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At least 649 records · Page 36Linked to original sources

USGS2014

Incubation stage and polychlorinated biphenyl (PCB) congener patterns in an altricial and precocial bird species

The composition of polychlorinated biphenyl (PCB) congeners was compared between non-incubated and embryonated eggs of tree swallows ( Tachycineta bicolor ) and little terns ( Sterna albifrons ) to determine if measurable changes in PCB congeners occurred during the embryonic period. There was no indication of changes in PCB congener patterns over the incubation period in tree swallows in 1999 and 2000 at a site with very high PCB exposure or a site with more modest PCB exposure. Additionally, congeners known to be either quickly metabolized or conserved based on experimental studies did not generally respond as predicted. Similarly, PCB congener patterns in eggs of little terns from Bottsand, Schleswig-Holstein, Germany, did not differ between non-incubated and embryonated eggs. The results from both species suggest that the stage of incubation is not an important consideration when evaluating PCB congener patterns; comparisons and assessments can be made with eggs collected at all stages of incubation.

Environmental Pollution↗

USGS2023

Gut microbiome composition associates with corticosteroid treatment, morbidity, and senescence in Chinook salmon (Oncorhynchus tshawytscha)

Pacific salmon experience prolonged elevation in corticosteroid hormones during important life history events including migration, reproduction, and senescence. These periods of elevated corticosteroids correspond with changes to immunity and energy metabolism; therefore, fish may be particularly vulnerable to mortality at these times. Recent studies found that stress-induced cortisol release associated with microbial community shifts in salmonids, raising the question of how longer-term corticosteroid dynamics that accompany life history transitions affect salmonid microbiomes. In this work, we experimentally evaluated the relationships between gut microbiome composition, chronically elevated corticosteroids, and mortality in juvenile Chinook salmon ( Oncorhynchus tshawytscha) . We found that treatment with slow-release implants of the corticosteroids cortisol or dexamethasone resulted in persistent changes to the gut microbiome. Morbidity was also associated with microbiome composition, suggesting that the gut microbiome reflects individual differences in susceptibility to opportunistic pathogens. Additionally, we analyzed a small number of samples from adult fish at various stages of senescence. Results from these samples suggest that microbiome composition associated with gut integrity, and that the microbial communities of corticosteroid treated juveniles shift in composition toward those of senescent adults. Overall, findings from this work suggest that the gut microbiome associates with mortality risk during periods of chronic corticosteroid elevation.

Scientific Reports↗

USGS2011

Experimental challenge and pathology of highly pathogenic avian influenza virus H5N1 in dunlin ( Calidris alpina ), an intercontinental migrant shorebird species

Background Shorebirds (Charadriiformes) are considered one of the primary reservoirs of avian influenza. Because these species are highly migratory, there is concern that infected shorebirds may be a mechanism by which highly pathogenic avian influenza virus (HPAIV) H5N1 could be introduced into North America from Asia. Large numbers of dunlin ( Calidris alpina ) migrate from wintering areas in central and eastern Asia, where HPAIV H5N1 is endemic, across the Bering Sea to breeding areas in Alaska. Low pathogenic avian influenza virus has been previously detected in dunlin, and thus, dunlin represent a potential risk to transport HPAIV to North America. To date no experimental challenge studies have been performed in shorebirds. Methods Wild dunlin were inoculated intranasally and intrachoanally various doses of HPAIV H5N1. The birds were monitored daily for virus excretion, disease signs, morbidity, and mortality. Results The infectious dose of HPAIV H5N1 in dunlin was determined to be 10 1.7 EID 50 /100 μl and that the lethal dose was 10 1.83 EID 50 /100 μl. Clinical signs were consistent with neurotropic disease, and histochemical analyses revealed that infection was systemic with viral antigen and RNA most consistently found in brain tissues. Infected birds excreted relatively large amounts of virus orally (10 4 EID 50 ) and smaller amounts cloacally. Conclusions Dunlin are highly susceptible to infection with HPAIV H5N1. They become infected after exposure to relatively small doses of the virus and if they become infected, they are most likely to suffer mortality within 3–5 days. These results have important implications regarding the risks of transport and transmission of HPAIV H5N1 to North America by this species and raises questions for further investigation.

