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Soil specific surface area (SA) reflects the quantity and quality of the soil mineral and organic fractions. For soil samples that have similar clay mineral types, clay content and or organic carbon (OC) contents are good predictors of SA. However, the magnitude of the OC contribution to SA for different soil types is unclear, particularly since SA varies depending on which method or probe molecule is used to measure it. Consequently, we set out to (i) quantify the contribution of soil OC to total SA for soils with varying clay mineralogy, and (ii) assess the effect of probe molecule (EGME or H2O) and water sorption direction on the contribution of OC to the total SA. We utilized 330 soil samples (clay = 1 to 89 %; silt = 2 to 78 %; OC = 0.03 to 34.9 %) that were grouped according to clay mineralogy and OC content. The total SA was measured using EGME adsorption (SAE) and water adsorption and desorption (SAH2O). The contribution of OC to SA was examined using regression and partial correlation analyses that combined clay, silt, and OC as explanatory variables. Results showed that SAE values were strongly correlated to SAH2O, except for OC-rich soils where SAE was significantly lower than SAH2O. Apart from montmorillonite-rich samples, OC contribution to SAE was smaller than for SAH2O. Organic carbon had a positive contribution to desorption SAH2O for all sample groups, except for montmorillonite-rich samples; OC contribution was 7.5, 10.7, and 13.9 m2/g per %C for illite-rich, kaolinite-rich and OC-rich samples, respectively. There was no significant effect of water sorption direction on the OC contribution to SAH2O. Partial correlation analyses that accounted for clay and silt contents confirmed a negative contribution of OC to SAE or SAH2O for soil samples dominated by montmorillonite clay minerals. We can conclude that for the soil types investigated here (except montmorillonitic soils), OC has a positive contribution to total SA, and the magnitude of the contribution depends on the clay type.

The nature of component layers of mixed-layer illite-smectite and their possible evolution in the course of illitization have been debated since the 1960s. The present study is a new attempt to solve these problems, using samples collected from diverse geological formations around the world. Twenty three purified illite-smectites from bentonites and hydrothermal rocks, covering the complete range of expandability, were analyzed chemically, including ${\rm{NH}}_4^ + $ determination, and their structural formulae were calculated. The exchangeable cations (EXCH) were plotted vs. the fixed cations (FIX) yielding the following experimental regression: $${\rm{EXCH}} = - 0.43 \times {\rm{FIX}} + 0.41\;\;\left( {{{\rm{R}}^2} = 0.98} \right)$$ FIX and EXCH depend on the charge of the illite (Qi) and smectite (Qs) interlayers, and the fractions of these interlayers in the bulk clay leading to: $${\rm{EXCH}} = - {Q_{\rm{s}}}{\rm{/}}{Q_{\rm{i}}} \times {\rm{FIX}} + {Q_{\rm{s}}}$$ Analysis of these relations and independent measurements of the total specific surface area (TSSA) indicate that the layer charges of both types do not change in the course of illitization. The smectite layer charge is equal to 0.41 and the illite layer charge is equal to 0.95 per O10(OH)2. End-member illite has a well defined composition that is close to intermediate between muscovite and phengite, with fixed cations content greater than that specified in the AIPEA classification of layer silicates: $${\rm{FI}}{{\rm{X}}_{0.95}}\left( {{\rm{S}}{{\rm{i}}_{3.25}}{\rm{A}}{{\rm{l}}_{0.75}}} \right)\left( {{\rm{A}}{{\rm{l}}_{1.81}}{\rm{F}}{{\rm{e}}_{0.01}}{\rm{M}}{{\rm{g}}_{0.19}}} \right){{\rm{O}}_{10}}{\left( {{\rm{OH}}} \right)_2}$$ The established relationship allows the calculation of the mean number (N) of 2:1 layers in all fundamental particles and also the fraction of smectitic layers (fs) from FIX: $$N = {Q_{\rm{i}}}{\rm{/}}({Q_{\rm{i}}} - {\rm{FIX}})\;\;{f_{\rm{s}}} = ({Q_{\rm{i}}} - {\rm{FIX}}){\rm{/}}{Q_{\rm{i}}}$$ N and fs can be used to calculate TSSA, and all three parameters can also be calculated from cation exchange capacity and from X-ray diffraction peak positions, utilizing the regressions established here.

The total specific surface area (TSSA) and smectitic layer charge (Q) calculated from the structural formulae and unit-cell dimensions of 12 pure smectite samples were used as a reference in the design and evaluation of TSSA and Q measurement techniques based on cation exchange capacity (CEC), HO retention at 47% RH, and ethylene glycol monoethyl ether (EGME) retention. A thermogravimetric analysis-mass spectrometry (TGA-MS) technique was used to study the release of HO from smectite on heating, and to introduce a correction for HO remaining in the smectite after heating to 110°C, because the sample weight at this temperature has been used routinely as a reference in CEC and EGME sorption measurements. A temperature of 200°C was found to be the optimum reference for such measurements. A good agreement between Q from the structural formula and from CEC was obtained when this correction was applied. The TSSA of smectite was measured with similar accuracy (mean error of ±5-7%) by three techniques: (1) using mean HO coverage; (2) using mean EGME coverage; and (3) using a combination of HO coverage and CEC. A reduction of the mean error from 5-7% to 4% can be obtained by averaging these measurements, and a further reduction to 3% by introducing corrections for the dependence of HO and EGME coverage on layer charge. The study demonstrates that Ca-smectite samples at 47% RH have HO contents corresponding to 88-107% of the theoretical mass of a monolayer and offers an explanation of this variation.