Selenium is usually known as the ‘double-edged sword element' for its dual toxic and beneficial character to health. Since the pioneer works by Schwarz and Foltz on the relationships between selenium deficiency and liver, muscle and heart diseases, many efforts have been undertaken to better understand the role of selenium in health. At the same time, an increasing number of publications have appeared during these last years on the selenium physico–chemical interactions within the environment. Both types of research represent ongoing efforts to correctly estimate the bioavailability of selenium species for health and the environment. Redox reactions, diffusion, adsorption and precipitation processes or interactions with organic matter and biota govern the speciation and mobility of selenium in the environment. This review intends to emphasize and collect the important advances made during these last years in the mechanistic understanding of processes which govern selenium cycling and bioavailability, like adsorption at the mineral/water interface, precipitation of elemental selenium, or bioavailability of nanoscaled precipitates. The advent of powerful spectroscopic techniques, like X-ray absorption spectroscopy, has allowed the structural description of adsorption and substitution processes that selenium undergoes in a variety of minerals. These and other structural details about selenium precipitates are reviewed here, together with their relationships to the bioavailability of the element in the environment.

Selenium environmental cycling and bioavailability: a structural chemist point of view

Variations in the isotopic composition of Fe in Late Archean to Early Proterozoic Banded Iron Formations (BIFs) from the Transvaal Supergroup, South Africa, span nearly the entire range yet measured on Earth, from –2.5 to +1.0‰ in 56Fe/54Fe ratios relative to the bulk Earth. With a current state-of-the-art precision of ±0.05‰ for the 56Fe/54Fe ratio, this range is 70 times analytical error, demonstrating that significant Fe isotope variations can be preserved in ancient rocks. Significant variation in Fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments, and the range measured for BIFs stands in marked contrast to the isotopic homogeneity of igneous rocks, which have δ56Fe=0.00±0.05‰, as well as the majority of modern loess, aerosols, riverine loads, marine sediments, and Proterozoic shales. The Fe isotope compositions of hematite, magnetite, Fe carbonate, and pyrite measured in BIFs appears to reflect a combination of (1) mineral-specific equilibrium isotope fractionation, (2) variations in the isotope compositions of the fluids from which they were precipitated, and (3) the effects of metabolic processing of Fe by bacteria. For minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis, the relative order of δ56Fe values appears to decrease in the order magnetite > siderite > ankerite, similar to that estimated from spectroscopic data, although the measured isotopic differences are much smaller than those predicted at low temperature. In combination with on-going experimental determinations of equilibrium Fe isotope fractionation factors, the data for BIF minerals place additional constraints on the equilibrium Fe isotope fractionation factors for the system Fe(III)–Fe(II)–hematite–magnetite–Fe carbonate. δ56Fe values for pyrite are the lowest yet measured for natural minerals, and stand in marked contrast to the high δ56Fe values that are predicted from spectroscopic data. Some samples contain hematite and magnetite and have positive δ56Fe values; these seem best explained through production of high 56Fe/54Fe reservoirs by photosynthetic Fe oxidation. It is not yet clear if the low δ56Fe values measured for some oxides, as well as Fe carbonates, reflect biologic processes, or inorganic precipitation from low-δ56Fe ferrous-Fe-rich fluids. However, the present results demonstrate the great potential for Fe isotopes in tracing the geochemical cycling of Fe, and highlight the need for an extensive experimental program for determining equilibrium Fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments.

Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton

Convergent plate motion at ∼320–210 Ma generated the Tongbai-Dabie-Sulu (east-central China)-Imjingang-Gyeonggi (central Korea)-Renge-Suo (Southwestern Japan)-Sikhote-Alin orogen along the paleo-Pacific edge of cratonal Asia. This amalgamated belt reflects collision between the Sino-Korean and Yangtze cratons on the SW portion, and accretion of outboard oceanic arcs ± sialic fragments against the NE margin. Subducted Proterozoic-Paleozoic continental and oceanic crustal complexes underwent high- and ultrahigh-pressure metamorphism at low to moderate temperatures. Tectonic slices of sialic crust episodically disengaged from the downgoing plate and, driven by buoyancy, ascended rapidly to midcrustal levels from depths exceeding 90–200 km after continental collision in east-central China plus or minus Korea, and from ∼30–50 km after arrival of far-traveled oceanic terranes in SW Japan and the Russian Far East. On achieving neutral buoyancy and stalling out at 10–20 km depth, later doming, gravitation...

Permo-Triassic Collision, Subduction-Zone Metamorphism, and Tectonic Exhumation Along the East Asian Continental Margin

which occurs in nature in the several steps of the food chain from plants to man. This section is followed by a comprehensive bibliography of American and European references on the subject, including some Russian authors. A presentation of the authors' own experimental methods in determining the distribution of radiostrontium in bone then follows. The techniques which have been utilized include differential x-ray absorption in microradiography and microinterferometry. The nature of the bone salt and effect of strontium replacement of calcium in the crystal was studied by x-ray. diffraction. It is concluded that the introduction of strontium into the living skeleton most likely results in its adsorption onto the surface of the apatite crystals. Localization of strontium-90 in bone was carried out also by means of autoradiography and the time relationship of distribution carefully elucidated. It is of interest to note the marked inhomogeneity in distribution with marked concentrations in areas of rapidly growing and poorly mineralized bone. Calculation of radiation dose from the radioisotope was carried out using a theoretical sandwich model which seemed justified by the above investigations into the distribution in bone. Account is taken of the possibility of marked local concentrations in placing a suggested value for the maximum permissable body burden of strontium-90 as 0.1 microcurie. The presentation is both precise and thorough. Numerous illustrations and charts supplement the text. The book is invaluable as a reference for experimental methods and systems of dosage calculation for those doing investigative work in this field.

