290 A TREATISE ON METAMORPHISM. 



The alteration of trernolite to talc, equation (1), is that of hydration 

 and carbonation. The alteration of actinolite to talc, equation (2), is that 

 of hydration, carbonation, desilication, and oxidation. The alteration of 

 actinolite to bastite, equations (3) and (4), is that of hydration, carbonation, 

 and desilication. The alteration of cummingtonite to bastite, equations (5) 

 and (6), is that of hydration and desilication. All these changes take place 

 with the liberation of heat and with expansion of volume, provided the 

 compounds which form mainly separate as solids. Whether or not there is 

 an actual increase in the volume as a result of the changes depends, of 

 course, upon the amounts of the secondary material which is dissolved. It 

 is therefore clear that all of these changes are those which are typical of the 

 zone of katamorphism, and especially the belt of weathering. Moreover, 

 some of the changes, like that of actinolite to talc and the accompanying- 

 compounds, illustrate all the processes normal to this position; i. e., hydration, 

 carbonation, oxidation, and desilication. The fact that calcite is so 

 frequently found associated with the talcs and serpentines secondary to 

 trernolite, cummingtonite, and actinolite, is rendered perfectly clear by the 

 equations; for there is always a residuum of calcium which evidently, under 

 the conditions of the upper physical-chemical zone, unites with the carbon 

 dioxide and produces calcium carbonate, which frequently separates as the 

 mineral calcite in large part, but which doubtless is frequently largely or 

 altogether carried away in solution. 



The alteration of hornblende into chlorite and accompanying minerals 

 is one of liberation of heat and expansion of volume. It is an alteration 

 also of carbonation, and of oxidation in case some of the ferrous iron be 

 changed to sesquioxide. It is therefore to be expected in the upper 

 physical-chemical zone, and as a matter of fact it occurs there. The change 

 from hornblende to biotite is a much deeper seated alteration. It involves 

 hydration, silication, and possibly carbonation, and thus includes an 

 unusual combination of reactions. Corresponding with these facts the 

 change of hornblende to biotite is one which takes place under rather 

 deep-seated conditions, particularly in connection with profound mechan- 

 ical action. The physics of the interchanges between hornblende and 

 augite are elsewhere discussed (see pp. 279-280); but it may be said that 

 the change of the first to the second involves decrease of volume, and, 

 corresponding with this fact, is known to take place under very deep- 

 seated conditions of metamorphism. 



