342 A TREATISE ON METAMOEPH1SM. 



Combining these equations, (3) and (4), and supposing the iron oxide to be 

 furnished by hematite, the reaction may be written as follows : 



(7) 60KHMg 2 Al 2 Si 3 O 12 +6Fe 2 O 3 4-40CaCO 3 +76H 2 O= 



30[H 2 Mg 4 Al,Si3O 12 .4(OH)]+4(H 5 Ca 10 Al 12 Fe 3 Si 15 O 6 5)+30SiO 2 +12AlO(OH) + 

 30K,CO 3 +10CO,+k. 



The increase in volume of the epidote, chlorite, quartz, and diaspore 

 together, as compared with the biotite and hematite, would be 1.81 per 

 cent. 



If biotite alters to hypersthene and sillimanite it may be presumed, in 

 order to furnish the necessary iron for the hypersthene, that the biotite is 

 an iron-bearing one. If the magnesium be to the iron as 3:1, and the 

 hypersthene be one in which the same ratio prevails, the reaction may be 

 written as follows: 



(8) H 2 K 2 Mg 3 FeAl 4 Si 6 24 - 1 -C0 2 =Mg 3 FeSi 1 12 +2Al 2 Si0 5 +H 2 0+K 2 C0 3 +k. 



The decrease in volume of the hypersthene and sillimanite, as compared 

 with the biotite, is 24.68 per cent, 



In the majority of the above reactions a formula for biotite is used 

 which contains no iron. The majority of biotites in nature do contain 

 some iron. If this material be present, simultaneously with the formation 

 of other minerals magnetite (isometric; sp. gr. 5.174), hematite (rhombohe- 

 dral; sp. gr. 5.225), and the other oxides of iron may be produced. The 

 abstraction of the iron oxides is accompanied commonly by a change in 

 color of the altering biotite from brown to green. The presence of these 

 compounds, however, in subordinate quantities will not alter the main 

 conclusions as to the volume relations above given. 



The alterations of biotite, serpentine, kaolin, and gibbsite, into hydro- 

 biotite and chlorite, equations (1) to (3), are all reactions which are 

 known to occur in the zone of katamorphism, corresponding with which 

 position they are all reactions of hydration, and the first two also of car- 

 bonation. Where chlorite and epidote form together, the reaction is that 

 of hydration, and doubtless this change also takes place in the zone of 

 katamorphism. The formation of hypersthene and sillimanite from biotite 

 usually occurs in connection with contact reactions of igneous rocks; it is, 

 therefore, a reaction requiring high temperature. Also the minerals 