Alaska↗

USGS2019

Effects of exercise and bioprocessed soybean meal diets during rainbow trout rearing

Background: Alternative protein sources to fishmeal in fish feeds are needed. Objectives: Evaluate rearing performance of adult rainbow trout (Oncorhynchus mykiss) (initial weight 139.0 ±1.5 g, length 232.9 ± 0.8 mm, mean ± SE) fed one of the two isonitrogenous and isocaloric diets (46% protein, 16% lipid) and reared at one of the two levels of exercise (water velocities of either 3.6 cm/s or 33.2 cm/s). Methods: Protein in the control diet was based on fishmeal. In the experimental diet, bioprocessed soybean meal replaced approximately 60% of the fishmeal. Fish were fed by hand once-per-day to near satiation, and the food was increased daily. The experiment lasted 90-days. Results: There were no significant differences in gain, percent gain, or specific growth rate between the dietary treatments. However, the amount of food fed and feed conversion ratio was significantly lower in the 60% bioprocessed soybean meal diet. Intestinal morphology, relative fin length, splenosomatic index, hepatosomatic index, and viscerosomatic index were not significantly different in the trout fed either diet. Fish reared at 3.6 cm/s had a significantly lower feed conversion ratio (1.02 ± 0.02) than fish reared at 33.2 cm/s (1.13 ± 0.02). However, there were no significant differences in gain, percent gain, specific growth rate, or percentage mortality in fish reared with or without exercise. No significant interactions were observed between diet and exercise (higher water velocity). Conclusion:Based on these results, at least 60% of the fishmeal in adult rainbow trout diets can be replaced by bioprocessed soybean meal, even if higher water velocities are used to exercise the fish.

The Open Biology Journal↗

USGS2022

Quantifying the stormwater runoff volume reduction benefits of urban street tree canopy

Trees in the urban right-of-way areas have increasingly been considered part of a suite of green infrastructure practices used to manage stormwater runoff. A paired-catchment experimental design (with street tree removal as the treatment) was used to assess how street trees affect major hydrologic fluxes in a typical residential stormwater collection and conveyance network. The treatment consisted of removing 29 green ash (Fraxinus pennsylvanica) and two Norway maple (Acer platanoides) street trees from a medium-density residential area. Tree removal resulted in an estimated 198 m3 increase in surface runoff volume compared to the control catchment over the course of the study. This increase accounted for 4% of the total measured runoff after trees were removed. Despite significant changes to runoff volume (p ≤ 0.10), peak discharge was generally not affected by tree removal. On a per-tree basis, 66 L of rainfall per m2 of canopy was lost that would have otherwise been intercepted and stored. Runoff volume reduction benefit was estimated at 6376 L per tree. These values experimentally document per-capita retention services rendered by trees over a growing season with 42 storm events. These values are within the range reported by previous studies, which largely relied on simulation. This study provides catchment scale evidence that reducing stormwater runoff is one of many ecosystem services provided by street trees. This study quantifies these services, based on site conditions and a mix of deciduous species, and serves to improve our ability to account for this important yet otherwise poorly constrained hydrologic service. Engineers, city planners, urban foresters, and others involved with the management of urban stormwater can use this information to better understand tradeoffs involved in using green infrastructure to reduce urban runoff burden.

Wisconsin↗

USGS2018

Effect of heat and singeing on stable hydrogen isotope ratios of bird feathers and implications for their use in determining geographic origin

Rationale Stable hydrogen isotope ( δ 2 H) ratios of animal tissues are useful for assessing movement and geographic origin of mobile organisms. However, it is uncertain whether heat and singeing affects feather δ 2 H values and thus subsequent geographic assignments. This is relevant for birds of conservation interest that are burned and killed at concentrating solar‐energy facilities that reflect sunlight to a receiving tower and generate a solar flux field. Methods We used a controlled experiment to test the effect of known heat loads (exposure to 200, 250 or 300°C for 1 min) on the morphology and δ 2 H values of feathers from two songbird species. Subsequently, we examined the effects of singeing on δ 2 H values of feathers from three other songbird species that were found dead in the field at a concentrating solar‐energy facility. Results Relative to control samples, heating caused visual morphological changes to feathers, including shriveling at 250°C and charring at 300°C. The δ 2 H values significantly declined by a mean of 27.8‰ in experimental samples exposed to 300°C. There was no statistically detectable difference between δ 2 H values of the singed and unsinged portions of field‐collected feathers from the same bird. Conclusions Limited singeing that did not dramatically alter the feather morphology did not substantially affect δ 2 H values of feathers from these songbirds. However, higher temperatures induced charring and reduced δ 2 H values. Therefore, severely charred feathers should be avoided when selecting feathers for δ 2 H‐based assessment of geographic origin.

