Ch. 30] AQUEOUS DISPERSION— MOBILITY 541 



of the ore. These are the only investigations of the method to date, 

 and much more work will be necessary before we can say that the 

 chemical approach to glacial prospecting will have general applic- 

 ability. 



Aqueous Dispersion — Mobility 



Water plays the dominant role in the dispersion of weathering prod- 

 ucts in unglaciated terranes. It is not only the medium in which most 

 of the chemical changes in the zone of weathering take place, but is also 

 the agency primarily responsible for the removal of weathering prod- 

 ucts, both soluble and insoluble, from the place of their origin. 



In the weathering cycle, each individual element is exposed to a 

 different series of physical, chemical, and biological reactions between 

 the time it first comes under the influence of weathering agents until 

 it reaches its ultimate site of resedimentation. Although in detail, the 

 individual reaction may be both complex and obscure, yet the summa- 

 tion of the reactions may be determined empirically by observing 

 the rate at which a given element tends to move from its source in 

 parent rock. This rate of movement has been referred to as "mobility" 

 in the cycle of weathering (Polynov, 1937, p. 162 ; Reiche, 1945, pp. 42- 

 45; Smyth, 1913). Elements that tend to remain in gossans and 

 residual soils near their source in the parent rock are said to have a 

 low mobility, whereas those that tend to move out in suspension or 

 solution in ground and surface waters and in the circulatory systems 

 of vegetation are said to have a high mobility. 



The initial factor that affects the mobility of an element is the 

 stability of its primary minerals in the zone of weathering. Thus 

 gold, platinum, tin in cassiterite, tungsten in wolframite, and chro- 

 mium in chromite will tend to have a low mobility, whereas metals in 

 the relatively unstable sulphide minerals will tend to have a corre- 

 spondingly high mobility. 



After the components of the primary mineral have gone into solu- 

 tion, the dominant factor governing mobility is the pH of the water in 

 which the elements are dissolved. As the pH of a dilute aqueous solu- 

 tion of a metal is raised, a point is reached at which that metal starts 

 to precipitate as hydroxide or basic salt. Table 1 shows the pH of 

 hydrolysis of a few common metals. 



Inasmuch as ground and surface waters in normal temperate climates 

 commonly have a pH. of 5.5 or higher, it will be readily seen that iron, 

 aluminum, and copper will tend to be relatively immobile. Experi- 

 mental studies of the copper content of natural waters in which the pH 

 exceeds 5.5 show that it is saturated at very low concentrations, re- 



