GYPSUM DEPOSITS OF NEW YORK 9 



(CaO) 32.5; sulfuric acid anhydrid (SO.) 46.6; water (H2O) 

 20.9. The crystallized variety of g}'psum may show a close ap- 

 proach to these percentages, but the ordinary rock and earthy g\'p- 

 sum employed in the industries contains a variable proportion of 

 foreign matter which amounts generally to several per cent of the 

 whole mass. 



The crystals of gypsum belong to the monoclinic system and are 

 usually formed by a simple combination of faces. According to 

 the relative development of the latter, they may be tabular or flat- 

 tened, prismatic, or elongated into acicular individuals. They are 

 sometimes twinned so as to yield arrowhead forms. The common 

 types of crystals are represented by the accompanying figures. The 



FiG. I Cryst .Is of gypsum. At ths right a twinned forn. (H. P. Whitlock, del.) 



crystals are characterized by an easy cleavage parallel to the prin- 

 cipal plane (face b in figures). Thin flakes so 'produced are flex- 

 ible, but not elastic like mica to which they bear some resemblance. 

 If bent sharply they break in a diagonal direction with the produc- 

 tion of fibers. 



The cleavage plates of gypsum can be. distinguished from foliated 

 talc by their greater hardness, which is inferior, ho^vever, to that 

 of anhydrite or calcite. Gypsum occupies the second place in the 

 Mobs scale of hardness, according to which certain minerals are 

 selected as standard and numbered in order of inicreasing hardness 

 from I to 10. In this scale talc is i and calcite 3. The specific 

 gravity of gypsum when pure is 2.3. 



Gypsum is only slightly soluble in pure water (one part dissolv- 

 ing in 415 parts of water at 32° F. and in 368 parts of water at 

 ioo.4°F.) but its solubility is considerably increased in the presence 

 of salts of the alkalis, such as sodium and potassium chlorids. Con- 

 centrated acids are generally poor solvents, sulfuric having no 



