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The microstructure of sedimentary rocks is studied by small-angle neutron scattering for length scales between 5 and 500 A. In limestones and dolomites, we find that the pore surfaces are effectively smooth above 50 A, but there is evidence for roughening on shorter length scales. In sandstones and shales, the pore surfaces show fractal character due to the presence of clay. The fractal dimension is nonuniversal. %e attribute these observations to impurity effects, which can lower the surface tension and maximize the surface area.Microgeometry of rocks is a subject of much scientific interest and practical importance. In the simplest model, one can view the rock as a random two-phase system. There is a pore space and a grain space, separated by a random interface. The disordered nature of this system suggests that it can be used to study fundamental statistical physics and to understand how randomness on a microscale can affect physical properties on a macroscale. This knowledge is important for extracting geological information from petrophysieal measurements.Two interesting recent studies suggest that the pore-grain interfaces in rocks are described by selfsimilar fractals with nonuniversal dimensions (2 & D & 3). In the first study, Avnir, Farin, andPfeifer examined surface-area-measurement data obtained by the molecular absorption technique. ' They showed that the area increases as the size of either the absorbent molecules or the absorbate particles decreases. In the second study, Katz and Thompson (KT) analyzed scanning electron microscope (SEM) and optical images of fractured sandstones, 2 from which they inferred a size distribution of "features. " They concluded that the number of features increases as the feature size decreases. The fractal interpretations of these two studies are not entirely consistent, however. For example, KT suggest that the lower limit of self-similarity is about 20 A in sandstones& awhile much of the absorption data extend down to 4 A. Unfortunately" there are no common samples in the two studies to make a firm comparison. KT also argue that the pore volume is a fractal with the same dimension D as the pore surface. This is an interesting idea not tested by the absorption experiments. In this paper, we report a detailed small-angle neutron-scattering (SANS) study on rocks to provide further understanding of their microgeometry. Specifically, we test the fractal and rough-surface ideas systematically in the three basic types of sedimentary rocks: sandstones, shales, and carbonates (limestones and dolomites).SANS is a powerful technique for the study of rocks or other porous materials, because neutrons can penetrate bulk samples to probe the virgin interior structure. Since the scattering amplitudes of common elements (carbon, oxygen, silicon, etc.) are comparable, for small wave vector q, the neutrons see the rock as a simple two-phase system. In the first Born approximation, the scattering cross section 1(q) is simply proportional to the Fourier transform of the geometric correla...

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Abstract--Sodium-saturated Wyoming bentonite was hydrothermally reacted at 150 ~ and 250~ for 30 to 180 days to determine smectite alteration rates that might be applied to nuclear-waste repository design. Na-Ca solutions deficient in K were used to determine the role of interlayer cations in the creation of high layer-charge in the smectites. The results provide insight into the mechanism and timing of various steps in the diagenetic alteration of smectite to illite. X-ray powder diffraction (XRD) analyses of the reacted clay showed little effect on the character of the 17-/~ reflection even after 180 days at 250~ Potassium saturation of these reacted clays and re-examination by XRD indicated collapse of some smectite layers, leaving at most only 60% expandable layers. The development of layer charge sufficient to cause collapse on saturation with a low hydration energy exchange-cation does not require K in the reacting fluid. Rate constants for the illitization reactions as determined by K-saturated collapse are between 1.0 x 10 -3 and 2.8 x 10-3/day with activation energies <3.5 kcal/mole. Ca in a Na-silicatebicarbonate solution slightly reduced the illitizafion rate constants. These rate constants are higher than expected from extrapolation of studies of beidellite-composition glasses at higher temperatures, but lower than values obtained in studies of natural clays in artificial sea water. The release of Si, A1, and Mg in the 150~ experiments suggests congruent dissolution of the smectite. In contrast, at 250~ the release of A1 was not stoichiometric with Si; as little as one half of the relative available Si was released. Rather than different mechanisms for dissolution at the two temperatures, the conclusion is that noncrystalline Al-rich phases formed at greater rates at higher temperatures. The cation-exchange capacities for several of these reacted smectites were significantly less than expected, suggesting a clogging of interlayer sites, perhaps by Al-complexes.

BackgroundAltered enteroendocrine hormone responses are widely believed to underlie the beneficial effects of bariatric surgery in type 2 diabetes. While elevated postprandial glucagon-like peptide-1 (GLP-1) is considered one of the mediators, increased postprandial glucagon levels have recently been implicated.ObjectivesWe investigated hormonal responses in lean patients after prophylactic total gastrectomy (PTG), as a model of Roux-en-Y gastric bypass without the confounding effects of obesity or massive weight loss.SettingUniversity hospital, United Kingdom.MethodsTen participants after PTG and 9 healthy volunteers were recruited for oral glucose tolerance tests. Plasma glucose, insulin, GLP-1, peptide YY, glucose-dependent insulinotropic-polypeptide, glucagon, oxyntomodulin, glucagon(1-61), and glicentin levels were assessed using immunoassays and/or mass spectrometry.ResultsPTG participants exhibited accelerated plasma glucose appearance, followed, in 3 of 10 cases, by hypoglycemia (<3 mM glucose). Plasma GLP-1, peptide YY, glucose-dependent insulinotropic-polypeptide, glicentin, and oxyntomodulin responses were elevated, and glucagon appeared to rise in PTG participants when measured with a glucagon-specific enzyme-linked immunosorbent assay. We revisited the specificity of this assay, and demonstrated significant cross-reactivity with glicentin and oxyntomodulin at concentrations observed in PTG plasma. Reassessment of glucagon with the same assay using a modified protocol, and by liquid chromatography-mass spectrometry, demonstrated suppression of glucagon secretion after oral glucose tolerance tests in both PTG and control cohorts.ConclusionsCare should be taken when assessing glucagon levels in the presence of elevated plasma levels of other proglucagon products. Substantial elevation of GLP-1 and insulin responses after PTG likely contribute to the observed hypoglycemia, and mirror similar hormone levels and complications observed in bariatric weight loss patients.