224 A TREATISE ON METAMORPHISM. 



the production of the other minerals require the addition of various other 

 constituents — in the case of spinel, magnesia ; in the case of sillimanite and 

 cyanite, silica; in the case of the complicated silicates, muscovite, margarite, 

 and zoisite, various bases and a large amount of silica. Therefore in these 

 cases it is clear that the common statement that corundum alters to the 

 minerals muscovite, margarite, and zoisite can have only the meaning that 

 the relations are such that corundum furnishes the alumina for the resultant 

 compound, and that the additional compounds are derived from another 

 source. It will be assumed in the alterations that the magnesia, lime, 

 and potash are derived from the solid carbonates and that the silica is 

 added as quartz. The equations for the reactions are as follows: 



(1) Al 2 8 +H 2 0=2[A10.(OH)]+k. 



(2) Al 2 3 f3H 2 0=2Al(OH) 3 +k. 



(3) Al,0 3 +MgC0 3 =MgAl 2 4 +C0 2 +k. 



(4) Al 2 3 +Si0 2 =Al 2 Si0 5 +k. 



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



(6) 2Al,0 3 +2Si0 2 +CaC0 3 +H 2 = H 2 CaAl 4 Si 2 12 +C0 2 +k. 



(7) 3Al 2 3 +6Si0 2 +4CaC0 3 +H 2 0=H 2 Ca,Al 6 Si 6 6 26 +4CO,+k. 



The increase in volume as compared with corundum is, for diaspore 

 (equation 1), 39.25 per cent; for gibbsite (equation 2), 161.83 per cent. 

 The volume of the corundum and the magnesite in passing to the spinel 

 (equation 3) is decreased 29.17 per cent. The volume of the corundum 

 and quartz in passing into sillimanite (equation 4) is increased 4.38 per 

 cent; into cyanite (equation 4) is decreased 6.59 per cent. If the volume 

 of the corundum be compared with that of the muscovite (equation 5), 

 with that of the margarite (equation 6), and with that of the zoisite 

 (equation 7), there will be great volume increases. If, on the other 

 hand, all the products which unite with the corundum in each case, with 

 the exception of the water, be counted as solid, there would be small 

 increase in the volume for muscovite, a considerable decrease for zoisite, 

 and a small decrease for margarite. On the first hypothesis the increase in 

 the volume in the production of muscovite is 264.25 per cent; in margarite, 

 159.02 per cent; in zoisite, 261.34 per cent. On the second hypothesis the 

 increase in volume in the production of muscovite is 1.62 per cent; to form 

 margarite the decrease is 1.22 per cent; to form zoisite the decrease is 23.58 

 per cent. 



It is reasonably certain that the passage of corundum to diaspore and 

 gibbsite is a reaction characteristic of the zone of katamorphitm, and 



