THE SOIL PARTICLE 



89 



The specific gravity 1 of a soil is obviously the average spe- 

 cific gravity of the particles. It is unaffected by the structure, 

 remaining the same whether the soil is loose and open or com- 

 pact and unaerated. Although a great range is observed in 

 the specific gravities of the common soil minerals 2 , the spe- 

 cific gravity of a purely mineral soil varies between the nar- 

 row limits of 2.6 and 2.7. This occurs because quartz and 

 feldspar, whose specific gravities are about 2.65 and 2.57, 

 respectively, usually make up the bulk of the mineral portion 

 of most soils. The fineness of the particles seems to have no 

 appreciable effect on specific gravity as shown by the follow- 

 ing data from Whitney and Smith 3 : 



1 Specific gravity is expressed as a ratio of the weight of any volume 

 of a substance to the weight of an equal volume of some other substance 

 taken as a standard unit. Liquids and solids are usually compared with 

 water at its maximum density (4° C). 



2 The specific gravities of some of the common soil minerals are as 

 follows: 



Quartz 2.60-2.70 



Orthoclase 2.57 



Plogioclase 2.62-2.76 



Muscovite 2.76-3.00 



Biotite 2.70-3.10 



Hornblende 3.05-3.47 



Augite 3.20-3.60 



Apatite 3.20 



Kaolinite 2.60-2.63 



Serpentine 2.50-2.65 



Chlorite 2.65-2.92 



Epidote 3.25-3.50 



Hematite 4.90-5.30 



Limonite 3.60-4.00 



3 Whitney, M., Some Physical Properties of Soils; U. S. Dept. Agr., 

 Weather Bur., Bui. 4, 1892. Smith, Alfred, Eelation of the Mechanical 

 Analysis to the Moisture Equivalent of Soils; Soil Sci., Vol. IV, No. 6. 

 p. 472, Dec, 1917. 



