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Results for “Journal of Raman Spectroscopy”

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USGS2014

Raman spectroscopic characterization of CH4 density over a wide range of temperature and pressure

The positions of the CH 4 Raman ν 1 symmetric stretching bands were measured in a wide range of temperature (from −180 °C to 350 °C) and density (up to 0.45 g/cm 3 ) using high-pressure optical cell and fused silica capillary capsule. The results show that the Raman band shift is a function of both methane density and temperature; the band shifts to lower wavenumbers as the density increases and the temperature decreases. An equation representing the observed relationship among the CH 4 ν 1 band position, temperature, and density can be used to calculate the density in natural or synthetic CH 4 -bearing inclusions.

Journal of Raman Spectroscopy↗

USGS2019

Shift in the Raman symmetric stretching band of N2, CO2, and CH4 as a function of temperature, pressure, and density

The Raman spectra of pure N 2 , CO 2 , and CH 4 were analyzed over the range 10 to 500 bars and from −160°C to 200°C (N 2 ), 22°C to 350°C (CO 2 ), and −100°C to 450°C (CH 4 ). At constant temperature, Raman peak position, including the more intense CO 2 peak ( ν +), decreases (shifts to lower wave number) with increasing pressure for all three gases over the entire pressure and temperature ( PT ) range studied. At constant pressure, the peak position for CO 2 and CH 4 increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N 2 first shows an increase in peak position with increasing temperature at constant pressure, followed by a decrease in peak position with increasing temperature. The inflection temperature at which the trend reverses for N 2 is located between 0°C and 50°C at pressures above ~50 bars and is pressure dependent. Below ~50 bars, the inflection temperature was observed as low as −120°C. The shifts in Raman peak positions with PT are related to relative density changes, which reflect changes in intermolecular attraction and repulsion. A conceptual model relating the Raman spectral properties of N 2 , CO 2 , and CH 4 to relative density (volume) changes and attractive and repulsive forces is presented here. Additionally, reduced temperature-dependent densimeters and barometers are presented for each pure component over the respective PT ranges. The Raman spectral behavior of the pure gases as a function of temperature and pressure is assessed to provide a framework for understanding the behavior of each component in multicomponent N 2 -CO 2 -CH 4 gas systems in a future study.

Journal of Raman Spectroscopy↗

USGS2021

Compositional evolution of organic matter in Boquillas Shale across a thermal gradient at the single particle level

The molecular composition of petroliferous organic matter and its compositional evolution throughout thermal maturation provides insight for understanding petroleum generation. This information is critical for understanding hydrocarbon resources in unconventional reservoirs, as source rock organic matter is highly dispersed, in contact with the surrounding mineral matrix, and may occur as multiple organic matter maceral types. Here, Raman spectroscopy and optical microscopy approaches were applied to a marginally mature (vitrinite reflectance ~0.5%) sample of the Late Cretaceous Boquillas Shale before and after hydrous pyrolysis (HP) at 300 °C and 330 °C for 72 h. This analytical approach allowed for correlative examination of micro-scale changes in organic matter compositional properties (e.g., aromaticity) for a variety of organic matter macerals across a thermal gradient (from marginally mature into the late oil/wet gas window) at the single particle level. Results indicate that while the examined amorphous organic matter, solid bitumen, and vitrinite particles exhibit different aromatic signatures in the unheated shale, they effectively progress along a similar trend through composition space with thermal maturation. Examined inertinite fragments were generally insensitive to the applied thermal stress, reinforcing the idea that reservoir temperature may be secondary for dictating the molecular composition of inertinite. Additional analysis of Raman spectra for individual organic matter macerals was performed using multivariate curve resolution (MCR) and correlation of standard Raman and reflectance-derived thermal maturity proxies against MCR parameters shows consistent trends. This trend suggests that MCR may be a fast and statistically robust method for extracting compositional information from Raman spectra of sedimentary organic matter, and can be used to construct thermal maturity relationships. These findings inform our understanding of how different petroliferous organic matter maceral types evolve throughout thermal reactions and further demonstrate that Raman spectroscopy combined with petrographic analysis can provide complementary estimates of organic matter composition and thermal maturity.

