358 A TREATISE ON METAMORPHISM. 



monly the sulphate is formed by the oxidation of a sulphide. A common 

 method is the production of iron sulphate by oxidation of pyrite, marcasite, 

 or p3^rrhotite, which reacts upon calcium carbonate, thus producing gypsum. 

 The reaction is — 



CaC0 3 +FeS0 4 +2H 2 0=CaS0 4 .2H 2 0+FeC0 3 +k. 



The development of gypsum by this method is illustrated at many mines. 

 Finally gypsum may be formed by trie hydration of anhydrite. All 

 these methods of formation of gypsum are characteristic of the zone of 

 katamorphism, and especially of the belt of weathering. 



Alterations. — An important alteration of gypsum is to anhydrite (ortho- 

 rhombic, sp. gr. 2.899-2.985). The reaction is — 



(1) CaSO,,.2H 2 0=CaSO.,+2H,0-K. 



The decrease in volume is 37.62 per cent. The other important alteration 

 of gypsum is into calcite (rhombohedral, sp. gr. 2.713-2.714). The 

 reaction is — 



(2) CaSOi.2H 2 0+C0 2 =CaC0 3 +H 2 S0 4 +H 2 0+k. 



The H 2 S0 4 produced may simultaneously react upon some other compound. 

 The decrease in volume is 50.29 per cent. 



Unless beds of gypsum have been deeply buried the alteration to 

 anhydrite has not extensivelv occurred. It is a reaction of diminution of 

 volume, absorption of heat, dehydration, increase in specific gravity, and 

 increase in symmetry, and therefore is one of the very rare cases which 

 illustrate all the tendencies of the lower physical-chemical zone. The 

 change of gypsum to calcite is a reaction with liberation of heat and con- 

 densation of volume. The change takes place near the surface, especially 

 in the belt of weathering, where carbon dioxide is abundant, and may also 

 occur in the lower zone. It therefore stands, in its physical-chemical 

 relations, in the same position as dolomitization. (See p. 240.) 



