METAMORPHISM OF IRON-BEARING CARBONATES. 837 



with a decrease of volume of 32 per cent, provided the silica be a solid 

 and the carbon dioxide escape. Where the original material was hydrous 

 ferrous silicate, greenalite, simple dehydration only is necessary to form 

 the grunerite. 



Where the iron-bearing carbonate bears calcium and magnesium in 

 considerable quantity, instead of grunerite being produced sahlite or 

 actinolite may be formed. Supposing the carbonate to be normal ankerite, 

 sahlite is produced, according to the following reaction: 



CaFeC 2 6 .CaMgC,0 B +4Si0 2 =Ca 2 MgFeSi i 12 +4C0 2 , 



with a decrease in volume of 37 per cent, provided the silica be solid and 

 the carbon dioxide escape. 



From ankerite actinolite may be jiroduced, according to the following 

 reaction : 



3 ( CaFeC A- CaMgC A ) +8Si0 2 =Ca,Mg 3 Fe 3 Si 8 24 4-8C0 2 +4CaC0 3 , 



with a decrease in volume of 23 per cent, provided the silica be a solid, the 

 CaC0 3 formed remain as a solid, and the carbon dioxide escape. 



If a more ferriferous and less calcareous iron-bearing carbonate be 

 taken, it would not be necessary to suppose any calcium carbonate to have 

 separated. 



The iron-bearing carbonates may be very impure, just as limestones 

 may be impure; and in this case there may develop various other minerals. 

 In proportion as impurities are mingled with the carbonates, other amphi- 

 boles and the pyroxenes, micas, garnets, and other heavy minerals such as 

 olivine, may abundantly develop ; and thus there may be produced a great 

 variety of rocks, such as garnetiferous magnetite rocks, micaceous grunerite 

 rocks, etc. As the impurities become abundant and the silicates other than 

 grunerite, sahlite, and actinolite more prominent, the alterations become 

 nearly those of the fragmental rocks, considered on page 853 et seq. 

 Between the two there are of course all gradations. 



But as a matter of fact, the two silicates which most extensively form 

 by the alterations of the iron-bearing carbonates in the zone of anamorphism 

 are actinolite and grunerite. Where these reactions are complete we may 

 have, in place of the iron-bearing carbonate, actinolite rocks, grunerite 

 rocks, and all gradations between them. 