Rapid Communications in Mass Spectrometry↗

USGS2025

Overwinter and prespawning movements by a vulnerable freshwater pelagophilic minnow

The decline of pelagophil minnows is related to river fragmentation across the southern Great Plains landscape. Because we know little about pelagophil movement patterns and timing, we aimed to quantify the movements of the vulnerable Arkansas River shiner (ARS) during the winter (November–March) and prespawning (April–June) seasons. We tagged 4233 ARS using visible implant elastomer, passive integrated transponder, or p-Chip micro-transponder tags in 2018–2020. We sampled to recapture tagged fish weekly during the winter and biweekly during the spring. Tagged fish exhibited a downstream movement bias and movement was weakly related to increasing temperature, discharge, and photoperiod during winter, however most of the variability was explained by a random individual effect. Larger individuals moved greater distances than smaller fish. Upstream movements by a migratory portion of the population appeared to begin around late February based on the presence of fish at previously unoccupied sites. However, the first long-distance (30-km) upstream movement by a tagged fish was documented in late May. We show evidence that some ARS may be resident fish at sites throughout winter and spring of multiple years. To conserve freshwater pelagophil minnows, our results indicate water management strategies improving river connectivity in late winter through the spawning season may benefit spawning by migratory individuals, whereas lateral connectivity might benefit the resident portion of the population. Research efforts under experimental flows could provide insight to improved recovery options.

Oklahoma↗

USGS1994

Cytonuclear genetics of experimental fish hybrid zones inside Biosphere 2

Two species of mosquitofish (family Poeciliidae ) known to hybridize in nature were introduced into freshwater habitats inside Biosphere 2, and their population genetics were monitored after 2 years. Within four to six generations, nuclear and cytoplasmic markers characteristic of Gambusia holbrooki had risen greatly in frequency, although some Gambusia affinis alleles and haplotypes were retained primarily in recombinant genotypes, indicative of introgressive hybridization. The temporal cytonuclear dynamics proved similar to population genetic changes observed in replicated experimental hybrid populations outside of Biosphere 2, thus indicating strong directional selection favoring G. holbrooki genotypes across the range of environments monitored. When interpreted in the context of species-specific population demographies observed previously, results suggest that the extremely rapid evolution in these zones of secondary contact is attributable primarily to species differences in life-history traits.

PNAS↗

USGS2007

Food availability affects the maternal transfer of androgens and antibodies into eggs of a colonial seabird

Mothers can improve the quality of their offspring by increasing the level of certain components in their eggs. To examine whether or not mothers increase deposition of such components in eggs as a function of food availability, we food‐supplemented black‐legged kittiwake females (Rissa tridactyla) before and during egg laying and compared deposition of androgens and antibodies into eggs of first and experimentally induced replacement clutches. Food‐supplemented females transferred lower amounts of androgens and antibodies into eggs of induced replacement clutches than did non‐food‐supplemented mothers, whereas first clutches presented no differences between treatments. Our results suggest that when females are in lower condition, they transfer more androgens and antibodies into eggs to facilitate chick development despite potential long‐term costs for juveniles. Females in prime condition may avoid these potential long‐term costs because they can provide their chicks with more and higher quality resources.

Alaska↗

USGS2012

Effects of resource chemistry on the composition and function of stream hyporheic biofilms.

Fluvial ecosystems process large quantities of dissolved organic matter as it moves from the headwater streams to the sea. In particular, hyporheic sediments are centers of high biogeochemical reactivity due to their elevated residence time and high microbial biomass and activity. However, the interaction between organic matter and microbial dynamics in the hyporheic zone remains poorly understood. We evaluated how variance in resource chemistry affected the microbial community and its associated activity in experimentally grown hyporheic biofilms. To do this we fed beech leaf leachates that differed in chemical composition to a series of bioreactors filled with sediment from a sub-alpine stream. Differences in resource chemistry resulted in differences in diversity and phylogenetic origin of microbial proteins, enzyme activity, and microbial biomass stoichiometry. Specifically, increased lignin, phenolics, and manganese in a single leachate resulted in increased phenoloxidase and peroxidase activity, elevated microbial biomass carbon:nitrogen ratio, and a greater proportion of proteins of Betaproteobacteria origin. We used this model system to attempt to link microbial form (community composition and metaproteome) with function (enzyme activity) in order to better understand the mechanisms that link resource heterogeneity to ecosystem function in stream ecosystems.

