GYPSUM DEPOSITS OF NEW YORK 



IS 



removal of water by the air the two dehydrations may go on side 

 by side as follows : 



CaSO,. 2 R,0:=CaS04. lA. H,0 + iJ. 

 CaSO,. Vo H/J---CaSO,.-i->4 H,0 



H.O 



This view is further substantiated by the heating of gypsum at 

 98° in an open crucible with the formation of half hydrate in nine 

 hoars and no further loss or change with eight hours heating. 



Connmercial plaster, Davis considers to be made up mainly of the 

 half hydrrite, not soluble anhydrite as held by Cloez, since the 

 water vapor in the mass would immediately hydrate any anhydrite 

 formed, or at least the moisture from the air would soo^n alter it to 

 the half hydrate. Bottled samples of freshly made plaster almost 



Fig. 7 Curves showing rates of dehydration of gypsum under different conditions. * 



(After Davis) 



always show 6 to 8 per cent water and are therefore the half 

 hydrate. 



In summing up then we may say that the change from gypsum 

 to anhydrite is brought about as follows : 



1 CaS04.2H'jO monoclinic to CaS04.2H20 ortliorhombic . . 



2 CaS04.2H20 orthorhombic to CaS04.iH20 orthorhombic 



3 CaS04.iH20 to CaS04 (soluble) orthorhombic 



4 CavS04 (soluble) to CaS04 insoluble (natural anhydrite) 



LOSS IN 

 WATER 



none 

 I ^mols 

 \ mol 

 none 