The Chemical Dynamics of Bone Mineral

Cycling of redox-sensitive elements such as Fe is affected by not only ambient Eh-pH conditions, but also by a significant biomass that may derive energy through changes in redox state (e.g., Nealson 1983; Lovely et al. 1987; Myers and Nealson 1988; Ghiorse 1989). The evidence now seems overwhelming that biological processing of redox-sensitive metals is likely to be the rule in surface- and near-surface environments, rather than the exception. The Fe redox cycle of the Earth fundamentally begins with tectonic processes, where “juvenile” crust (high-temperature metamorphic and igneous rocks) that contains Fe which is largely in the divalent state is continuously exposed on the surface. If the surface is oxidizing, which is likely for the Earth over at least the last two billion years (e.g., Holland 1984), exposure of large quantities of Fe(II) at the surface represents a tremendous redox disequilibrium. Oxidation of Fe(II) early in Earth’s history may have occurred through increases in ambient O2 contents through photosynthesis (e.g., Cloud 1965, 1968), UV-photo oxidation (e.g., Braterman and Cairns-Smith 1987), or anaerobic photosynthetic Fe(II) oxidation (e.g., Hartman 1984; Widdel et al. 1993; Ehrenreich and Widdel 1994). Iron oxides produced by oxidation of Fe(II) represent an important sink for Fe released by terrestrial weathering processes, which will generally be quite reactive. In turn, dissimilatory microbial reduction of ferric oxides, coupled to oxidation of organic carbon and/or H2, is an important process by which Fe(III) is reduced in both modern and ancient sedimentary environments (Lovley 1991; Nealson and Saffarini 1994). Recent microbiological evidence (Vargas et al. 1998), together with a wealth of geochemical information, suggests that microbial Fe(III) reduction may have been one of the earliest forms of respiration on Earth. It therefore seems inescapable that biological redox cycling of Fe has occurred for at least several billion years of Earth's history.

http://www.geology.wisc.edu/~eroden//Publications/Johnsonetal_RiMG55Chap10B_2004.pdf

Isotopic Constraints on Biogeochemical Cycling of Fe

Sideritic “coprolites” from the late Miocene of southwest Washington, the Upper Cretaceous of Saskatchewan and Madagascar, and the Permian of China have often been claimed to be pseudofossils. They are here interpreted as intestinal casts (cololites) prefossilized by bacterial activity and later transformed into siderite with no traces of original food particles left. All occurrences are found within fluvial overbank deposits that carry no other vertebrate remains. Their absence could be due to aquifer roll-fronts that destroyed phosphatic bones and teeth but favored siderite precipitation.

A fresh look at sideritic "coprolites"

Insidious hazards that affect humans and other living creatures are addressed in this review. Debilitation and disease that may arise from naturally occurring gases, minerals, and elements create problems with characteristics that inhibit progress toward adequate and immediate solutions: (a) Human or animal reactions are delayed, not detected until long after the original exposure; (b) sporadic or continual low-level exposures to the minor or trace amounts of offending materials via the atmosphere, hydrosphere, and lithosphere are difficult to evaluate; (c) the air, water, and food consumed may contain hazardous substances that go unnoticed; and (d ) health reactions depend on bioavailability of the hazard and susceptibility of the individual. The etiology of many chronic diseases from exposure to earth materials—asbestos disease, As- and Se-induced cancers, and other disabilities—are suspected, but precise mechanisms of induction are presently unknown. Stipulation of disease from Earth-based hazardous materials requires collaboration of Earth and biomedical scientists to avert, or at least ameliorate, future disease and debilitation.

/pdf/the-earth-source-of-health-and-hazards-an-introduction-to-4knhsm87v6.pdf

The Earth, Source of Health and Hazards: An Introduction to Medical Geology

Over the past fifty years the mineralogy and crystal chemistry of the materials called asbestos have been refined leading to better identification through new and more sensitive techniques. The health aspects of the exposure to fibrous inorganic species are also equally well determined from the anatomical to the biochemical levels. Integrating the efforts of many researchers from a diversity of disciplines from mineralogy and medicine, we can be more specific and detailed in the questions we raise. Coordinated research should add to our basic understanding of one area of disease induction and treatment: trauma related to inorganic materials.

Mineralogy of Asbestos Minerals

High-resolution computed tomography (CT) scans through two fossilized lesions in hadrosaur bone guided paleohistological and stable-isotopic sampling to examine the stages and structures of bone re...

https://bioone.org/journals/Journal-of-Vertebrate-Paleontology/volume-29/issue-2/039.029.0211/Bone-Lesions-in-Hadrosaurs--Computed-Tomographic-Imaging-as-a/10.1671/039.029.0211.pdf

Bone lesions in Hadrosaurs: computed tomographic imaging as a guide for paleohistologic and stable-isotopic analysis

Skeletal carbonate hydroxylapatite, heretofore unknown in modern coelenterates, has been found in the axes and holdfasts of the gorgonian octocoral family Gorgoniidae. This carbonate apatite is a distinctive feature of this family of octocorals and may indicate a previous history of phosphatic skeletal mineralization within this phylum.

H. Catherine W. Skinner

Papers

A fresh look at sideritic "coprolites"

The Earth, Source of Health and Hazards: An Introduction to Medical Geology

Mineralogy of Asbestos Minerals

Bone lesions in Hadrosaurs: computed tomographic imaging as a guide for paleohistologic and stable-isotopic analysis

Possible vestige of early phosphatic biomineralization in gorgonian octocorals (Coelenterata)