ALTERATIONS OF PLAGTOCLASE FELDSPARS. 263 



Supposing the sodium carbonate is dissolved and the other compounds 

 are solids, the increase in volume for (10) is 37.14 per cent, for (11) is 

 43.50 per cent, and for (12) is 46.76 per cent. 



Scolecite (monoclinic; sp. gr. 2.16-2.40) may be derived from anorthite, 

 according to the following reaction: 



(13) 3CaAl 2 Si 2 O s +9H 2 O+CO 2 =2CaAl 2 Si 3 O 10 .3H 2 O+2Al(OH)3+CaCO 3 +k. 



The increase in volume is 35.23 per cent, provided the gibbsite separates 

 as a solid and the CaC0 3 is dissolved. 



Mesolite (monoclinic or triclinic; sp. gr. 2.29), according to Clarke," 

 is an isomorphous mixture of equal quantities of natrolite and scolecite; 

 therefore the reaction for this compound may be expressed by the following: 



(14) 4NaAlSi 3 O s +3CaAl 2 Si 2 8 H-13H 2 0+C0 2 = 



2 (H 8 Na 2 CaAl 4 Si 6 24 . H,0 ) +6Si0 2 +2Al ( OH ) 3 + CaC0 3 + k. 



The expansion in volume is 24.96 per cent, provided the silica and gibbsite 

 separate as solids and the CaC0 3 is earned away in solution. If all products 

 are solid the increase in the volume is 30.19 per cent. 



Turning now from the zeolites to other minerals, the plagioclases are 

 recorded as altering into prehnite (orthorhombic; sp. gr. 2.875). Since 

 albite is recorded as simultaneously separating, the most probable reaction 

 is by hydration of the anorthite molecule. 



(15) 4CaAl 2 Si 2 3 +8H 2 0=2H 2 Ca 2 Al 2 Si 3 12 +4Al(OH) 3 +2Si0 2 +k. 



Supposing the compounds formed to be solids, the increase in volume is 

 14.85 per cent. 



Another important alteration of the plagioclases is into zoisite 

 (orthorhombic; sp. gr. 3.25-3.37) or epidote (monoclinic; sp. gr. 3.25-3.5), 

 with the simultaneous formation of kaolinite or gibbsite. The reaction for 

 anorthite in the case of zoisite is probably: 



( 16 ) 4CaAl 2 Si 2 8 +3H 2 0= H 2 Ca 4 Al 6 Si 6 26 +H 4 Al 2 Si 2 9 + k, 



or 



(17) 4CaAl 2 Si 2 8 +4H 2 0=H 2 Ca 4 Al 6 Si 6 26 +2Al(OH) 3 +2Si0 2 +k. 



The decrease in volume of the solids in (16) is 7.77 per cent, and in (17) 

 is 4.58 per cent. 



In the formation of epidote the reactions are of a similar kind, but 

 Fe 2 3 replaces some AL0 3 of the feldspar molecule. Supposing the 



"Clarke, cit., Bull. 125, pp. 35-36. 



