288 A TREATISE ON METAMORPHISM. 



The change of hornblende to biotite requires the addition of potassium. 

 The potassium can be derived from some other mineral. It is perhaps 

 most frequently derived from orthoclase, although it is undoubtedly in 

 many cases derived from leucite. Supposing it is derived from orthoclase, 

 and therefore is in the form of potassium silicate, K,Si0 3 , and that the 

 actinolite molecule in the hornblende is to the sesquioxide molecule as 2 : 3, 

 the change may be represented as follows : 



(8) 2Ca(MgFe) 3 Si 4 12 .3(MgFe) 2 (AlFe) 4 Si 2 12 +3K 2 Si0 3 +Si0 2 +3H 2 0+2C0 2 = 



6HK ( MgFe), ( AlFe) 2 Si 3 I2 +2CaC0 3 + k. 



In order to ascertain the volume relations it is necessary to make assump- 

 tions with reference to the proportions of the Mg and Fe, and of the Al and 

 Fe. Supposing the magnesia is to the iron protoxide as 2:1, and the 

 alumina is to the iron sesquioxide as 2 : 1, the equation is: — 



(9) 2(0aMg 2 FeSi 4 12 ).Mg 4 Fe 2 Al 8 Fe 4 Si 6 O 36 +3K 2 SiO 3 +Si0 2 +3H 2 O+2C0 2 = 



H 6 K 6 Mg 8 Fe 4 Al s Fe 4 Si 18 7 , + 2CaC0 3 +k. 



The increase in volume of the biotite and calcite as compared with the 

 hornblende and quartz is 41 13 per cent, It has been noted, however, 

 that the alteration of hornblende to biotite is often accompanied by the 

 separation of epidote; and this is natural, since there is residual calcium in 

 the hornblende not needed by the biotite, which could pass into the 

 epidote. Supposing this residual calcium to pass into the epidote, the 

 reaction may be written as follows: 



(10) 8Ca(MgFe) 3 Si 4 12 .18(MgFe) 2 (AlFe) 4 Si 2 12 +15K 2 Si0 3 +19Si0 2 +17H 2 0= 



30HK(MgFe) 2 (AlFe) 2 Si 3 O 12 +4HCa 2 (AlFe) 3 Si s O 13 +k. 



In order to calculate the volume relations it may be supposed that the 

 MgO : FeO as 2 : 1, and the A1 2 3 : Fe 2 3 as 2:1, the equation being — 



(11) 8CaMg,FeSi 4 12 .6Mg 4 Fe 2 Al 8 Fe 4 Si 6 36 +15K 2 Si0 3 +19Si0 2 +17H 2 = 



10H 3 K 3 Mg 4 Fe 2 Al 4 Fe 2 Si 1) O 3 6+H i Ca 8 Al 8 Fe 4 Si 1 ,O 6: , + k. 



The increase in volume of the biotite and epidote as contrasted with the 

 hornblende and quartz is 30.05 per cent. The increase in volume for 

 equations (9) and (11) would be much less if the K 2 Si0 3 were taken into 

 account. 



Where serpentine also occurs as an alteration product accompanying- 

 the chlorite, biotite, epidote, and other products, this mineral is doubtless 

 derived from the actinolite part of the molecule, and an equation may be 

 readily written which represents the simultaneous formation of the bastitic 