Frontiers in Microbiology↗

USGS2020

Detecting cover crop end-of-season using VENµS and sentinel-2 satellite imagery

Cover crops are planted during the off-season to protect the soil and improve watershed management. The ability to map cover crop termination dates over agricultural landscapes is essential for quantifying conservation practice implementation, and enabling estimation of biomass accumulation during the active cover period. Remote sensing detection of end-of-season (termination) for cover crops has been limited by the lack of high spatial and temporal resolution observations and methods. In this paper, a new within-season termination (WIST) algorithm was developed to map cover crop termination dates using the Vegetation and Environment monitoring New Micro Satellite (VENµS) imagery (5 m, 2 days revisit). The WIST algorithm first detects the downward trend (senescent period) in the Normalized Difference Vegetation Index (NDVI) time-series and then refines the estimate to the two dates with the most rapid rate of decrease in NDVI during the senescent period. The WIST algorithm was assessed using farm operation records for experimental fields at the Beltsville Agricultural Research Center (BARC). The crop termination dates extracted from VENµS and Sentinel-2 time-series in 2019 and 2020 were compared to the recorded termination operation dates. The results show that the termination dates detected from the VENµS time-series (aggregated to 10 m) agree with the recorded harvest dates with a mean absolute difference of 2 days and uncertainty of 4 days. The operational Sentinel-2 time-series (10 m, 4–5 days revisit) also detected termination dates at BARC but had 7% missing and 10% false detections due to less frequent temporal observations. Near-real-time simulation using the VENµS time-series shows that the average lag times of termination detection are about 4 days for VENµS and 8 days for Sentinel-2, not including satellite data latency. The study demonstrates the potential for operational mapping of cover crop termination using high temporal and spatial resolution remote sensing data.

Remote Sensing↗

USGS2013

The influence of precipitation, vegetation and soil properties on the ecohydrology of sagebrush steppe rangelands on the INL site

The INL Site and other landscapes having sagebrush steppe vegetation are experiencing a simultaneous change in climate and floristics that result from increases in exotic species. Determining the separate and combined/interactive effects of climate and vegetation change is important for assessing future changes on the landscape and for hydrologic processes. This research uses the 72 experimental plots established and initially maintained for many years as the “Protective Cap Biobarrier Experiment” by Dr. Jay Anderson and the Stoller ESER program, and the experiment is also now referred to as the “INL Site Ecohydrology Study.” We are evaluating long-term impacts of different plant communities commonly found throughout Idaho subject to different precipitation regimes and to different soil depths. Treatments of amount and timing of precipitation (irrigation), soil depth, and either native/perennial or exotic grass vegetation allow researchers to investigate how vegetation, precipitation and soil interact to influence soil hydrology and ecosystem biogeochemistry. This information will be used to improve a variety of models, as well as provide data for these models.

Report↗

USGS2023

Proceedings of the Fiscal Year 2022 Annual Reporting Meeting to the Glen Canyon Dam Adaptive Management Program

(Hartwell) This report is prepared primarily to account for work conducted and products delivered in FY 2022 by GCMRC and to inform the Technical Work Group of science conducted by GCMRC and its cooperators in support of the Glen Canyon Dam Adaptive Management Program (GCDAMP). It includes a summary of accomplishments, modifications to work plans, results, and recommendations related to projects included in GCMRC’s FY 2021-23 Triennial Work Plan. This work was done to support the 11 resource goals identified in the Glen Canyon Dam Long-Term Experimental and Management Plan (LTEMP) Environmental Impact Statement and Record of Decision (Table 1).

