216 A TREATISE ON METAMORPHISM. 



chemical reactions can take place there is a tendency in the lower zone for 

 pyrrhotite to alter to pyrite. 



The natural conditions for the transformation of pyrite, marcasite, and 

 pyrrhotite to limonite are those of abundance of oxygen and moisture. 

 These conditions are found in the zone of katamorphism, and especially in 

 the belt of weathering. In this belt the process goes on with such rapidity 

 that pyrite, marcasite, and pyrrhotite have generally been completely 

 oxidized where the rocks have been long exposed to the reactions of the 

 belt. The reactions are oxidation and hydration. They take place with 

 great liberation of heat and, for pyrite and pyrrhotite, with some expansion 

 of volume, and these changes may therefore be taken as typical illustrations 

 of alterations of the belt of weathering. 



The conditions for the formation of magnetite from pyrite, marcasite, 

 and pyrrhotite are the presence of some oxygen, but not a sufficient amount 

 to fully oxidize the iron, and considerable pressure. Where iron carbonate 

 is present, which also alters to magnetite, oxygen is not necessary. This 

 reaction is of great consequence. (See p. 244.) The alterations of the 

 sulphides to magnetite involve a decrease of volume of 24 to 39 per cent 

 and liberation of heat. Corresponding with this fact, the changes take 

 place in the belt of cementation or in the zone of anamorphism. 



THE FLUORIDES. 



Among the fluorides the only important rock-making mineral is fluorite. 



FLUORITE. 



Fluorite: 

 CaF 2 . 

 Isometric. 

 Sp. gr. 3.01-3.25. 



-Fluorite occurs as an accessory constituent, especially in 

 granitic and syenitic rocks. It is also found in other eruptive rocks, and 

 in metamorphic rocks, such as the schists and marbles. It therefore has a 

 somewhat widespi'ead occurrence, but is of very subordinate importance. 



Alterations — By tlie action of alkaline waters fluorite alters into calcite 

 (rhombohedral; sp. gr. 2.7135). Supposing the alkaline compound to be 

 sodium carbonate, the reaction is : 



CaF 2 +Na 2 C0 3 =CaC0 3 +2NaF+k. 



The increase in volume of the calcite as compared with the fluorite is 47.66 

 per cent. 



