ALTERATIONS OF HORNBLENDE. 287 



Supposing that the Mg : Fe : : 2 : 1, the equation is — 



(4) CaMg 2 FeSi 4 12 +2H 2 0+C0 2 =H 4 Mg 2 FeSi 2 9 +CaC0 3 4-2Si0 2 +k. 



The increase in volume, supposing all the compounds to separate as solids, 

 is 38.67 per cent. If only the bastite remains as a solid, the decrease in 

 volume is 18.06 per cent. 



The alteration of cummingtonite into bastite is as follows: 



(5) 3(MgFe)Si0 3 +2H 2 0=H 4 (MgFe) 3 Si 2 9 +Si0 2 +k. 



Supposing that the Mg : Fe : : 3 : 1, the equation is — 



(6) 3Mg s FeSi 4 12 +8H 2 0=H I6 Mg 9 Fe 3 Si 8 Oa 6 +4Si0 2 +k. 



The increase in volume of the bastite and quartz as compared ' with the 

 cummingtonite is 36.76 per cent; of the bastite alone, 14.2 per cent. 



In writing equations for the alteration of hornblende into chlorite and 

 accompanying minerals the soda-bearing part of the molecule will be 

 omitted, since the amount of soda present in ordinary hornblende is small. 

 Supposing that there are eight actinolite molecules in the hornblende to 

 two of the sesquioxide molecules, that the MgO : FeO : : 2 : 1, and the 

 A1 2 3 : Fe 2 3 : : 2 : 1, that the chlorite produced is on the border line between 

 prochlorite and clinochlore, and that the Al : Fe in the epidote as 2 : 1, the 

 reaction may be written as follows: 



(7) 8CaMg 2 FeSi 4 12 .2Mg 4 Fe 2 Al 8 Fe 4 Si 6 36 +21H 2 0+16C0 2 = 



2(H 20 Mg 12 Al 6 Si 7 O 4 . 5 )+2HCa 2 Al 2 FeSi 3 Oi 3 -|-4CaCO 3 -)-i2FeCO 3 +24SiO 2 +3Fe 2 O3+k. 



Provided all the compounds separate as solids the increase in volume is 

 25.39 per cent. 



It is noticeable that the equation for the alteration of the hornblende 

 to chlorite as a chief resultant product demands that epidote, calcite, sid- 

 erite, quartz, and hematite be produced; and corresponding with this, 

 Lacroix noted all of these minerals as accompaniments of the chloritic 

 alteration with the exception of hematite." Of course all or a larger part 

 of the iron may pass into the form of iron oxide — magnetite, hematite, or 

 limonite — in which case some oxygen would need to be added to the equa- 

 tion; the amount of C0 2 required will be less; and the oxide of iron will 

 replace the iron carbonate partly or wholly. As these modifications can 

 easily be made in the equation, it hardly seems necessary to write out 

 formulas for them. 



a Lacroix, cit., pp. 667-668. 



