OCCURRENCE OF ZEOLITES. 



331 



(Dana's formula is one of the molecules given.) 



Gisniondite: 



Ca 3 Al 6 Si 4 24 .12H,0. 

 Ca 3 Al 6 SiA 4 -12H 2 0. 

 Monoclinic. 

 Sp. gr. 2.265. 



Chabaztte: 



Ca 3 Al 6 Si 10 O 32 .16H 2 O. (Dana agrees with Clarke as to the amount of Si. His formula 



is (CaNa 2 )Al 2 Si 4 1 2-6H 2 0.) 

 Ca 3 Al 6 Si 12 36 .18H,0. 

 Rhombohedral. 

 Sp. gr. 2.08-2.16. 



Gmelinite: 



Na 6 Al 6 Si 10 O 32 .16H 2 O. 

 Na 3 Al 3 Si 7 O 20 .9H 2 O. 

 Rhombohedral. 

 Sp. gr. 2.04-2.17. 



Laumontite: 



H i CaAl 2 Si 1 O u .2H 2 0. 

 Ca 3 Al 6 Si 12 36 .12H 2 0. 



Monoclinic. 



Sp. gr. 2.25-2.36. 



(Dana varies from each. His formula is (Na_,Ca)Al 2 Si 4 O u ,.6H 2 0. 



(Dana agrees with Groth.) 



■■■ — The zeolites are not known as original pyrogenic constitu- 

 ents of igneous rocks. As secondary minerals they are mosL abundantly 

 found in basic lavas and allied rocks, including both glassy and crystallized 

 kinds. The zeolites occur in these rocks, both in the ordinary feldspathic 

 varieties, such as basalt and trachyte, and in those containing leucite, 

 nephelite, sodalite, etc., and especially in those which are somewhat vesicu- 

 lar. The zeolites also develop abundantly in the deep-seated equivalents 

 of the basalts in such rocks as the diabases, gabbros, etc. The zeolites are 

 less prevalent, although far from rare, in the diorite and syenite families, 

 including both the ordinary syenites and nepheline-syenites. Finally, the 

 zeolites are far from uncommon in the more acid rocks, such as those of 

 the granite family. In short, the zeolites may occur in almost any variety 

 of the igneous rocks, but, as already said, are most prevalent in the basic 

 group. In the sedimentary rocks the zeolites may be allogenic constituents 

 separate from other minerals, or an alteration product of partly decomposed 

 minerals. Also, after the deposition of material in the fragmental rocks 

 the zeolites may develop as alteration products. Hence the zeolites are 

 constituents of the altered sedimentary rocks, of the semi metamorphosed 