International Journal of Coal Geology↗

USGS2019

Understanding organic matter heterogeneity and maturation rate by Raman spectroscopy

Solid organic matter (OM) in sedimentary rocks produces petroleum and solid bitumen when it undergoes thermal maturation. The solid OM is a ‘geomacromolecule’, usually representing a mixture of various organisms with distinct biogenic origins, and can have high heterogeneity in composition. Programmed pyrolysis is a common method to reveal bulk geochemical characteristics of the dominant OM, while detailed organic petrography is required to reveal information about the biogenic origin of contributing macerals. Despite the advantages of programmed pyrolysis, it cannot provide information about the heterogeneity of chemical compositions present in the individual OM types. Therefore, other analytical techniques such as Raman spectroscopy are necessary. In this study, we compared geochemical characteristics and Raman spectra of two sets of naturally and artificially matured Bakken source rock samples. A continuous Raman spectral map on solid bitumen particles was created from the artificially matured hydrous pyrolysis residues, in particular, to show the systematic chemical modifications in microscale. Spectroscopic data was plotted for both sets against thermal maturity to compare maturation rate/path for these two separate groups. The outcome showed that artificial maturation through hydrous pyrolysis does not follow the same trend as naturally-matured samples although having similar solid bitumen reflectance values (%SBRo). Furthermore, Raman spectroscopy of solid bitumen from artificially matured samples indicated the heterogeneity of OM decreases as maturity increases. This may represent an alteration in chemical structure towards more uniform compounds at higher maturity. This study may emphasize the necessity of using analytical methods such as Raman spectroscopy along with conventional geochemical methods to better reveal the underlying chemical structure of OM. Finally, observation by Raman spectroscopy of chemical alteration of OM during artificial maturation may assist in the proposal of improved pyrolysis protocols to better resemble natural geologic processes.

North Dakota↗

USGS1997

Measurement of clathrate hydrates via Raman spectroscopy

Raman spectra of clathrate hydrate guest molecules are presented for three known structures ( I ( sI ), II ( sII ), and H ( sH )) in the following systems: CH 4 ( sI ), CO 2 ( sI ), C 3 H 8 ( sII ), CH 4 + CO 2 ( sI ), CD 4 + C 3 H 8 ( sII ), CH 4 + N 2 ( sI ), CH 4 + THF- d 8 ( sII ), and CH 4 + C 7 D 14 ( sH ). Relative occupancy of CH 4 in the large and small cavities of sI were determined by deconvoluting the ν 1 symmetric bands, resulting in hydration numbers of 6.04 ± 0.03. The frequency of the ν 1 bands for CH 4 in structures I, II, and H differ statistically, so that Raman spectroscopy is a potential tool to identify hydrate crystal structure. Hydrate guest compositions were also measured for two vapor compositions of the CH 4 + CO 2 system, and they compared favorably with predictions. The large cavities were measured to be almost fully occupied by CH 4 and CO 2 , whereas only a small fraction of the small cavities are occupied by CH 4 . No CO 2 was found in the small cavities. Hydration numbers from 7.27 to 7.45 were calculated for the mixed hydrate.

Journal of Physical Chemistry B↗

USGS2021

A review of spatially resolved techniques and applications of organic petrography in shale petroleum systems

This review examines new techniques and applications of organic petrography in source-rock reservoir petroleum systems that have occurred along with development of the global ‘shale revolution’ in energy resources. The review is limited to techniques and instrumentation that provide spatially resolved information, typically at or below microscales, for dispersed organic matter occurring in situ in samples of shale and mudrock. A brief summary of ion beam sample preparation is followed by discussion of the most common analytical techniques and applications. Advantages and limitations of each technique, including requisite sample preparation, types of information generated [e.g., molecular or elemental (isotopic) abundance], sensitivity, and resolution are discussed. In a few cases, techniques not yet applied to organic petrology of shale or mudrock are described (e.g., X-ray photoelectron spectroscopy imaging), in anticipation of near-term future application. The most common in situ techniques applied for characterization of organic matter in shale and mudrock include optical (including fluorescence) and electron microscopies, Raman, fluorescence, and infrared spectroscopies, and surficial measurements via force microscopy. Techniques that show growing application to organic petrography of shale include tip-enhanced photothermal infrared spectroscopy, mass spectrometry imaging, and synchrotron-based spectroscopies, among others. It is anticipated that the future of dispersed organic matter petrography will hold continued development of integrated instrument techniques (e.g., simultaneous or sequential correlative microscopies and/or spectroscopies of the same location), increased instrumental resolution, increased use of multiscale and multimodal organic petrography investigations, and three-dimensional imaging and chemical speciation mapping applications via multiple analytical approaches.

