osite Group of Minerals. 217 



the crystals being exactly like those of natrojarosite and plum- 

 bojarosite, except that they are a trifle smaller and white, or 

 colorless when seen under the microscope. 



From a chemical standpoint the most interesting feature 

 of the new minerals is the light they throw upon the iso- 

 morphism of potassium, sodium and lead. Ordinarily, even 

 potassium and sodium are not isomorphous, as shown by the 

 fact that their simple salts seldom crystallize in the same form. 

 Although KCl and I^aCl both crystallize in cubes, it is not 

 certain that both salts belong to the same group of the iso- 

 metric system. It has been shown, for example, by etching, 

 that KCl crystallizes like NH^Cl in the plagihedral group of 

 the isometric system, while the etchings produced on halite 

 seem to indicate that it crystallizes in the normal group. 

 Again at Stassfurt, Germany, sylvite and halite both occur 

 crystallized s*de by side upon the same hand specimen, instead 

 of mixing as isomorphous molecules. Even in such complex 

 molecular compounds as the feldspars, the potassium and 

 sodium salts crystallize as orthoclase and albite, rather than as 

 isomorphous mixtures. Lastly potassium has a strong tend- 

 ency to form alums which is not shared by sodium. In con- 

 trast to these differences in chemical nature, we have in the 

 jarosite-alunite group of minerals not only the alkali-metals, 

 potassium and sodium, but, what seems still more remarkable, 

 lead., playing the same role in the compounds, and yielding 

 crystals which are surprisingly alike in all their physical prop- 

 erties. The writers can at present offer no other reason for 

 the isomorphism in the group of minerals under consideration 

 than that the alkalies and lead play so small a role, and the 

 remaining constituents so prominent a part in the complex 

 chemical molecules, that the latter control or dominate the 

 crystallization by virtue of what may be called their mass effect. 



The alunite from Red Mountain, described by Hurlburt, 

 was analyzed in the Shefiield Mineralogical Laboratory under 

 the direction of one of the present writers, and it was found 

 that water was first expelled from the compound at a rather 

 high temperature, thus indicating that the mineral contains 

 hydroxyl and no water of crystallization : accordingly it was 

 shown that the seemingly complex formula of the mineral, 

 expressed by the ratio K\fi, : K,0 : SO3 : H,0 = 3:1:4:6, may 

 be much simplified to K[Ai(0HX]3[S0J,. In the light of the 

 present investigation it now seems best to abandon the above 

 simple formula and adopt one containing double the number 

 of atoms, in order to make clear the isomorphism between K^, 

 Na^ and Pb. The formulas of the minerals of the group 

 would then be expressed as follows : 



