ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 279 



like the change to talc, serpentine, and bastite, one characteristic of the 

 upper physical-chemical zone. The change of pyroxene to the fibrous 

 amphibole known as uralite occurs in the belt of cementation on an exten- 

 sive scale, and to this position the volume change corresponds. 



But the passage of pyroxene into definite amphibole individuals is one 

 of the most common alterations in the zone of anamorphism, and especially 

 under conditions of mashing. The rule for this zone is for alterations to 

 occur which result in minerals of higher specific gravity. The alteration 

 of pyroxene to amphibole seems to be an exception to this rule; for the 

 specific gravity of the pyroxenes ranges between 3.2 and 3.6, while that of 

 the amphiboles varies from 2.9 to 3.4. 



Mainly in consequence of this decrease in specific gravity the increase 

 in volume, as already seen, of all compounds entering into the reactions in 

 the change from diopside to tremolite, equation (7), is 10.55 per cent; of 

 sahlite to actinolite, equation (8), 10.81 per cent; of augite to hornblende, 

 equation (9), 6.14 per cent, supposing that the necessary chemical constitu- 

 ents added to the pyroxene are solid carbonates and the other compounds 

 produced are solid carbonates. 



Unlike the previous alterations, these changes do not involve oxidation, 

 hydration, or carbonation ; nor, on the other hand, do they involve deoxi- 

 dation, dehydration, or silication. They are substitution reactions, by which 

 magnesium, or iron, or both take the place of calcium. They are, there- 

 fore, analogous to the dolomitization or ferritization of the limestones; but 

 the volume change is in an opposite sense from those alterations. 



But another factor may enter into the problem, the effect of which is 

 hard to estimate. The exchange of the magnesium and iron for calcium is 

 supposed to take place with the separation of a carbonate. If such carbon- 

 ate were simultaneously silicated, the entire volume change for all the fac- 

 tors concerned would be decrease. It is necessary to consider the volume 

 relations of all the resultant minerals rather than those of the pyroxene and 

 amphibole alone, and hence it may be that in the change of pyroxene to 

 amphibole in the lower physical-chemical zone, if one could ascertain the 

 entire effect of this alteration in connection with other alterations, the 

 volume would not be expanded but contracted, and thus there be no real 

 exception to the law that the reactions here take place with condensation 

 of volume. 