Arizona↗

USGS2004

Microforms in gravel bed rivers: Formation, disintegration, and effects on bedload transport

This research aims to advance current knowledge on cluster formation and evolution by tackling some of the aspects associated with cluster microtopography and the effects of clusters on bedload transport. The specific objectives of the study are (1) to identify the bed shear stress range in which clusters form and disintegrate, (2) to quantitatively describe the spacing characteristics and orientation of clusters with respect to flow characteristics, (3) to quantify the effects clusters have on the mean bedload rate, and (4) to assess the effects of clusters on the pulsating nature of bedload. In order to meet the objectives of this study, two main experimental scenarios, namely, Test Series A and B (20 experiments overall) are considered in a laboratory flume under well-controlled conditions. Series A tests are performed to address objectives (1) and (2) while Series B is designed to meet objectives (3) and (4). Results show that cluster microforms develop in uniform sediment at 1.25 to 2 times the Shields parameter of an individual particle and start disintegrating at about 2.25 times the Shields parameter. It is found that during an unsteady flow event, effects of clusters on bedload transport rate can be classified in three different phases: a sink phase where clusters absorb incoming sediment, a neutral phase where clusters do not affect bedload, and a source phase where clusters release particles. Clusters also increase the magnitude of the fluctuations in bedload transport rate, showing that clusters amplify the unsteady nature of bedload transport. A fourth-order autoregressive, autoregressive integrated moving average model is employed to describe the time series of bedload and provide a predictive formula for predicting bedload at different periods. Finally, a change-point analysis enhanced with a binary segmentation procedure is performed to identify the abrupt changes in the bedload statistic characteristics due to the effects of clusters and detect the different phases in bedload time series using probability theory. The analysis verifies the experimental findings that three phases are detected in the bedload rate time series structure, namely, sink, neutral, and source. ?? ASCE / JUNE 2004.

Journal of Hydraulic Engineering↗

USGS2014

Evaluation of catch-and-release regulations on Brook Trout in Pennsylvania streams

In 2004, the Pennsylvania Fish and Boat Commission implemented catch-and-release (CR) regulations on headwater stream systems to determine if eliminating angler harvest would result in an increase in the number of adult (≥100 mm) or large (≥175 mm) Brook Trout Salvelinus fontinalis . Under the CR regulations, angling was permitted on a year-round basis, no Brook Trout could be harvested at any time, and there were no tackle restrictions. A before-after–control-impact design was used to evaluate the experimental regulations. Brook Trout populations were monitored in 16 treatment (CR regulations) and 7 control streams (statewide regulations) using backpack electrofishing gear periodically for up to 15 years (from 1990 to 2003 or 2004) before the implementation of the CR regulations and over a 7–8-year period (from 2004 or 2005 to 2011) after implementation. We used Poisson mixed models to evaluate whether electrofishing catch per effort (CPE; catch/100 m 2 ) of adult (≥100 mm) or large (≥175 mm) Brook Trout increased in treatment streams as a result of implementing CR regulations. Brook Trout CPE varied among sites and among years, and there was no significant effect (increase or decrease) of CR regulations on the CPE of adult or large Brook Trout. Results of our evaluation suggest that CR regulations were not effective at improving the CPE of adult or large Brook Trout in Pennsylvania streams. Low angler use, high voluntary catch and release, and slow growth rates in infertile headwater streams are likely the primary reasons for the lack of response.

Pennsylvania↗

USGS2016

Silicic lunar volcanism: Testing the crustal melting model

Lunar silicic rocks were first identified by granitic fragments found in samples brought to Earth by the Apollo missions, followed by the discovery of silicic domes on the lunar surface through remote sensing. Although these silicic lithologies are thought to make up a small portion of the lunar crust, their presence indicates that lunar crustal evolution is more complex than originally thought. Models currently used to describe the formation of silicic lithologies on the Moon include in situ differentiation of a magma, magma differentiation with silicate liquid immiscibility, and partial melting of the crust. This study focuses on testing a crustal melting model through partial melting experiments on compositions representing lithologies spatially associated with the silicic domes. The experiments were guided by the results of modeling melting temperatures and residual melt compositions of possible protoliths for lunar silicic rocks using the thermodynamic modeling software, rhyolite-MELTS. Rhyolite-MELTS simulations predict liquidus temperatures of 950–1040 °C for lunar granites under anhydrous conditions, which guided the temperature range for the experiments. Monzogabbro, alkali gabbronorite, and KREEP basalt were identified as potential protoliths due to their ages, locations on the Moon (i.e., located near observed silicic domes), chemically evolved compositions, and the results from rhyolite-MELTS modeling. Partial melting experiments, using mixtures of reagent grade oxide powders representing bulk rock compositions of these rock types, were carried out at atmospheric pressure over the temperature range of 900–1100 °C. Because all lunar granite samples and remotely sensed domes have an elevated abundance of Th, some of the mixtures were doped with Th to observe its partitioning behavior. Run products show that at temperatures of 1050 and 1100 °C, melts of the three protoliths are not silicic in nature (i.e., they have <63 wt% SiO 2 ). By 1000 °C, melts of both monzogabbro and alkali gabbronorite approach the composition of granite, but are also characterized by immiscible Si-rich and Fe-rich liquids. Furthermore, Th strongly partitions into the Fe-rich, and not the Si-rich glass in all experimental runs. Our work provides important constraints on the mechanism of silicic melt formation on the Moon. The observed high-Th content of lunar granite is difficult to explain by silicate liquid immiscibility, because through this process, Th is not fractionated into the Si-rich phase. Results of our experiments and modeling suggests that silicic lunar rocks could be produced from monzogabbro and alkali gabbronorite protoliths by partial melting at T < 1000 °C. Additionally, we speculate that at higher pressures ( P ≥ 0.005 GPa), the observed immiscibility in the partial melting experiments would be suppressed.

