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Rehydroxylation of the previously dehydroxylated dioctahedral 2:1 layer clay mineral occurs preferentially in specific sites within the former octahedral sheet. The rehydroxylation of dehydroxylated Al-rich and Al,Mg-rich 2:1 layers occurs as trans-vacant (tv) structural arrangements, regardless of whether the initial structure was tv or cis-vacant (cv). In nontronite (Fe-rich 2:1 layer clay), the dehydroxylate pseudo-cv structure is probably directly reconstructed into the rehydroxylated cv structure without migration of octahedral cations. Rehydroxylation occurs preferentially in the R 3+ -O r -R 3+  former octahedral structural arrangements (Or = residual oxygen) over R 2+ -O r -R (R = R 3+  or R 2+  = Al 3+ , Fe 3+  or Mg 2+ , Fe 2+ ). In the case of the R 2+  octahedral substitution, the interlayer cation is attracted to the electrostatically undersaturated residual oxygen of the R 2+ -O r -R arrangement, which blocks the ability of water molecules to pass through the ditrigonal cavity and rehydroxylate the previously dehydroxylated local arrangement. The pyrophyllite-like type of octahedral R 3+ -O r -R 3+  arrangements, formed due to the lack of tetrahedral substitution and resulting in the absence of interlayer cations, is thus favored for rehydroxylation over the mica-like R 3+ -O r -R 3+  arrangements where Al occurs in the tetrahedral sheet. The valence of the interlayer cation and the charge density of the 2:1 layer clay mineral, which controls the interlayer cation content, also affect the degree of rehydroxylation.

Nature of rehydroxylation in dioctahedral 2:1 layer clay minerals