366 A TREATISE ON METAMORPHISM. 



REVERSIBLE REACTIONS. 



On the foregoing pages numerous reactions have been written by 

 which the minerals characteristic of the zone of katamorphism are pro- 

 duced; very few reactions have been written by which the minerals of the 

 zone of anamorphism are reproduced. It is certain that when the minerals 

 formed in the belts of weathering and cementation are altered under the 

 conditions of the zone of anamorphism the minerals characteristic of that 

 zone develop; therefore it is believed that many of the reactions for the 

 development of the minerals of the zone of katamorphism are reversible. 

 To illustrate, in the zone of katamorphism olivine may alter into the min- 

 erals serpentine, magnetite, magnesite, and quartz, according to the fol- 

 lowing equation: 



3Mg,FeSi,0 8 +3C0 2 +4H.,0+0=2H 4 Mg 3 Si,0 9 +FeA+3MgC0 3 +2SiO ; ,+k. 



It is believed that when these four minerals are brought together in proper 

 proportions under favorable conditions in the deep-seated zone the reverse 

 reaction occurs, and that« the equation may be read from right to left 

 instead of left to right, thus reproducing the olivine. 



The above illustration is chosen because the change from left to right 

 involves carbonation, desilication, hydration, and oxidation; and the change 

 from right to left involves silication, decarbonation, dehydration, and deoxi- 

 dation. Of course, where deoxidatiou takes place in the zone of anamor- 

 phism some reducing agent must be present to utilize the abstracted oxygen. 

 The principle of the reversibility of the reactions in the two opposing zones 

 is actually illustrated in a few cases where the products of the zone of kata- 

 morphism have been observed to alter in the zone of anamorphism. For 

 instance, it is recorded (p. 261) that analcite is derived from albite according 

 to the following equation: 



2NaAlSi 3 8 +2H 2 0=N& 2 Al 2 Si i 1 2-2H 2 0+2SiO,+k; 



whereas we find (p. 334) that analcite alters to albite by the reaction: 



Na,Al 2 SiA,.2H 2 0+2Si0 2 =2NaAlSi 3 8 +2H 2 0-k. 



In other words, the reaction is exactly reversible; for while the k is plus 

 in the first equation and minus in the second, it is on opposite sides in the 

 two equations. The feldspars alter into many zeolites, and a number of 