International Journal of Coal Geology↗

USGS1977

Feasibility and technology for making remote measurements of solutes in water

An indepth evaluation of the available technology in the field of laser-Raman spectroscopy indicates that a TV-type detector, a single monochromator with a holographic grating, an entrance slit filter blocking the Rayleigh light, and a pulsed laser coupled to signal averaging electronics is the best combination of commercial equipment that is presently available for building a remote water-quality sensor. The resultant sensor would be capable of measuring oxyanions in water at concentrations from 10 to 50 milligrams per liter at distances from ground level to 30 meters above the sample. The main interferences would be ambient light, bioluminescence, and natural fluorescence, all of which are minimized when taking advantage of the signal generating and readout capability contained in this equipment package.

Journal of Research of the U.S. Geological Survey↗

USGS2001

Search for life on Mars in surface samples: Lessons from the 1999 Marsokhod rover field experiment

The Marsokhod 1999 field experiment in the Mojave Desert included a simulation of a rover-based sample selection mission. As part of this mission, a test was made of strategies and analytical techniques for identifying past or present life in environments expected to be present on Mars. A combination of visual clues from high-resolution images and the detection of an important biomolecule (chlorophyll) with visible/near-infrared (NIR) spectroscopy led to the successful identification of a rock with evidence of cryptoendolithic organisms. The sample was identified in high-resolution images (3 times the resolution of the Imager for Mars Pathfinder camera) on the basis of a green tinge and textural information suggesting the presence of a thin, partially missing exfoliating layer revealing the organisms. The presence of chlorophyll bands in similar samples was observed in visible/NIR spectra of samples in the field and later confirmed in the laboratory using the same spectrometer. Raman spectroscopy in the laboratory, simulating a remote measurement technique, also detected evidence of carotenoids in samples from the same area. Laboratory analysis confirmed that the subsurface layer of the rock is inhabited by a community of coccoid Chroococcidioposis cyanobacteria. The identification of minerals in the field, including carbonates and serpentine, that are associated with aqueous processes was also demonstrated using the visible/NIR spectrometer. Other lessons learned that are applicable to future rover missions include the benefits of web-based programs for target selection and for daily mission planning and the need for involvement of the science team in optimizing image compression schemes based on the retention of visual signature characteristics.

California↗

USGS1994

Raman spectroscopy of garnet-group minerals

The Raman spectra of the natural end members of the garnet-group minerals, which include pyrope, almandine and spessarite of Fe-Al garnet series and grossularite, andradite and uvarovite of Ca-Fe garnet series, have been studied. Measured Raman spectra of these minerals are reasonably and qualitatively assigned to the internal modes, translational and rotatory modes of SiO 4 tetrahedra, as well as the translational motion of bivalent cations in the X site. The stretch and rotatory Al g modes for the Fe-Al garnet series show obvious Raman shifts as compared with those for the Ca-Fe garnet series, owing to the cations residing in the X site connected with SiO 4 tetrahedra by sharing the two edges. The Raman shifts of all members within either of the series are attributed mainly to the properties of cations in the X site for the Fe-Al garnet series and in the Y site for the Ca-Fe garnet series.

