Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. II. Rate constants,effective surface area,and the hydrolysis of feldspar

Analysis of experimental data reported by Lagache (1965, 1976), Evans (1965), Busenberg (1975), Busenberg and Clemency (1976), Holdren and Berner (1979), Siegel and Pfannkuch (1984), and Chou and Wollast (1984) with the aid of irreversible thermodynamics and transition state theory (Aagaard and Helgeson, 1977, 1982) suggests that at temperatures at least up to 650°C, the rate of both congruent and incongruent feldspar hydrolysis in aqueous solutions far from equilibrium at pH ? 10.6 ? (2300/T), where T stands for temperature in kelvins, is a function solely of effective surface area and pH at constant pressure and temperature. At higher pH, the rate is apparently pH-independent up to ~pH 8 at 25°C, where it again becomes pH-dependent at higher pH. Observations of scanning electron micrographs indicate that the cross-sectional area of etch pits on hydrolyzed feldspar grains is of the order of 10^?9 to 10^?8 cm² and that the ratio of the effective to total surface area (which may or may not change with reaction progress) ranges from <0.01 to 1, depending on the grain size, dislocation density, and the extent of comminution damage on the surfaces of the grains. Apparent rate constants retrieved from experimental data reported in the literature for feldspar hydrolysis in the lower pH-dependent range extend from ~10^?13 to ~10^?7 moles cm^?2 sec^?1 at temperatures from 25° to 200°C, which is consistent with activation enthalpies for albite and adularia of the order of 20 kcal mole^?1. In contrast, the apparent rate constants for the pH-independent rate law range from ~10^?16 to ~10^?11 moles cm^?2 sec^?1 at temperatures from 25° to 650°C, which requires an activation enthalpy for adularia of ~ 9 kcal mole^?1. These observations are consistent with surface control of reaction rates among minerals and aqueous solutions. The rate-limiting step in the pH-dependent case apparently corresponds at the lower end of the pH scale to breakdown of a protonated configuration of atoms on the surface of the reactant feldspar, but at higher pH the rate is limited by decomposition of an activated surface complex corresponding in stoichiometry to hydrous feldspar. In highly alkaline solutions, an activated complex containing hydroxyl ions apparently controls the rate of feldspar hydrolysis. Nevertheless, near equilibrium, regardless of pH the rate is proportional to the chemical affinity of the overall hydrolysis reaction.     

Book review

Offshore Soil Mechanics edited by Philip George and David Wood. Cambridge University Engineering Department and Lloyd's Register of Shipping, London, 1976. 468 pp. £7.50 + shipping.

Beach Processes and Sedimentation by Paul D. Komar. Prentice‐Hall, Inc. Englewood Cliffs, New Jersey, 1976. 429 pp., $26.95.     

Regional seismic travel times for central Asian epicentres

Summary. Travel times for central Asia studied by Gogna, Jeffreys and Shimshoni are modified. Corrected surface times of P and S are tabulated and a structure is computed. We tabulate also full travel-time tables for all usual depths of focus.     

TREND SURFACE ANALYSIS OF GLACIALLY BURIED PLEISTOCENE ORGANIC DEPOSITS IN CENTRAL MICHIGAN*

Trend surfaces were determined and compared for the bedrock surface, two characteristics of the present topography, and 42 water well log reports of glacially buried carbonaceous horizons within drift from an area in central Michigan. Data almost certainly indicate that the glacially buried Pleistocene organic deposits represent two or more pre-Woodfordian paleosurfaces. Trend surface analysis of buried organic horizons may be useful for recognizing paleosurfaces and separating drift sheets in other areas with limited subsurface data.     

Occurrence of the endangered shortnose sturgeon,Acipenser brevirostrum,in the Delaware River estuary

All available capture records of the endangered shortnose sturgeon,Acipenser brevirostrum, for the Delaware River Estuary from the early nineteeth century to the present were compiled. During 1817 through 1913 some 1,949 captures were reported, most as a bycatch of the shad gill net fishery. No documented captures during 1913 to 1954 were reported in the literature. Thirty-seven shortnose sturgeon were reported captured from 1954 through 1979, mostly incidental to fishery and ecological studies. Most specimens were taken in the upper tidal freshwater portion of the estuary (rkm 200–214). Seasonal-spatial distribution appeared similar to that observed for northern shortnose sturgeon populations. Taxonomic data obtained from seven specimens generally agreed with those from other drainages.     

Considerations on the magnitude distributions of the Kuiper belt and of the Jupiter Trojans

By examining the absolute magnitude (H) distributions (hereafter HD) of the cold and hot populations in the Kuiper belt and of the Trojans of Jupiter, we find evidence that the Trojans have been captured from the outer part of the primordial trans-neptunian planetesimal disk. We develop a sketch model of the HDs in the inner and outer parts of the disk that is consistent with the observed distributions and with the dynamical evolution scenario known as the ‘Nice model’. This leads us to predict that the HD of the hot population should have the same slope of the HD of the cold population for 6.5<H<9, both as steep as the slope of the Trojans' HD. Current data partially support this prediction, but future observations are needed to clarify this issue. Because the HD of the Trojans rolls over at H∼9 to a collisional equilibrium slope that should have been acquired when the Trojans were still embedded in the primordial trans-neptunian disk, our model implies that the same roll-over should characterize the HDs of the Kuiper belt populations, in agreement with the results of Bernstein et al. [Bernstein, G.M., and 5 colleagues, 2004. Astron. J. 128, 1364-1390] and Fuentes and Holman [Fuentes, C.I., Holman, M.J., 2008. Astron. J. 136, 83-97]. Finally, we show that the constraint on the total mass of the primordial trans-neptunian disk imposed by the Nice model implies that it is unlikely that the cold population formed beyond 35 AU.     

THE PARENT STRATUM AND DISTRIBUTION OF THE GEORGIA TEKTITES

Seventy-six field expeditions for Georgia tektites have been made to the vicinity of Dodge County, Georgia in an effort to determine the parent formation and distribution pattern by locating new specimens. About 67 new specimens have been recovered; they represent about 9 percent of all known specimens. The primary area of distribution is a fan shaped area approximately 30 by 72 kilometers. It is likely that these tektites have been transported by water from the parent formation. No correlation could be found between size, shape, soil type, elevation and tektite location. Georgia tektites have previously been found only in strata much younger than their 34 m.y. radiometric age. Here, however, the parent formation is identified as the Tobacco Road Sand of this age.