K. N. Dalby

Resolution dependence of petrophysical parameters derived from X-ray tomography of chalk

X-ray computed tomography data from chalk drill cuttings were taken over a series of voxel dimensions, ranging from 320 to 25 nm. From these data sets, standard petrophysical parameters (porosity, surface area, and permeability) were derived and we examined the effect of the voxel dimension (i.e., image resolution) on these properties. We found that for the higher voxel dimensions, they are severely over or underestimated, whereas for 50 and 25 nm voxel dimension, the resulting values (5%–30% porosity, 0.2–2 m2/g specific surface area, and 0.06–0.34 mD permeability) are within the expected range for this type of rock. We compared our results to macroscopic measurements and in the case of surface area, also to measurements using the Brunauer-Emmett-Teller (BET) method and found that independent of the degree of compaction, the results from tomography amount to about 30% of the BET method. Finally, we concluded that at 25 nm voxel dimension, the essential features of the nanoscopic pore network in chalk are captured but better resolution is still needed to derive surface area.

Elements of Eoarchean life trapped in mineral inclusions

Metasedimentary rocks from Isua, West Greenland (over 3,700 million years old) contain 13C-depleted carbonaceous compounds, with isotopic ratios that are compatible with a biogenic origin. Metamorphic garnet crystals in these rocks contain trails of carbonaceous inclusions that are contiguous with carbon-rich sedimentary beds in the host rock, where carbon is fully graphitized. Previous studies have not been able to document other elements of life (mainly hydrogen, oxygen, nitrogen and phosphorus) structurally bound to this carbonaceous material. Here we study carbonaceous inclusions armoured within garnet porphyroblasts, by in situ infrared absorption on approximately 10−21 m3 domains within these inclusions. We show that the absorption spectra are consistent with carbon bonded to nitrogen and oxygen, and probably also to phosphate. The levels of C–H or O–H bonds were found to be low. These results are consistent with biogenic organic material isolated for billions of years and thermally matured at temperatures of around 500 °C. They therefore provide spatial characterization for potentially the oldest biogenic carbon relics in Earth’s geological record. The preservation of Eoarchean organic residues within sedimentary material corroborates earlier claims for the biogenic origins of carbon in Isua metasediments.

Repulsive hydration forces between calcite surfaces and their effect on the brittle strength of calcite‐bearing rocks

The long-term mechanical strength of calcite-bearing rocks is highly dependent on the presence and nature of pore fluids, and it has been suggested that the observed effects are due to changes in nanometer-scale surface forces near fracture tips and grain contacts. In this letter, we present measurements of forces between two calcite surfaces in air and water-glycol mixtures using the atomic force microscope. We show a time- and load-dependent adhesion at low water concentrations and a strong repulsion in the presence of water, which is most likely due to hydration of the strongly hydrophilic calcite surfaces. We argue that this hydration repulsion can explain the commonly observed water-induced decrease in strength in calcitic rocks and single calcite crystals. Furthermore, this relatively simple experimental setup may serve as a useful tool for analyzing surface forces in other mineral-fluid combinations.

Adhesion of Alkane as a Functional Group on Muscovite and Quartz: Dependence on pH and Contact Time

The interactions between mineral surfaces and organic molecules in water control many processes in nature and in the production of modern materials. To improve the understanding of fluid-surface interactions, we investigated the interface behavior of quartz and muscovite, a model for clay minerals, in aqueous solutions where the pH and composition were controlled. We used atomic force microscopy (AFM) in chemical force mapping (CFM) mode to measure adhesion using tips functionalized with alkyl, -CH3. By combining adhesion forces measured as a function of pH, with data from streaming potential experiments and DLVO calculations, we were able to determine the surface charge density. We observed increased adhesion between the mineral surface and the hydrophobic tips as the contact time increased from 7 ms to ∼2 s. The diffusion of dissolved ions takes time, and density functional theory (DFT) calculations did not indicate a strong hydration of the mineral surfaces. Therefore, we interpret that the loss of ions from the confined space between the tip and sample is a likely explanation of the correlation between the dwell time and adhesion. The maximum adhesion increase with dwell time for muscovite, i.e., 400 ± 77 pN, was considerably larger than for quartz, 84 ± 15 pN, which fits with the different surface structure and composition of the two minerals. We propose two mechanisms to explain these results: (1) cations that are structured in the solution and on the surface remain associated at the tip-sample interface initially but diffuse away during extended contact time and (2) adventitious carbon, the organic material that comes spontaneously from air and solution, can diffuse to the tip-sample interface during contact. This material decreases the surface energy by aggregating near the alkyl tip and increases adhesion between the tip and sample.

A one-step delamination procedure to form single sheet iron(III)-(oxy)hydroxides

The dispersion of a layered iron(III)-(oxy)hydroxide intercalated with dodecanoate (oxGRC12, Fe3IIIO2.18(OH)3.13(C12H23O2)0.56(SO4)0.47; derived from the corresponding layered iron(II)–iron(III)-hydroxide) in 0.1 M sodium hydroxide results in delamination of oxGRC12 with formation of separate planar layers of iron(III)-(oxy)hydroxides, here termed single sheet iron-(oxy)hydroxides (SSI). Delamination is confirmed by powder X-ray diffraction while Fourier transform infrared spectroscopy reveals the removal of dodecanoate from the parent compound. Atomic force microscopy shows that SSI has a thickness of 1 nm and the lateral size ranging from 100 to 200 nm. The observed thickness of SSI is double the thickness of the iron(III)-(oxy)hydroxide layers (0.48 nm) which is attributed to water layers adsorbed on both sides of SSI. The SSI shows a high tendency to aggregate. High resolution transmission electron microscopy and selected area electron diffraction confirm that SSI inherits the in-plane crystal structure from its parent layered compound. Both the hyperfine parameters and the X-ray absorption spectrum show a similar local iron coordination in SSI before and after delamination. The extended X-ray absorption fine structure spectra also show that the FeO6 octahedron has expanded to some extent (1–2%) after delamination. O1s X-ray photoelectron spectra of SSI reveal extensive deprotonation of hydroxyl groups in SSI, reflecting a layer charge reversal during delamination. To the best of our knowledge, this is the first report on planar 2D nanosheets of iron-(oxy)hydroxide with the thickness of 1 nm.