218 Hillehrand and Penfield — Additions to the 



Alunite, K,[Al(OH)J,[SOJ, or K,A1,[0H],,[S0J, 



Natroalanite, Ka,[Al(OH)J^[SOJ, or Na,Al^[OH]^ JSO 1^ 



Jarosite, K,[Fe(OH) J,[SOJ, or K,Fe,[OH],,[SOJ, 



Natrojarosite, Na,[Fe(OH)J,[SOJ, or Na,Fe,[OHJ,,[SOJ, 



Plumbojarosite, Pb[Fe(0H)j3[S0J, or PbFe,[OH],,[SOJ, 



In the case of the lead compound one atom of lead, and in 

 the others two atoms of either potassium or sodium, are com- 

 bined in complex molecules containing fifty other atoms ; 

 hence that the complex of fifty atoms, to the right of the K^, 

 [Ra^ and Pb in the foregoing formulas, should control or domi- 

 nate crystallization by virtue of -mass effect^ and condition an 

 isomorphism between such unlike elements as sodium, potas- 

 sium 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 formiUas, of which 

 the following are perhaps the simplest and most satisfactory : 



0= s = o 

 HO>Fe-0^ // \o^ 0-^<0H 

 hC>''«-07 S -O -Pb- O- S - 0-Fe<OH 

 l^>eV \ / ^0-Fe<gH 



wn ..TT 



^^ S— 0-K KO-S ^^ 



o/" \ / \o 



*^ o o o ^ 



HO^ ^ ^l^r. ^ /OH 



gg>Fe— O— S— O— Fe 



OH 



It is interesting to note that although K^^ ISTaa and Pb play so 

 small a role in the alunite-jarosite molecules, the substitution 

 of 1^32 for K2 is attended by quite a marked variation in the 

 angles of the crystals, greater in fact than is generally observed 

 in isomorphous replacements. That alunite and jarosite con- 

 taining 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 : 



Axial length. r ^r' c i^r 



Corundum, Al^Og 1'3630 93° 56' 57° 34' 



Hematite, Fe.Og 1-3656 94 00 57 37 