Chinese Journal of Geochemistry↗

USGS2024

Deep syntectonic burial of the Anthracite belt, Eastern Pennsylvania

Fluid inclusion microthermometry and Raman spectroscopy of fluid inclusions in quartz veins from the Pennsylvanian rocks of the Anthracite belt, eastern Pennsylvania support a deep burial model of coalification in favor of focused orogenic hot fluid flow. High-temperature (250 to 255 °C) trapping of CH 4 ± CO 2 saturated aqueous fluids and CH 4 ± CO 2 inclusions indicate fluid trapping at depths of 11.5 to 13.4 km under a cover of Pennsylvanian to Permian(?) syntectonic load. In the folded rocks to the south of the Anthracite belt, CH 4 ± CO 2 fluid inclusions indicate a sediment load that was up to 16.3 km thick. Re -equilibrated aqueous fluid inclusions from veins in Silurian through Devonian rocks give the same range of trapping conditions but a wide range of fluid salinities suggesting that folding, fracturing, and meteoric recharge resulted in the intermixing of fluids from throughout the stratigraphic succession.

Pennsylvania↗

USGS2022

Relating systematic compositional variability to the textural occurrence of solid bitumen in shales

This study presents Raman spectroscopic data paired with scanning electron microscopy (SEM) images to assess solid bitumen composition as a function of solid bitumen texture and association with minerals. A series of hydrous pyrolysis experiments (1–103 days, 300–370 °C) using a low maturity (0.25% solid bitumen reflectance, BR o ), high total organic carbon [(TOC), 14.0 wt%] New Albany Shale sample as the starting material yielded pyrolysis residues designed to evaluate the evolution of solid bitumen aromaticity with increasing temperature and heating duration. Solid bitumen was analyzed by Raman spectroscopy wherein point data were collected from accumulations that ranged in size and degree of association with the mineral matrix. Raman spectroscopy results show that with increasing temperature and experimental duration, solid bitumen aromaticity increases and compositional variability decreases. With regards to texture and composition, coarser-grained solid bitumen (>1.3 μm from nearest mineral grain) has consistently higher, but less variable aromaticity than thinner, wispy solid bitumen which is more intimately associated with the mineral matrix. Collocated scanning electron microscope images were used to provide qualitative assessments of porosity hosted by the organic matter. These paired data sets suggest that solid bitumen porosity development and molecular composition are linked, and these parameters are related to the textural relationships of the organic matter within the whole rock. These results are discussed with perspective towards understanding how rock fabric and texture can influence organic matter evolution during thermal maturation of organic-rich marine shales and inform our broader understanding of these important energy resources.

International Journal of Coal Geology↗

USGS2022

Evaluating aromatization of solid bitumen generated in the presence and absence of water: Implications for solid bitumen reflectance as a thermal proxy

Geological models for petroleum generation suggest thermal conversion of oil-prone sedimentary organic matter in the presence of water promotes increased liquid saturate yield, whereas absence of water causes formation of an aromatic, cross-linked solid bitumen residue. To test the influence of hydrogen from water, organic-rich (22 wt% total organic carbon , TOC) mudrock samples from the Eocene lacustrine Green River Formation Mahogany zone oil shale were pyrolyzed under hydrous and anhydrous conditions in closed system batch reactors at temperatures between 300 and 370 °C for 72 h. Pre- and post-pyrolysis samples were characterized using petrographic approaches including optical microscopy , reflectance, Raman spectroscopy , and scanning electron and transmission electron microscopy to quantify differences in relative appearance, abundance, and composition of solid bitumen newly generated during the pyrolysis experiments. Petrographic analyses were supplemented by geochemical screening measurements (TOC content and programmed temperature pyrolysis). Results show post-hydrous pyrolysis residues contain lower TOC, are comprised of solid bitumen with greater aromaticity , and have textures indicative of lower viscosities, relative to anhydrous residues from the same temperature pyrolysis conditions. These observations suggest solid bitumen forming from thermal conversion of oil-prone sedimentary organic matter under anhydrous conditions may be less aromatic, although more cross-linked, than solid bitumen forming under hydrous conditions at the same time-temperature combination. To explain these results, we suggest that a radical disproportionation mechanism is favored in the presence of hydrogen donated from water, and that this disproportionation promotes aromatization in the solid residue with concomitant expulsion of saturated hydrocarbons.

