COMPOSITION, DISTRIBUTION, AND EXCAVATION OF GYPSUM. 15 



etc. Table 8, on page 53, gives a number of analyses of the gypsum 

 used at various plaster-plants; and a glance at this table will show 

 the kind and amount of impurities which may be expected to occur in 

 commercial gypsum. 



Varieties of gypsum. Owing to differences in form, texture, color, 

 etc., gypsum presents several varieties, some of which have been given 

 distinct names. The ordinary form in which gypsum occurs in the 

 workable deposits is as massive or rock gypsum. Alabaster is a pure 

 white, fine-grained massive gypsum, occasionally used for statuary, etc. 

 The term selenite is applied to the crystalline, white, almost transparent 

 gypsum which occurs frequently, but in relatively small quantity, scattered 

 through a deposit of massive gypsum. 



Aside from these various forms of rock gypsum, two less massive 

 forms of the mineral are to be noted as being of commercial importance. 

 In certain Western States and Territories deposits of earthy gypsum, gyp- 

 sum earth, or gypsite occur. These deposits contain an impure, earthy, 

 granular form of gypsum. Deposits of gypsum sands are also found 

 in the West, being dunes or heaps of fine grains of gypsum. 



Physical properties. Pure gypsum is white and, when in the crystal- 

 line form, translucent. The impurities which it commonly contains 

 usually destroy its translucency and affect its color, so that the mineral 

 as mined is an opaque, fine-grained mass, varying from white to reddish, 

 gray, or brown in color. 



Gypsum can be distinguished from most other minerals by its extreme 

 softness, for even when in the crystalline form it can be readily scratched 

 by the finger-nail. When treated with acids it does not effervesce. 

 On heating it loses its water of crystallization and, if previously trans- 

 lucent, becomes a chalky, opaque white. Pure crystalline specimens 

 have a specific gravity * of 2.30 to 2.33. 



Anhydrite. The mineral anhydrite is closely related to gypsum, 

 as it is an anhydrous lime sulphate, with the formula CaSO4. It there- 

 fore corresponds in composition to the product obtained by heating 

 gypsum so strongly as to drive off all of its water of combination (see 

 pages 31, 32). Anhydrite occurs, but in relatively small amounts, in 

 almost all gypsum deposits. Pure specimens have a specific gravity * 

 of 2.92 to 2.98. 



Occurrence and origin of gypsum deposits. Rock gypsum occurs 

 in the form of beds, frequently closely associated with beds of rock 

 salt, and almost always interstratified with thin beds of limestone and 



* Clarke, F. W. Constants of Nature, Part I, pp. 81, 82. 



