SULPHATES 109 



tion of the feldspars. These minerals, as explained elsewhere, 

 consist of silicates of alumina and lime, with more or less of 

 the alkalies potash and soda, and iron oxides. In the process 

 of decomposition new compounds are formed, the more soluble 

 of which are leached out, leaving the less soluble silicates, 

 including kaolin, behind in a condition of more or less purity. 

 The material is of great value for fictile purposes, and is de- 

 scribed more fully under the head of argillaceous fragmental 

 rocks. 



(4) SULPHATES 



Gypsum. — The rock gypsum is chemically a hydrous sul- 

 phate of lime, that is to say, consists of sulphur, lime, and 

 water, in the proportion of 32.6 parts of lime and 20.9 parts 

 of water, combined with 46.5 parts of sulphur trioxide. When 

 crystallized, the mineral is nearly colorless and transparent, 

 and splits readily into thin, inelastic sheets. The compact 

 massive varieties are white, gray to black, and sometimes pink 

 from various impurities. The most characteristic feature is 

 its softness, which is such that it can be readily cut with a 

 knife or even by the thumbnail. 



Four varieties of gypsum are recognized: (1) The common 

 massive form, dull in color and often more or less impure; 

 (2) the pure white, fine-grained variety, alabaster; (3) the 

 fibrous variety, satin spar; and (4) the broadly foliated, trans- 

 parent variety, selenite, so called from the Greek word ceXsvi, 

 the moon, in allusion to its soft and pleasing lustre. 



The following is an analysis of a commercial gypsum from 

 Ottawa County, Ohio, as given by Professor Orton :^ 



Lime (CaO) 32.52% 



Sulphurie acid (SOs) .... 45.56 



Water (H^O) 20.14 



Magnesia (MgO) 0.56 



Alumina (AI2O3) 0.16 



Insoluble residue 0.68 



99.62% 



Gypsum occurs mainly associated with stratified rocks, and 

 is regarded as a chemical deposit resulting from the evapora- 

 tion of waters of inland seas and lakes; it may also originate 

 through the decomposition of sulphides and the action of the 



^ Geology of Ohio, 1888, Yol. YI, p. 700. 