International Journal of Coal Geology↗

USGS2024

Depths in a day - A new era of rapid-response Raman-based barometry using fluid inclusions

Rapid-response petrological monitoring is a major advance for volcano observatories, allowing them to build and validate models of plumbing systems that supply eruptions in near-real-time. The depth of magma storage has recently been identified as high-priority information for volcanic observatories, yet this information is not currently obtainable via petrological monitoring methods on timescales relevant to eruption response. Fluid inclusion barometry (using micro-thermometry or Raman spectroscopy) is a well-established petrological method to estimate magma storage depths and has been proposed to have potential as a rapid-response monitoring tool, although this has not been formally demonstrated. To address this deficiency, we performed a near-real-time rapid-response simulation for the September 2023 eruption of Kīlauea, Hawaiʻi. We show that Raman-based fluid inclusion barometry can robustly determine reservoir depths within a day of receiving samples — a transformative timescale that has not previously been achieved by petrological methods. Fluid inclusion barometry using micro-thermometric techniques has typically been limited to systems with relatively deep magma storage (>0.4 g/cm 3 or >7 km) where measurements of CO 2 density are easy and accurate because the CO 2 fluid homogenizes into the liquid phase. Improvements of the accuracy of Raman spectroscopy measurements of fluids with low CO 2 density over the past couple of decades has enabled measurements of fluid inclusions from shallower magmatic systems. However, one caveat of examining shallower systems is that the fraction of H 2 O in the fluid may be too high to reliably convert CO 2 density to pressure. To test the global applicability of rapid response fluid inclusion barometry, we compiled a global melt inclusion dataset (>4000 samples) and calculate the fluid composition at the point of vapor saturation (⁠X H 2 O⁠ ). We show that fluid inclusions in crystal-hosts from mafic compositions (<57 wt. % SiO 2 ) — likely representative of magmas recharging many volcanic systems worldwide — trap fluids with X H 2 O low enough to make fluid inclusion barometry useful at many of the world’s most active and hazardous mafic volcanic systems (e.g., Iceland, Hawaiʻi, Galápagos Islands, East African Rift, Réunion, Canary Islands, Azores, Cabo Verde).

Hawaii↗

USGS2023

Thermal evolution of graptolite and solid bitumen properties at high maturity under natural and artificial conditions

To refine the use of graptolite and solid bitumen as thermal proxies at overmature conditions, we evaluated their evolution via Raman and infrared (IR) spectroscopies, reflectance, and geochemical screening using high-temperature pyrolysis experiments in comparison to naturally matured samples. Naturally matured samples included marine shales from the overmature Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations (herein referred to as Wufeng-Longmaxi) of the Sichuan Basin, China. Immature samples for pyrolysis experiments included Mesoproterozoic (Ectasian) Xiamaling marine shale from Hebei, China (graptolite absent) and graptolite-bearing Ordovician (Tremadocian) Alum marine shale from Grönhögen, Sweden. Pyrolysis experiments at 360 °C for 10 days and 27 days (hydrous conditions), and 450 °C 3 days, 550 °C 3 days, and 550 °C 10 days (all anhydrous) created Xiamaling and Alum residues with solid bitumen and graptolite, respectively, of similar and higher maturity compared to the naturally matured Wufeng-Longmaxi samples. Compositional proxies from IR spectroscopy were inconclusive. Raman spectral properties including Raman band separation (RBS) and the full-width at half-maximum of the G-band (G-FWHM) exhibited robust positive and inverse relationships with increasing reflectance in pyrolysis residues, respectively. RBS values were systematically lower in artificially matured samples versus naturally matured samples with equivalent reflectance values, whereas G-FWHM values were systematically higher. This observation indicates that the limited duration or low internal reactor pressure of pyrolysis experiments is insufficient to allow alignment or ordering of aromatic carbon arrays in both organic matter types, relative to natural maturation occurring at geologic time scales. Reflectance of graptolite was typically higher than co-occurring solid bitumen in Wufeng-Longmaxi samples and the untreated Alum sample, suggesting inherently higher aromaticity. However, aromaticity in pyrolysis residues was systematically higher in solid bitumen compared to graptolite with equivalent reflectance values, indicating a higher kinetic barrier to molecular rearrangement in graptolite versus solid bitumen. These data further clarify differences in the molecular evolution of sedimentary organic matter in natural versus analogue laboratory environments and distinguish the properties of individual organic matter types in response to thermal stress.