American Mineralogist↗

USGS2011

Effects of black bear relocation on elk calf recruitment at Great Smoky Mountains National Park

Previous research from 2001 to 2006 on an experimentally released elk ( Cervus elaphus ) population at Great Smoky Mountains National Park (GSMNP or Park) indicated that calf recruitment (i.e., calves reaching 1 yr of age per adult female elk) was low (0.306, total SE = 0.090) resulting in low or negative population growth ( λ  = 0.996, 95% CI = 0.945–1.047). Black bear ( Ursus americanus ) predation was the primary calf mortality factor. From 2006 to 2008, we trapped and relocated 49 bears (30 of which were radiocollared) from the primary calving areas in the Park and radiomonitored 67 (28 M:39 F) adult elk and 42 calves to compare vital rates and population growth with the earlier study. A model with annual calf recruitment rate correlating with the number of bears relocated each year was supported (ΔAIC c  = 0.000; β  = 0.070, 95% CI = 0.028–0.112) and a model with annual calf recruitment differing from before to during bear relocation revealed an increase to 0.544 (total SE = 0.098; β  = −1.092, 95% CI = −1.180 to −0.375). Using vital rates and estimates of process standard errors observed during our study, 25-yr simulations maintained a mean positive growth rate in 100% of the stochastic trials with λ averaging 1.118 (95% CI = 1.096–1.140), an increase compared with rates before bear relocation. A life table response experiment revealed that increases in population growth were mostly (67.1%) due to changes in calf recruitment. We speculate that behavioral adaptation of the elk since release also contributed to the observed increases in recruitment and population growth. Our results suggest that managers interested in elk reintroduction within bear range should consider bear relocation as a temporary means of increasing calf recruitment.

North Carolina, Tennessee↗

USGS1987

Molecular orbital (SCF-Xα-SW) theory of metal-metal charge transfer processes in minerals

A number of mixed valence iron oxides and silicates (e.g., magnetite, ilvaite) exhibit thermally induced electron delocalization between adjacent Fe 2+ and Fe 3+ ions and optically induced electronic transitions which are assigned to Fe 2+ →Fe 3+ intervalence charge transfer. In this paper, the mechanism of electron delocalization (i.e., polarons versus itinerant electrons) and the nature of optically induced intervalence charge-transfer in minerals are investigated using molecular orbital theory. SCF-Xα-SW molecular orbital calculations were done for several mixed-valence (Fe 2 O 10 ) 15− clusters corresponding to edgesharing Fe 2+ and Fe 3+ coordination polyhedra. A spinunrestricted formalism was used so that the effect of ferromagnetic versus antiferromagnetic coupling of adjacent Fe 2+ and Fe 3+ cations could be determined. The molecular orbital results can be related to the polaron theory of solid state physics and the perturbation theory formalism used by Robin and Day (1967) and others to describe electron transfer in mixed valence compounds. Intervalence charge-transfer results from the overlap of Fe(3d) orbitals across the shared edges of adjacent FeO 6 polyhedra to give weak Fe-Fe bonds. Electron delocalization, however, requires that adjacent Fe cations be ferromagnetically coupled. Antiferromagnetic coupling results in distinguishable Fe 2+ and Fe 3+ cations. Electronic transitions between the Fe-Fe bonding and Fe-Fe antibonding orbitals results in the optically-induced intervalence charge transfer bands observed in the electronic spectra of mixed valence minerals. Such transitions are predicted to be polarized along the metal-metal bond direction, in agreement with experimental observations.

Physics and Chemistry of Minerals↗