Nicholas E. Pingitore

The coprecipitation of Sr2+ with calcite at 25°C and 1 atm

The incorporation of Sr2+ into calcite at earth surface aqueous conditions is affected by the absolute concentration of Sr2+, the presence of Ba2+ and NaCl in the solution, and the rate of precipitation. At solution ratios (molar) of Sr2+ to Ca2+ in the low 10−3 range, which yield calcites with several hundred ppm Sr2+, Ksrcalcite typically assumes a value between 0.10 and 0.20. Above these concentrations the value of kSrcalcite drops to approximately 0.06. Furthermore, if minor amounts of Ba2+ or large amounts of Na+ (0.48 M) are added to a dilute Sr2+ solution, a value around 0.06 for kSrcalciteis found. This “strontium concentration effect” and the associated “competitive cation effect” suggest that small amounts of Sr2+ may be incorporated into a limited number of nonlattice sites in calcite. Incorporation of Sr2+ into these sites, presumably defects, noticeably affects kSrcalcite only at low Sr2+ concentrations and in the absence of competition from other large cations. An increase in kSrcalcite with rate of precipitation, qualitatively similar to that found in other studies, was observed only when precipitation times were decreased from days to hours. For many geologic settings a partition coefficient for Sr2+ into calcite of 0.06 appears appropriate, but there are situations—very low Sr2+ concentrations, the presence of Mg2+, and fast precipitation rates—in which a larger value might better approximate natural partitioning.

Identification of sulfate in natural carbonates by x-ray absorption spectroscopy

Abstract We have analyzed sulfur K-edge X-ray absorption near-edge structures (XANES) and demonstrated, with a high degree of certainty, that sulfur is present in a variety of biogenic and diagenetic carbonates as sulfate. The sulfate is clearly not in the form of gypsum or anhydrite inclusions, and there is good evidence that the sulfate substitutes for carbonate. This finding validates systematic investigations of sulfate distribution in modern and ancient carbonate minerals. Further, it documents that carbonate sedimentation is a sink for marine sulfate, accounting for perhaps 15% of the annual sulfur sedimentary depositional flux.

The experimental partitioning of Ba2+ into calcite

Abstract In a series of 19 runs, the partition coefficient for barium into calcite, k Ba 2+ calcite , was determined to be 0.06 ± 0.01 at 25°C and 1 bar. The partition coefficient is independent of precipitation rate over the range of rates tested. Our value of k Ba 2+ calcite is substantially lower than that previously reported by Y. Kitano and coworkers. This is attributed to our seeding of the solutions with calcite to examine partitioning during calcite growth, rather than relying on spontaneous nucleation. The value of 0.06 for Ba is consistent with values reported for Sr ( k Sr 2+ calcite = 0.05 to 0.14) upon consideration of the radii of the respective divalent ions. The geologic behavior of Ba, as predicted for such a small partition coefficient, is consistent with observations of its preferential exclusion from natural diagenetic calcite relative to the parent aqueous solution.

Barium partitioning during the transformation of corals from aragonite to calcite

Examination of fresh (aragonite) and altered (calcite) corals from the Pleistocene reef terraces exposed on Barbados, West Indies, reveals the natural partitioning of Ba2+ during the aragonite—calcite transformation. The Ba2+ decreased from typical values of 8–15 ppm in the aragonites to 1–3 ppm in the calcites, a finding consistent with the experimentally determined partition coefficient of Ba2+ into calcite (0.06 ± 0.02 at 25°C). Quantitatively, the degree of partitioning of Ba2+ in these samples was compatible with the simultaneous partitioning of Sr2+.

Valences of Iron and Copper in Coral Skeleton: X-ray Absorption Spectroscopy Analysis

Synchrotron-based X-ray absorption spectroscopy (XAS) indicates that both iron and copper are present in their normal fully oxidized states, Fe(III) and Cu(II), in the aragonite (CaCO3, orthorhombic) skeletons of scleractinian reef corals. The Fe(III) may substitute for Ca(II) at its structural sites; alternatively, it may be present in clay or other detritus incorporated in the coral skeleton. The XAS-determined Cu(II) valence is consistent with copper substitution for calcium; Cu(II) is known to coprecipitate preferentially with aragonite. Independent evidence from stepwise dissolution of coral aragonite also suggests substitution of Cu(II) for Ca(II) at the latters structural sites. The low concentrations of both Fe and Cu in the corals (<100 ppm) prevents more detailed structural analysis. Nonetheless, in situ XAS analysis has proved valuable in constraining the general nature of incorporation of these elements into coral.

