822 A TREATISE ON METAMORPHISM. 



SILK ATK KOCKS. 



Li certain localities the limestones and dolomites may be wholly 

 replaced by silicates. Of these silicates the most important are the pyrox- 

 enes and amphiboles. Of the pyroxenes the most common are wollastonite, 

 diopside, and sablite. Of the amphiboles the most important are tremolite 

 and actinolite. Of course other pyroxenes and amphiboles may be present, 

 such as the augites and hornblende. Other silicates which rather frequently 

 replace limestones are olivine, mica (including muscovite, biotite, and 

 phlogopite), chondrodite, vesuvianite, and tourmaline. In fact, almost any 

 of the dense silicates may develop. The limestones are transformed to 

 silicate rocks only in the zone of anamorphism. The silicates are the 

 ultimate products of alteration under the conditions there obtaining. The 

 process is one of simple silication ; the silica originally present in the lime- 

 stone, or contributed to it by ground waters, replaces the carbon dioxide 

 and forms the silicate. 



If the rock be pure limestone, wollastonite forms, according to the 

 following reaction: 



CaC0 3 +Si0 2 +nH 2 0=CaSi0 3 +C0 2 4-nH 2 0, 



with a decrease in volume of 31 per cent, provided the silica is a solid 

 and the carbon dioxide escapes. 



If the rock be magnesian, diopside or tremolite forms, according to the 

 following reactions, respectively : 



CaMg(C0 3 ) 2 +2Si0 2 =CaMgSi 2 6 +2C0 2 , 

 3CaMg(C0 3 ) 2 +4Si0 2 =CaMg 3 Si 4 1 2+2CaC0 3 +4C0 2 , 



with a decrease in volume of 40.11 and 25.20 per cent, respectively, 

 provided the silica is a solid, the calcium carbonate remains as a solid 

 when tremolite forms, and the liberated carbon dioxide escapes. 



If the limestone be one which contains many bases in important 

 amounts, other more complex silicates develop, with variable decreases in 

 volume; and thus we have the explanation of subordinate amounts of 

 augite, hornblende, mica, chondrodite, vesuvianite, tourmaline, etc., which 

 are so common as replacement products of marble. 



The localities in which the calcium-carbonate formations have been 

 completely changed to silicate formations are comparatively few, and at 



