ANHYDRITE AND GYPSUM, 357 



THE SULPHATES. 

 ANHYDRITE AND GYPSUM. 



The only important rock-making sulphates are anhydrite and gypsum. 



Anhydrite: 

 CaS0 4 



Orthorhombic. 

 Sp. gr. 2.899-2.985. 



Gypsum: 



CaS0 4 .2H 2 



Monoclinic. 



Sp. gr. 2.314-2.328. 



ANHYDRITE. 



occurrence. — As explained below, the main source of anhydrite is by the 

 alteration of gypsum in the zone of anamorphism. Although I do not 

 know the facts, I conjecture that the anhydrite deposits of Switzerland 

 have had such a history. 



Alterations. — The chief alteration of anhydrite is to gypsum (monoclinic; 

 sp. gr. 2.314-2.328). The reaction is: 



(1) 0aSO 4 +2H 2 O=CaSO 4 .2H 2 O+]£ 



The increase in volume is 60.30 per cent. This alteration is one which 

 takes place in the zone of katamorphism. An interesting case is that of 

 Bex, Switzerland, where the transformation from anhydrite to gypsum has 

 taken place completely to a depth of from 18 to 30 meters, and where 

 below this depth the material is anhydrite. The change of anhydrite to 

 gypsum is with liberation of heat, expansion of volume, hydration, lowering 

 of specific gravity, and lessening of symmetry, and thus stands as a rare 

 example of all the tendencies of the upper physical-chemical zone. 



occurrence. — The most important source of gypsum is as a chemical 

 precipitate, especially in salt lakes having no outlets. It therefore natu- 

 rally occurs in association with halite, calcite, and mechanical detritus. 

 Gypsum also is produced in a subordinate way through fumarole action. 

 The calcium sulphate for the gypsum in either case is produced by the 

 reaction of sulphuric acid or sulphates upon calcium-bearing salts. Com- 



