788 A TREATISE ON METAMORPHISM. 



SULPHATE ORDER. 



GYPSUM AND ANHYDRITE FAMILY. 



Gypsum is hydrated calcium sulphate (CaS0 4 .2H 2 0). Anhydrite is 

 calcium sulphate (CaS0 4 ). The calcium for gypsum is released from other 

 combinations, mainly silicates, in the zone of katamorphism, and especially 

 in the belt of weathering. Sulphur, as sulphide, and especially as pyrite, 

 is a widespread but not abundant mineral in the original igneous rocks. 

 In the zone of katamorphism, and especially in the belt of weathering, 

 the sulphur is oxidized to sulphuric acid. The sulphuric acid unites with 

 bases and forms sulphates. Sulphates formed in the belt of weathering are 

 transported by the underground circulating waters to the sea or to lakes 

 without outlets. In these lakes, by the continual addition of sulphates and 

 evaporation, supersaturation is reached, and the sulphates are thrown down 

 as calcium sulphate, because this is the most insoluble of the sulphates of 

 the bases which abundantly occur in such bodies of water. Where locally 

 barium and strontium are found, these sulphates form in preference. 

 Gypsum is less soluble than niter and sodium chloride, and more soluble 

 than calcium carbonate; therefore it is mainly precipitated after the tufas 

 and before the rock salts. Gypsum is now forming on a somewhat exten- 

 sive scale in lakes with no outlets. In the past it has formed on a very 

 extensive scale in such lakes and has been buried under later deposits. 

 Some of the more important gypsum deposits of the United States are those 

 of Grand Rapids, Michigan, and Fort Dodge, Iowa, and the widespread 

 gypsums which occur in connection with the Red Beds (Permian and 

 Triassic) of western America, 



Gypsum, like rock salt, may by recrystallization become coarsely 

 crystalline. In many localities, because of its somewhat ready solubility, 

 it has formed crystals of great size. The most famous of the localities for 

 large gypsum crystals is that of the Paris Basin. However, the most 

 gigantic crystals known have recently been discovered in Utah, some of 

 them measuring 150 cm. in greatest dimension. 



Gypsum, like niter and rock salt, is a product of the zone of kata- 

 morphism, and mainly of the belt of weathering. 



When gypsum beds become so deeply buried that pressure is dominant, 

 and the heat is somewhat higher than at the surface, the process of dehy- 



"Talmage, J. E., A remarkable occurrence of selenite: Science, vol. 21, 1S93, pp. 85-86. 



