uz. 



1.4 This report was first circulated under date of July 20, 1956. It 

 has since been reviewed by Dr. Floyd L. Culler, Oak Ridge National 

 Laboratory, Oak Ridge , Tennessee, Dr. M. King Hubbert, Shell Oil 

 Company, Inc., Houston, Texas, and Dr. C. V. Theis, U. S. Geolog- 

 ical Survey, Albuquerque, New Mexico. I am greatly indebted to these 

 associates on the Princeton Committee for their critical comments on 

 the paper, which have generally been incorporated in the present re- 

 vision of the report. Any responsibility for errors or other inadequa- 

 cies and for opinions expressed in the report are, however, my own. 



2. CHARACTERISTICS OF SALT DEPOSITS 



2. 1 Rock salt in its crystalline form is the mineral halite (NaCl; 

 sodium 39.4, chlorine 60.6%). Halite is isometric and occurs in crys- 

 tals with cubical cleavage, which are transparent or translucent. 

 Hardness is 2.5. Specific gravity of pure crystal salt is about 2.17 

 (136 lbs. per cu. ft.). Index of refraction is 1.5442. It is highly non- 

 conductive of electricity. The melting point of salt is 801° C. and the 

 boiling point, 1413° C. Solubility in water in grams per 100 ml. is 

 35.7 at 0° C. and 39.12 at 100° C.< 2 H 3 ) 



2.2 In its usual occurrence, rock salt contains impurities. As mined 

 for commercial purposes, it is generally not less than 97% pure, with 

 grades used in the chemical industry over 99% pure. As mined, the 

 specific gravity ranges from 2.1 to 2.6, depending upon the degree of 

 purity. It has a coarse granular to compact structure. Its toughness 

 makes it resistant to mining with power machines and explosives are 

 used in its production in solid form. Its solubility in water permits 

 its solution and extraction as brine . 



2.3 From the geological standpoint, salt is plastic and flows under 

 pressure. In that respect it is similar to ice, but the pressure and 

 time required to produce observable plastic flow in salt are very much 

 greater. The pressure required for the rapid deformation of rock salt 

 is very great but, over long periods of time, much lower pressure may 

 be expected to result in flowage. Plastic movement of rock salt has 

 apparently not been observed in the pillars left in salt mines in the 

 United States, with the amount of overburden as much as 2,000 feet. 



In mining potash in New Mexico, where the depth of the deposit is about 

 900 feet, the sylvinite ore (a mixture of halite , NaCl, and sylvite , KC1) 

 shows positive evidence of plastic flow. Horizontal drill holes in the 

 sylvinite ore show vertical compression of about 25% in about ten years, 

 Sylvite, the principal potash-bearing mineral in the ore, is apparently 

 more plastic than halite . ^ ' 



