Base Exchange in Relation to Composition of Clay with Special Reference to Effect of Sea Water

The term “base exchange,” as commonly used, refers to the alteration in cation composition of a solid resulting from treatment with a salt solution. Many minerals undergo cation exchange to some extent, certain types of clays being especially reactive in this regard. Treatment with ocean water produces notable cation exchange in clays. Although sodium is the dominant base of ocean water, its magnesium plays a greater part in base exchange with clays than sodium. For example, soils recently submerged in ocean water and samples of soil and a bentonitic clay, after treatment with ocean water, have been found to contain somewhat more replaceable magnesium than replaceable sodium. On the other hand, Taylor and Case reported that clays, overlying oil deposits, contain little replaceable magnesium and relatively much replaceable sodium. These facts suggest that the oil-field clays have been in contact with salines quite different from ocean water. The composition of oil-field brines is in harmony with this conclusion. However, it is possible that the base-exchange constituents of the oil-field clays have undergone molecular re-arrangement or crystallization since being deposited in ocean water, with the result that the magnesium has passed into nonreplaceable forms.