• journal_title：American Mineralogist
• Contributor：Arkadiusz Derkowski ; Victor A. Drits ; Douglas K. McCarty
• Publisher：Mineralogical Society of America
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2012.3871
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：97
• issue：4
• firstpage：610
• section：Articles

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 R³⁺-O_r-R³⁺ former octahedral structural arrangements (O_r = residual oxygen) over R²⁺-O_r-R (R = R³⁺ or R²⁺ = Al³⁺, Fe³⁺ or Mg²⁺, Fe²⁺). In the case of the R²⁺ octahedral substitution, the interlayer cation is attracted to the electrostatically undersaturated residual oxygen of the R²⁺-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 R³⁺-O_r-R³⁺ 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 R³⁺-O_r-R³⁺ 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.

Dehydroxylated 2:1 layer minerals with a high-rehydroxylation potential, including beidellite and illite, use all the adsorbed water molecules that persist above 200 °C for rehydroxylation; the water vapor from the ambient environment also becomes a source of H₂O molecules for rehydroxylation. The high demand for water molecules to use for rehydroxyltion results in a noticeable gain of mass in the temperature interval between 200 and 350 °C even during heating.