276 A TREATISE ON METAMOEPHISM. 



average compositions. With all these hypotheses, and supposing- the extra 

 silica to separate as quartz, the magnesia to separate as magnesium carbonate, 

 and the iron as sesquioxide of iron, the equation may be written as follows: 



(6 ) 6 [2 ( Ca 3 Mg,FeSi 6 18 ) . Mg 4 Fe 2 Al 9 Fe 3 Si 6 S6 ] -4- 1 2C0 2 + 39H,0 + 120 = 



18(HCa 2 Al 2 FeSi 3 O ls )+3(H 20 Mg 12 Al 6 Si 7 O 45 )+33SiO i! +12Fe 2 O3+12MgCO 3 +k. 



If all the compounds remain as solids the increase in volume is 15.43 per 

 cent. If the magnesium carbonate be dissolved the increase in volume is 

 8.58 per cent, 



It is evident that many other equations could be written if other sup- 

 positions be made as to the relative proportions of the magnesium to the 

 iron and the aluminum to the iron in the respective compounds, and if other 

 chlorites than the particular one chosen be produced. For the complex 

 silicates, present knowledge is not sufficient to determine whether or not 

 particular equations written accurately represent the alterations which take 

 place, although closer study in the future may possibly determine this. 

 But there is little doubt that substantially the change represented by equa- 

 tion (6) has occurred in many instances, whether it can be verified in an 

 individual case or not, as doubtless have also a multitude of alterations 

 which might be represented by other possible equations. The difficulty is 

 to ascertain in a given instance which of the equations represents a given 

 alteration. It is hoped that the quantitative statement of the problem given 

 by equation (6) and following equations will lead to closer study of the 

 compounds which enter into new compounds and the compounds which are 

 produced, and thus to more exact knowledge of the various alterations of 

 augite. 



According to the above reactions, as would be expected from the nature 

 of the compounds, the alteration of diopside and sahlite more frequently 

 produces talc, serpentine, and bastite, while the alteration of augite more 

 frequently produces chlorite and epidote. 



As already noted, perhaps the most characteristic of the alterations of 

 the pyroxenes is to the amphiboles. This alteration involves the substitu- 

 tion of magnesium, or magnesium and iron, for calcium. It is supposed 

 that the iron and magnesium are added in the form of carbonate, and that 

 the liberated calcium separates in the form of carbonate. Parallel equa- 

 tions can, however, readily be written on the basis of any other magnesium 

 compound being added and similar iron and calcium compounds being 



