278 A TREATISE ON METAMORPHISM. 



supposed that the last half of the augite and hornblende molecules are 

 present in greater proportion than given in the equations, the chemical 

 change would be of still less relative importance. This is of interest because 

 the alteration of augite to hornblende is a far more common phenomenon 

 than the alterations of diopside and sahlite to tremolite and actinolite The 

 equations, also give reasons for the very frequent occurrence of calcite with 

 uralite. The nature of the alterations is such that calcium carbonate must 

 be produced, and very naturally a portion of this substance frequently 

 separates as calcite. 



In the change of augite to biotite it is necessary that potassium be 

 derived from some source. Supposing it to be furnished in the form of 

 potassium carbonate, as a result of the decomposition of some of the potas- 

 sium-bearing silicates, the simplest form of reaction may be written as 

 follows : 



(10) 2[Ca(MgFe)Si 2 6 .(MgFe)(AlFe) 2 Si0 6 ]+K 2 C0 3 +H 2 0+C0 2 = 



2HK(MgFe),(AlFe) 2 Si 3 1 ,+2CaC0 3 +k. 



Supposing the MgO: FeO: : 2: 1, and the A1 2 3 : Fe 2 3 : : 3: 1 — these ratios 

 being chosen because they represent about an average of the analyses — 

 and multiplying the above equation by 6, we have : 



( 11 ) 2[Ca 6 Mg 4 Fe 2 Si 12 36 .Mg 4 Fe 2 Al 9 Fe 3 Si 6 36 ] +6K 2 C0 3 +6H 2 0+6C0 2 = 



2 ( H 6 K r ,Mg s Fe 4 Al 3 Fe 3 Si 18 72 ) + 12CaC0 3 +k. 



Disregarding all other compounds, the increase in volume of the biotite as 

 compared with the augite is 17.26 per cent. 



The alteration of diopside and sahlite to talc, serpentine, and bastite, 

 equations (1), (3), and (4), all involve increase in volume and liberation of 

 heat; also they are alterations involving carbonation and hydration. Equa- 

 tions (2) and (5) involve carbonation, hydration, and oxidation. In all 

 except equation (1), even if all of the separated quartz and calcite is 

 dissolved, there is still an increase in volume. They therefore stand as 

 alterations that are typical of all the principles of metamorphism in the 

 zone of katamorphism. 



The changes of the pyroxenes, especially augite, to chlorite and epidote, 

 equation (6), involve hydration, carbonation, and oxidation. The change 

 occurs with increase in volume and liberation of heat, even if the resultant 

 oxide of iron and magnesium carbonate be ignored. If these separate as 

 solids, the increase in volume is considerable. The alteration is, therefore, 



