838 A TREATISE ON METAMORPHISM. 



With the griinerite, actinolite, sahlite, or other silicates, magnetite 

 usually develops to some extent, probably as follows: 



2FeC0 3 +FeS 2 +2H 2 0=Fe 3 4 +2H 2 S+2CO ; ,. 



The following reactions are also possible: 



3FeCO,=Fe 3 4 +CO+2C0 2 ; 



or where there is sufficient oxygen: 



3FeC0 3 +0=Fe 3 4 +3C0 2 . 



The first reaction produces a decrease in volume of 47 per cent, and the 

 second and third a decrease of 50 per cent, provided the hydrogen sulphide, 

 oxygen, carbon monoxide, and carbon dioxide be ignored. 



Observation in the field shows beyond question that the change from 

 iron carbonate to magnetite takes place on an extensive scale. Which of 

 the above reactions is the more important may be an open question. Since 

 the change takes place in the zone of anamorphism, where the conditions 

 are reducing instead of oxidizing, and since, as has already been explained 

 (p. 828), pyrite is a very common associate of iron carbonate, I think it 

 probable that the first of the three reactions written is the dominant one. 

 The reactions of the second and third equations are identical with those 

 which take place when iron carbonate is heated in a closed tube in the 

 laboratory where oxygen is absent or deficient in amount. It is explained 

 below that the development of magnetite from iron carbonate takes place 

 on the most extensive scale where igneous rocks have been intruded. This 

 suggests at first that the reactions may be those of dry heat, but this idea 

 is probably not warranted, for the magnetite which forms from carbonate is 

 usually segregated into crystals or clusters of crystals, and this leads me to 

 believe that the change is accomplished through the agency of water, with 

 heat as a promoting force and iron sulphide as one of the active agents. 



In the group of rocks under consideration the relative proportions of 

 the silicates, of magnetite, and of quartz are very variable. In some 

 instances the actinolite or the griinerite is predominant, with quartz as a 

 subordinate constituent, and we have a quartzose actinolite or quartzose 

 griinerite rock. In other instances the predominant constituents are mag- 

 netite and quartz, and in still other cases quartz is subordinate in amount; 

 hence we may have a magnetite-quartz rock, or a quartzose magnetite 



