262 A TREATISE ON METAMORPHISM. 



G-ismondite (monoclinic; sp. gr. 2.265) is derived from anorthite 

 according to the following reaction: 



(7) 3CaAl 2 Si 2 8 +12H 2 0=Ca 3 Al 6 Si 6 24 .12H 2 0+k. 



The increase in volume is 52.76 per cent, 



For laumontite (monoclinic; sp. gr. 2.305) the change does not appear 

 to have been determined with reasonable certainty. It may be supposed 

 to be derived from anorthite by the simultaneous union of freed calcium 

 and aluminum with other compounds, the calcium perhaps passing into the 

 carbonate and the aluminum into the hydrate. On this hypothesis the 

 reaction is: 



(8) 2CaAl 2 Si 2 8 +7H 2 0+C0 2 =H 4 CaAl 2 Si. 1 H .2H,0+CaGO :1 +2[Al (OH) 3 ] +k. 



The increase in volume is 33.65 per cent, supposing the calcium carbonate 

 to be dissolved and the aluminum hydroxide to remain as gibbsite. How- 

 ever, Clarke regards laumontite as derived from equal quantities of anorthite 

 and the hypothetical compound trisilicic anorthite" (Ca 3 Al 6 (Si 3 8 ) 6 ). 



The zeolite phillipsite (monoclinic; sp. gr. 2.20) may be regarded as 

 derived from albite, anorthite, and leucite, as follows: 



(9) 6CaAl 2 Si 2 8 +4NaAlSi 3 8 +6KAlSi 2 6 +48H 2 0+2C0 2 = 



3(K 2 Ca 2 Al 6 Si I2 36 .14H 2 0)+2Na 2 C0 3 +4Al(OH) 3 +k. 



The leucite is added as a source of the potassium. The increase in volume 

 of the three compounds in passing into phillipsite is 31.98 per cent. 



Heulandite (epistilbite) (monoclinic; sp. gr. 2.20) and stilbite (mono- 

 clinic; sp. gr. 2.1495) are regarded by Clarke as derived from the 

 hypothetical compound trisilicic anorthite. Chabazite (rhombohedral ; sp. 

 gr. 2.12) is regarded by him as derived from this compound and from 

 normal anorthite. All four, however, may be equally well considered 

 as derived from intermediate plagioclases with carbonation of the excess 

 of calcium and hydration of the excess of aluminum. On these hypotheses 

 the reactions for the four may be written as follows: 



(10) 4NaAlSi 3 8 +3CaAl 2 Si 2 8 +21H 2 0+2C0 2 = 



3 ( H 4 CaAl 2 Si 6 I8 . 3H 2 ) +2Na 2 C0 3 + 4 [ Al (OH) 3 ] +k. 



(11) 4NaAlSi 3 8 +3CaAl 2 Si 2 8 +24H 2 0+2C0 2 = 



Ca 3 Al 6 (Si 3 8 ) 6 .18H 2 0+2Na 2 C0 3 +4Al(OH) 3 +k. 



(12) 6NaAlSi 3 8 +6CaAl 2 Si 2 8 +3C0 2 +45H 2 0= 



2[Ca 3 Al 6 (Si0 4 ) 3 (Si 3 8 ) 3 .18H 2 0]-(-3Na 2 C0 3 4-6[Al(OH) 3 ]4-6Si0 2 +k. 



a Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, p. 42. 



