Clarke — Constitution of Ptilolite and Mordenite. 101 



Si0 2 65-67 



A1„0 3 13-48 



Fe"„0 3 1-51 



MnO tr. 



BaO -32 



CaO 2-41 



MgO - -31 



K 2 2-42 



Na 2 . 1-52 



S0 3 -28 



PA - tr. 



HO ._ 12-27 



100-19 



Appendix. — Note on the Constitution of Ptilolite and 

 Mordenite ; by F. W. Claeke. 



The data given in the foregoing paper by Cross and Eakins, 

 especially as regards the dehydration of ptilolite, lead to quite 

 simple formulae for the three minerals under discussion. The 

 moment that we recognize the fact that part of the water in 

 each mineral may be basic, the apparent anomalies disappear ; 

 and the compounds reduce at once to similar salts of the acid 

 H,Si,0 5 , which acid is well known in the species petalite and 

 milarite. Both ptilolite and mordenite become simply repre- 

 sentable by the general formula 



Al 2 (Si 2 5 ) 5 R' 4 + rcAq. 

 the only serious uncertainties being in respect to the hydra- 

 tion. 



In brief, the following salts appear to be represented in the 

 three minerals, commingled in simple ratios : 



1. Al a (Si a 6 ),CaH a . 3 aq. 



2. Al 2 (Si 2 5 ) 6 CaH 2 . 6 aq. 

 8. Al 2 (Si 2 6 ) 5 K 2 H 2 .6aq. 

 4. Al 2 (Si 2 OJ 5 Na 2 H 2 .6aq. 



Three of these are entirely alike in type : but the first, con- 

 taining only half the normal quantity of crystalline water, is 

 assumed in order to account for the low hydration of the orig- 

 inal, Green Mt. ptilolite. Upon this basis the three minerals 

 investigated may be assigned the following theoretical compo- 

 sitions : 



Ptilolite, Green Mt., molecules 1 and 3, in ratio of 3 : 2 

 Silver Cliff " 2 " 4, " "1:1 



Mordenite, " 2, 3, 4, " "1:1:1 