Surface organic geochemical prospecting for hydrocarbons: multivariate analysis

Abstract A database of analyses of C 1 and C 7 hydrocarbons from an oil and gas producing region in Mexico has been assembled from gas samples collected at depths of 3, 15 and 30 meters from surficial holes drilled in traverses over producing and barren structures. The surface consisted of subtropical swamps; depth to structure was 3500 to 5800 meters. Hydrocarbon analysis from six structures (three producing and three barren) selected from the database were subjected to multiple discriminant function analysis to produce a retrospective statistical test of the ability of geochemical prospecting to distinguish producing from non-producing structures. The hydrocarbon spectra from 3 meters depth yielded ambiguous results; those from 30 meters produced clear distinction between barren and producing structures. Further, the discriminant functions established a base of geochemical characteristics, founded on known areas (retrospective), to which additional unknown areas (prospective) may be compared for classification. This suggests a bootstrapping approach to exploration in which an increasing number of “known” results can be used to continually update and refine the predictive power of the discriminant function. This indicates the practical ability of a combined geochemical-multivariate statistical prospecting approach as an exploration tool, particularly within a single geochemical/geological province. Geochemical prospecting, perhaps with relatively deep (30 m) penetration, combined with multivariate data analysis is a rapid, potent and relatively inexpensive additional tool for petroleum exploration.

X-Ray absorption spectroscopy of uranium at low ppm levels in coral skeletal aragonite

Abstract We performed X-ray absorption spectroscopy of uranium in three coral specimens and present results of analysis of U L-II edge XANES (X-ray absorption near edge structure) spectra. The XANES of two specimens, Pavona gigantea (collected live at Champion Island, Galapagos) and Pavona clavus (collected live from Bartolme, Galapagos) were most similar to XANES of model compounds of U6+. In contrast, the XANES of a specimen of Montastrea annularis (Pleistocene age from an exposed reef terrace on Marie Galante, Guadeloupe) showed some features typical of spectra of model compounds of U4+. These data and interpretations suggest that uranium is incorporated into coral skeletons chiefly in its oxidized U6+ form. The uranium may be subject to postmortem subaerial reduction to U4+, or may be replaced or enhanced by addition of U4+ compounds.

Discrimination of topaz rhyolites by major-element composition: a statistical routine for geochemical exploration

Abstract The recognition of topaz-bearing, calc-alkaline, and peralkaline rhyolites at an early stage of an exploration program may be of both geologic interest and of economic significance due to the different mineral deposits characteristically associated with each of these rock suites. Such discrimination could result in better definition of target areas and commodities to explore within a selected region. A geochemical database of major element analyses of calc-alkaline, peralkaline (comendites), and topaz rhyolites from western North America was assembled. Multiple discriminant function analysis assigned each sample statistically to the calc-alkaline, topaz, or peralkaline groups, using only major element composition. The assignment correctly identified 90% of the rhyolites as members of the groups to which they actually belong. In effect, then, major-element composition can serve as a proxy for fluorine content either when no fluorine analysis is available or when it is suspected that fluorine was not preserved in the rock due to its loss during emplacement or subsequent devitrification. We have included a simplified procedure to use our discrimination model to identify silicic rocks of unknown affiliation. This model makes it possible to evaluate the economic potential of both new or previously explored areas by analyzing data files containing major-element chemical analyses.

Cracks And Fractures in Rock: Geological Analysis with the Acoustic Microscope

Because of its ability to image in three dimensions, the acoustic microscope can become an important tool for the analysis of cracks and fractures in rock. Most rock exhibits fracture, indicating material failure at scales from the global fault systems of plate tectonics to minuscule shears visible only with a microscope. Fractures are of great interest to the geologist, whether they are still open or have been healed by precipitates of such natural cements as calcium carbonate or silica. Healed fractures record the effects of one or more generations of stress fields, from which the tectonic history of the rock may be reconstructed. Open fractures are potential reservoirs and conduits for the storage and migration of water and hydrocarbons, and thus have major economic and social significance.

Imaging Geological Materials with the Acoustic Microscope

Acoustic microscopy, a technique best known in materials science for nondestructive testing, presents exciting and intriguing opportunities for imaging geological materials and for micro-characterization of their elastic properties. In the last dozen years, acoustic microscopy has won broad acceptance in materials science and hightechnology manufacturing, particularly for non-destructive testing of electronic devices. Several vendors market commercial instruments designed and used chiefly to detect delaminations in layered electronic components. Due to the scarcity of commercial or purpose-built instruments in universities, very few studies of geological materials have been made by acoustic microscopy. Notable exceptions include examination of mineral specimens by Bonner (1978) and Morales et al. (1991), of rocks by Dansburg and Yuhas (1978) and Rodriguez-Rey (1990), of fossils by Kustov et al. (1987) and Scott and Hemsley (1991), and of a variety of geologic materials by Pingitore et al. (1993). The objective of our research program, then, is to explore the possible applications of acoustic microscopy in the geosciences, appraise their merit, and pursue the most promising lines of investigation.