236 A TEEATISE ON METAMORPHISM. 



alterations from diaspore and gibbsite have taken place on an extensive 

 scale, even if they have not been recorded. There is no doubt about the 

 formation of gibbsite abundantly in the zone of katamorphism, especially 

 in the belt of weathering. To my mind there is as little doubt that the 

 widespread corundum of the schists, gneisses, and marbles is derived in 

 large measure from gibbsite. I am confident that the hydrous aluminum 

 oxides furnish the bases for much of the spinel (isometric; sp. gr. 3.80), 

 sillimanite (orthorhombic; sp. gr. 3.235), and cyanite (triclinic; sp. gr- 

 3.615) which occur in these rocks. And it is little short of a certainty that 

 gibbsite furnishes alumina for the silicates, muscovite (monoclinic; sp. gr. 

 2.88), margarite (monoclinic; sp. gr. 3.035), and zoisite (orthorhombic; 

 sp. gr. 3.31). As with diaspore, the reactions producing all the above- 

 mentioned silicates are those of dehydration and silicifiation. The following 

 etpiations may be written for the above supposed reactions: 



(1) Al(OH) 3 =A10(OH)+H 2 0-k. 



(2) 2Al(OH) 3 =Al 2 3 +3H 2 0-k. 



(3) 2Al(OH) 3 +MgC0 3 =MgAl 2 4 +C0 2 +3H 2 0-k. 



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



(5) 6Al(OH) 3 +6Si0 2 +K 2 C0 3 =2H 2 KAl 3 Si 3 12 +C0 2 -7H 2 0— k. 



(6) 4Al(OH) 3 +2Si0 2 +CaC0 3 =H 2 CaAl i Si,0 1 ,+C0 2 +5H 2 0-k. 



(7) 6Al(OH) 3 +6Si0 2 +4CaC0 3 =H 2 Ca 4 Al 6 Si 6 2 6+ 4 C0 2 +8H 2 0-k. 



Regarding all the minerals as solid, the decrease of volume for diaspore (1) 

 is 46.82 per cent; for corundum (2), 61.81 per cent; for sjmiel (3), 60.12 

 per ceut; for sillimanite (4), 43.68 per cent; for cyanite (4), 49.61 per cent; 

 for muscovite (5), 64.99 per cent; for margarite (6), 38.92 jjer cent; for 

 zoisite (7), 43.06 per cent. The decreases of volume are greater for the 

 corresponding minerals than for diaspore because of the greater amount of 

 water in the gibbsite. In all the reactions heat is absorbed. The reactions 

 are therefore typical of the zone of anamorphism. 



THE CARBONATES. 



The important carbonates which occur as rock-making constituents 

 are the calcite group, including calcite, dolomite, ankerite and parankerite, 

 magnesite, and siderite, and the aragonite group, of which aragonite is the 

 only important rock-making member. 



