ALUMINUM-SILICATE GROUP. 319 



The decrease in volume of the talc as compared with the andalusite is 32.37 

 per cent; as compared with the sillimanite, 31.20 per cent; as compared 

 with the cyanite, 23.12 per cent. But if the gibbsite be included as a solid 

 the increases in volume are 1)7.67 per cent, 101.09 per cent, and 124.71 per 

 cent, respectively. 



The change of the three minerals to kaolin may be written as follows: 



(2) 2Al 2 Si0 5 +5H 2 0=H 4 Al 2 Si 2 9 +2Al(OH) 3 +k. 



The change in volume of the kaolin as compared with the andalusite is a 

 decrease of 3.15 per cent; as compared with, the sillimanite, a decrease of 

 1.47 per cent; and as compared with the cyanite, an increase of 10.11 per 

 cent. But if the gibbsite be a solid, the increases in volume are 61.87 

 per cent, 64.67 per cent, and 84.02 per cent, respectivel}". 



The alterations of the same minerals to muscovite (damourite) may be 

 written as follows: 



(3) 6Al 2 Si0 5 +K 2 C03+llH 2 0=2H 2 KAl 3 Si 3 12 +6Al(OH)3 + C0 2 +k. 



The decrease in volume of the muscovite as compared with the andalusite 

 is 9.55 per cent; as compared with the sillimanite, 7.98 per cent; the 

 increase as compared with the cyanite is 2.83 per cent. But if the gibbsite 

 be regarded as a solid, the increases in volume are 55.47 per cent, 58.16 

 per cent, and 76.74 per cent, respectively. 



The alterations of the aluminum-silicate minerals to talc, kaolin, or 

 muscovite, with the accompanying gibbsite, are all reactions of hydration. 

 They involve great increase of volume, from 55 to 125 per cent. To 

 produce the original heavy aluminum-silicate minerals in the zone of 

 anamorphism undoubtedly required great condensation of volume. When 

 the reactions are reversed in the zone of katamorphism, there is a corre- 

 spondingly great expansion of volume. The change of the heavy aluminum- 

 silicate minerals to the much lighter hydrous minerals gives one of the best 

 illustrative cases of typical reactions of the zone of katamorphism. 



The change of andalusite to cyanite, as already explained, being a 

 molecular one, involves a volume relation inversely as the specific gravity, 

 and therefore by the change the volume is decreased 12.03 per cent. The 

 change of andalusite to cyanite is a reaction of the zone of anamorphism. 



