GYPSUM DEPOSITS OF NEW YORK II 



Anhydrite. This mineral is mentioned here on account of its 

 close relation to gypsum. It differs chemically in the absence of 

 any water of crystallization and like overburned plaster lacks the 

 property of setting when mixed with water. If exposed to weather- 

 ing influences for a long time, however, it will absorb moisture and 

 change to gypsum, a process that has prohably taken place fre- 

 quently in nature. The change involves an increase of 60 per cent 

 in volume. The reverse reaction — the change of gypsum to anhy- 

 drite — may also occur under the influence of heat and pressure 

 superinduced for instance by the burial of gypsum beds beneath a 

 great thickness of overlying strata. 



Anhydrite crystallizes in the orthorhombic system and cleaves in 

 three directions normal to each other. Cleavage fragments are thus 

 rectangular. Its hardness is from 3 to 3.5, noticeably greater than 

 that of gypsum. 



Beds of anhydrite occur frequently in association with gypsum, 

 as might be expected from their similar origin. From a solution of 

 calcium sulfate which is saturated with sodium chlorid ether anhy- 

 drite or gypsum may be deposited according to the temperature.^ 



THE USES OF GYPSUM 

 Crude and ground gypsum 



Ornamental and building stone. Alabaster, the semitranslucent 

 white gypsum, which comes mostly from England, Italy and Spain, 

 finds a demand for ornamental uses, less in this country than 

 abroad. Very little gypsum is now used for ornamental or build- 

 ing purposes in the United States. According to the Iowa Geologi- 

 cal Survey,- the gypsum around Fort Dodge was quite extensively 

 quarried at one time for various kinds of structural work and for 

 sidewalks. The stone has a tendency, it is said, to bleach and crack 

 on the surface when exposed to the sun though it does not actually 

 disintegrate to any harmful extent. 



Agricultural plaster. The most important use of raw gypsum 

 is as a soil amendment, for which purpose the rock, pure or impure 

 as it may be, is simply crushed and ground to a powder. The em- 

 ployment of land plaster is of very ancient origin, going back at 

 least to Roman times, and its beneficial influence has been advo- 

 cated repeatedly by prominent writers on agriculture. But of late 



iVan't Hoff, J. H. & Weigert, F. Sitzungsber. Akad. Berlin, iqoi. 

 p. 1 140. The deposition of anhydrite from sea water takes place at a tem- 

 perature of 25° C. or 70° F. 



2An. Rep't. 1895. v. 3, pt 2. p. 293. 