International Journal of Coal Geology↗

USGS2026

Petrological, textural, and volatile constraints on the architecture of Mauna Loa’s magmatic system over 170 years (1852–2022)

Mauna Loa’s reawakening in 2022 highlighted the limited availability of petrological constraints characterizing the geometry and dynamics of pre-eruptive storage at this volcano. In particular, there has been relatively little prior work constraining pre-eruptive volatile budgets, mineral zoning and textures. To address this, we examine olivine- and orthopyroxene-hosted melt inclusions (MIs), their mineral hosts, and matrix glasses from seven Mauna Loa eruptions spanning 170 years (1852–2022). We present the first extensive MI dataset from Mauna Loa with direct Raman measurements of vapor bubble CO 2 . Olivine- and orthopyroxene-hosted MIs contain 0.3–0.5 wt % H 2 O and 100–600 μg/g CO 2 . MI saturation pressures reveal that the dominant reservoir is located at ~2–5 km depth, consistent with fluid inclusion barometry and geophysical constraints. This depth has remained consistent over the past 170 years, although some eruptions appear to have tapped a narrower range of depths than others. Even the high-Mg# orthopyroxene phenocryst-hosted MIs record saturation pressures of <150 MPa (<6 km depth), which is inconsistent with a deep sub-Moho origin proposed by previous studies that rely on thermodynamic modeling. Our Raman measurements also provide an opportunity to assess the accuracy of different methods to reconstruct the amount of CO 2 in vapor bubbles, with applications to MI datasets published without direct Raman measurements. In general, models that partition CO 2 between the melt and bubble using the bubble volume prior to pre-eruptive quenching (e.g. Melt Inclusion Modification Corrections, MIMiC) systematically underestimate CO 2 contents by a median of ~100–200 μg/g (depending on the model and K D choice) and thus underestimate storage depths by ~0.5–1.5 km. While these methods are useful for first-order estimates of bubble CO 2 in published datasets, new studies should measure bubble CO 2 by Raman spectroscopy or perform MI rehomogenization. The zoning and textures of the crystals that host the MIs provide additional constraints on the plumbing system to supplement MI barometry. Orthopyroxene crystals exhibit varied zoning patterns, including oscillatory zoning that records repeated fluctuations in magmatic conditions, and sector zoning indicating rapid, kinetically controlled crystal growth. While faster Fe-Mg diffusion in olivine results in a simpler zoning record, complexly zoned olivines with higher-Fo ‘shoulders’ from 1852, 1868, and 1950 also record the injection of more primitive melts at shallow levels. Electron backscatter diffraction reveals that touching crystals in orthopyroxene glomerocrysts exhibit only very weakly systematic crystallographic orientation relationships, perhaps due to rapid aggregation in a dynamic system. Overall, our data are consistent with a large, shallow (~2–5 km) pre-eruptive reservoir repeatedly recharged by primitive magma injections that homogenized prior to eruption due to their small volume relative to the overall size of this reservoir. This chemical complexity is effectively recorded in mineral and MI compositions (orthopyroxene zoning in particular) but is obscured in glass and whole-rock analyses.

Hawaii↗

USGS2018

High microscale variability in Raman thermal maturity estimates from shale organic matter

Raman spectroscopy has recently received attention as a means to estimate thermal maturity of organic matter in petroleum generating source rocks to complement more traditional approaches such as vitrinite reflectance and programmed pyrolysis. While many studies have observed positive correlations between source rock thermal maturity and Raman spectral parameters, little attention has been given to the degree of variation in the Raman response across individual organic grains, especially for shales or mudrocks with highly dispersed organic matter. Here the spatial variation in Raman estimates of thermal maturity within individual organic grains is assessed from shales from the Boquillas, Marcellus, Niobrara, and Woodford Formations. The thermal maturity parameters extracted from Raman spectra can vary widely across distances of ≤5 μm within the same organic grain. These results illustrate the high degree of chemical heterogeneity inherent to the organic matter within these source rocks. Additionally, the spatial pattern of the Raman parameters, as revealed by 2D Raman mapping, imply that organic matter structure is influenced by associations with mineral surfaces within the surrounding rock matrix. Chemical heterogeneity and matrix effects directly impact the Raman response from these types of materials and thus the extracted thermal maturity estimate. These findings highlight the care which must be adopted when making Raman measurements of organic matter within source rock matrices, especially for samples which feature highly dispersed, heterogeneous organic matter as found in petroliferous mudrocks.

