cvrsu:*! deposits of new york 



75 



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

 by side as follows : 



CaSO,. 2 H,0 = CaSO,. lA H,0 + iJ.^ H,0 



CaSO,. 14 H,0 



CaSO,J->^ 



H.,0 



This view is further substantiated by the heating of gA'psum at 

 c;8° in an ooen crucible with the formation of half hydrate in nine 

 ho'Urs and no further loss or change with eight hours heating. 



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

 half hydr.-te, not soluble anhydrite as held by Cloez, since the 

 water vapor in the mass would immediately hydrate any anhydrite 

 formed, cr at least the moisture from the air would soon alter it to 

 the half hydrate. Bottled samples of freshly made plaster almost 



/6 flOSAJ-i 



Fig. 7 Curves sliowi g rates cf dehyrlration of gypsum under different condition?. * 



(Af .er 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 g\'psum 

 to anhydrite is brought about as follows : 



1 CaS04.2H20 monoclinic to CaS04.2H20 orthorhombic . . 



2 CaS )4.2n20 orthorhombic to CaS04.Ml20 orthorhombic 



3 ('aS04.|Il20 to CaS()4 (soluble) orthorhombic 



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



LOSS IN 

 WATER J 



none 

 1 Amols 

 \ niol 

 none 



