340 A TREATISE ON METAMORPHISM. 



goes into the serpentine, and that all the silica not required for the pro- 

 duction of this mineral passes into the kaolin, the reaction is as follows : 



(1) 6KHMg 2 Al 2 Si 3 O 12 +18H,0+3C0 2 = 



4H 4 Mg s Si 2 9 +5H 4 Al 2 Si 2 9 +2Al(OH) s +3K 2 CO s +k. 



Supposing all the serpentine, kaolin, and gibbsite to remain as solids, and 

 the potassium carbonate to go into solution, the increase in volume is 14.26 

 per cent. 



The change of biotite into hydrobiotite may be written: 



(2) 2HKMg 2 Al 2 Si 3 IZ 4-7H 2 0+C0 2 =2(H 2 Mg 2 Al 2 Si30 12 .3H 2 0)+K J C0 3 +k. 



The increase in volume is 3.8 per cent. 



The alteration into chlorite, supposing all the alumina and silica 

 to remain in the altered mineral, and the additional magnesia to be added 

 in the form of a carbonate, may be written as follows : 



(3) 2KHMg 2 Al 2 Si 3 12 +4MgC0 3 - r 5H 2 0=2[H 2 JIg 4 Al. 2 Si 3 1 ,.4(OH)]+K 2 C0 3 +3C0 2 +k. 



The increase in volume, supposing the magnesium carbonate is added in 

 solution and the potassium carbonate goes into solution, is 22.92 per cent. 

 The reaction by which biotite passes into epidote is uncertain. If the 

 ferrous iron of the biotite be changed to sesquioxide during the alteration, 

 and if the proportion of magnesium to iron be supposed to be 3 : 1 in the 

 biotite, and of aluminum to iron 4:1 in the epidote, the reaction may be 

 written : 



(4) 6H 2 K 2 Mg 3 FeAl 4 Si 6 O 24 4-20CaCO 3 +4CO 2 +3O= 



2(H 5 Ca u ,A1 12 Fe 3 Si I5 G5 )+6Si0 2 +18MgC0 3 +6K 2 C0 3 +H 2 0+k. 



The ratios assumed of the magnesium and iron for the biotite, and of 

 aluminum and iron for the epidote, are near means. If it be assumed that 

 the iron of the biotite is not changed to sesquioxide, but that the sesquioxide 

 of iron for the epidote must be derived from another source, the reaction 

 takes a very different form. Under such an assumption the epidote may 

 be produced from normal biotite, and the equation stand as follows: 



(5) 30KHMg 2 Al 2 Si 3 O 12 +6Fe 2 O 3 +40CaCO 3 +35CO 2 + H,0= 



4( H 5 Ca u Al I2 Fe 3 Si 15 65 ) +30SiO 2 + 12A10 ( OH ) +60MgCO 3 + 15K 2 C0 3 +k. 