International Journal of Coal Geology↗

USGS2025

Relating systematic molecular and textural properties of graptolite pyrolyzed via gold tube hydrous pyrolysis: Implications for thermal proxies in lower Paleozoic marine shales

A series of gold tube pyrolysis experiments (72 h, 300–550 °C, 50 MPa) conducted on a graptolite-rich lower Paleozoic marine shale generated pyrolysis residues for a comprehensive evaluation of the molecular and structural variability of three types of graptolite periderm. Organic petrology, Raman spectroscopy, and field emission scanning electron microscopy (FE-SEM) with energy dispersive spectroscopy (EDS) were combined to evaluate the thermal evolution process. The three types of graptolite periderm, namely granular, non-granular, and nodular graptolite, were analyzed by Raman spectroscopy wherein point measurements were obtained after the maceral was identified and the location verified by organic petrology. Distinct thermal evolution pathways among non-granular, granular, and nodular graptolite periderms were recorded. The evolution patterns of the Raman parameters, particularly D1 and G bands, highlight the differences in geochemical composition of the graptolite periderm types and the alteration of molecular structure with increasing thermal maturity. Raman parameters D1 (position of the D1 peak), G-FWHM (full width at half maximum of the G peak), and ratios D1-FWHM/G-FWHM (full width at half maximum of the D1 peak ratioed to G-FWHM) and A D1 /A G (ratio of D1 and G peak intensities) showed effectiveness in assessing thermal maturity. Bireflectance with increasing gold tube pyrolysis temperature followed a hierarchy: non-granular > granular > nodular, reflecting different molecular alignment intensities. Qualitative FE-SEM evaluation showed that fine-grained mineral inclusions (primarily Fe-sulfide as determined via EDS) were associated with the graptolite populations, with granular graptolite containing greater amounts of coarser-grained (e.g., ∼300–1400 nm) mineral inclusions relative to non-granular and nodular graptolite, which contain finer-grained (e.g., ∼100–200 nm) inclusions difficult to resolve with optical microscopy. These findings are investigated to highlight the mechanisms that drive organic matter evolution within graptolite during thermal maturation, as well as to explore some of the limitations of using spectroscopic parameters as thermal maturity proxies.

International Journal of Coal Geology↗

USGS2013

2D IR spectra of cyanide in water investigated by molecular dynamics simulations

Using classical molecular dynamics simulations, the 2D infrared (IR) spectroscopy of CN − solvated in D 2 O is investigated. Depending on the force field parametrizations, most of which are based on multipolar interactions for the CN − molecule, the frequency-frequency correlation function and observables computed from it differ. Most notably, models based on multipoles for CN − and TIP3P for water yield quantitatively correct results when compared with experiments. Furthermore, the recent finding that T 1 times are sensitive to the van der Waals ranges on the CN − is confirmed in the present study. For the linear IR spectrum, the best model reproduces the full widths at half maximum almost quantitatively (13.0 cm −1 vs. 14.9 cm −1 ) if the rotational contribution to the linewidth is included. Without the rotational contribution, the lines are too narrow by about a factor of two, which agrees with Raman and IR experiments. The computed and experimental tilt angles (or nodal slopes) α as a function of the 2D IR waiting time compare favorably with the measured ones and the frequency fluctuation correlation function is invariably found to contain three time scales: a sub-ps, 1 ps, and one on the 10-ps time scale. These time scales are discussed in terms of the structural dynamics of the surrounding solvent and it is found that the longest time scale (≈10 ps) most likely corresponds to solvent exchange between the first and second solvation shell, in agreement with interpretations from nuclear magnetic resonance measurements.

Journal of Chemical Physics↗