Effect of Sea-Water on Clay Minerals

Abstract

Samples of a montmorillonite, a mixed-layer mineral (mica and montmorillonite) “illite”, kaolinite, and halloysite were immersed in 50 ml sea-water for 10 days, and additional samples of the first three were immersed for 150 days. The exchangeable cations were determined both before and after treatment. It was found that Mg 2+ ions from sea-water moved into the exchange positions in the minerals in preference to Ca 2+ and Na + ions. The H-form of these minerals showed a gradual adjustment to sea-water as measured by change in pH and filling of the exchange positions with cations other than H + . Kaolinite adjusted very rapidly, but montmorillonite and the mixed-layer mineral were slow. All the minerals reacted to yield appreciable amounts of SiO 2 , Al 2 O 3 , and Fe 2 O 3 to the sea-water. The quantities yielded are in the order: montmorillonite > mixed-layer mineral > “illite” > kaolinite > halloysite The solubility is considered to be due to direct solution of SiO 2 in the sea-water and to removal of Al 2 O 3 from the octahedral layer of the minerals. When H-clays were titrated with sea-water three distinct kinds of curves were obtained: (a) kaolinite; (b) mixed-layer mineral, “illite,” and halloysite; and (c) montmorillonite. The curves are similar to those obtained with clay minerals titrated with other alkaline solutions. Kaolinite reacts somewhat like a number of simple acids, but the curves for the other minerals are more complex and are related to the neutralization of H and its replacement in the exchange sites by metallic cations. The exchangeable cations were determined in the minerals after titration, and the results are similar to those obtained after immersing the minerals in sea-water. The volume of sea-water required to reach an end point at about pH 7.6 varies from 11 ml for kaolinite to 135 ml for montmorillonite and is related to the titratable alkalinity of the sea water and to the exchange capacity of the minerals.