THE NONMETALLIC MINEBALS. 415 



5. GLAUBERITE. 



Composition sodium and calcium sulphate. Na 2 SO 4 .CaSO 4 ,=sul- 

 phurtrioxide, 57. 6 per cent; lime, 20.1 per cent; soda, 22. 3 per cent. This 

 is a pale yellow to gray salt, partially soluble in water leaving a white 

 residue of sulphate of lime and with a slightly saline taste. On long 

 exposure to moisture it falls to pieces, and hence is to be found only 

 in protected places or arid areas. It occurs associated with other sul- 

 phates and carbonates, as with thenardite and mirabilite at Borax 

 Lake, in San Bernardino County, California, and with halite in rock 

 salt at Stassfurt (Specimen No. 40229, U.S.N.M.) and other Euro- 

 pean localities. 



6. THENAKDITE. 



Composition anhydrous sodium sulphate. Na 2 SO 4 ,= sulphur triox- 

 ide, 43.7 per cent; soda, 56.3 per cent. Color when pure, white, trans- 

 lucent to transparent; hardness, 2 to 3; specific gravity, 2.68; brittle. 

 In cruciform twins or short prismatic forms roughly striated. Readily 

 soluble in water. Is found in various arid countries, as on the Rio 

 Verde in Arizona, at Borax Lake, California, and Rhodes Marsh in 

 Nevada, associated with other salts of sodium and boron. 



7. EPSOMITE; EPSOM SALTS. 



Composition sulphate of magnesia MgSO 4 -f7H 2 O.= sulphur tri- 

 oxide, 32.5 per cent; magnesia, 16.3 per cent; water, 51.2 per cent. 



This is a soft white or colorless mineral readily soluble in water and 

 with a bitter saline taste. It is a constant ingredient of sea water and of 

 most mineral waters as well. Being readily soluble, it is rarely met with 

 in nature except as an effervescence in mines and caves. In the dry parts 

 of the limestone caverns of Kentucky, Tennessee, and Indiana it occurs 

 in the form of straight acicular needles in the dirt of the floor and in pecu- 

 liar recurved fibrous and columnar forms or in loose snow-white masses 

 on the roofs and walls. (Specimens Nos. 68145, 68153, U. S. N. M. , from 

 Wyandotte Cave, Indiana. ) In all these cases it is doubtless a product of 

 sulphuric acid set free from decomposing pyrites combining with the 

 magnesia of the limestone. It is stated that at the so-called "alum 

 cave" in Sevier County, Tennessee, masses of epsomite very pure and 

 nearly a cubic foot in volume have been obtained. The material in all 

 these cases is of little value, the chief source of the commercial supply 

 being that obtained as a by-product during the manufacture by evapora- 

 tion of common salt (sodium chloride). 



In Albany County, Wyoming, are several lakes, the largest of which 

 has an area of but some 90 acres, in which deposits of epsom salts are 

 formed on a very large scale, but which are of little commercial value, 

 owing to cost of transportation. The material forms compact, almost 



