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A rsenic has diverse chemical behavior in the natural environment. It has the ability to readily change oxidation state and bonding configuration, which creates rich inorganic and organic chemistry. This behavior is a consequence of the electronic configuration of its valence orbitals, with partially filled states capable of both electron donation and overlap in covalent bonds. In natural compounds, arsenic bonds primarily to oxygen and sulfur, generating a variety of aqueous species and minerals. The affinity of arsenic for these two elements, along with its stable bonding to methyl groups, constitutes the structural basis for most organic and biosynthetic compounds. The agile chemistry of arsenic helps to explain its contradictory action as both a toxin and a curative, and its sometimes-elusive behavior in the environment. KEYWORDS: chemical bonding, arsenic sulfides, arsenic oxides, organoarsenicals, arsenic toxicity Chemistry and Mineralogy of Arsenic 78 E L E M E N T S APRIL 2006 Nordstrom and Archer 2003). The fact that the stable equilibrium state of arsenic changes over the range of Eh and pH conditions commonly encountered in natural environments, roughly within the range of NO 3 -/N 2 and Fe 3+ /Fe 2+ redox potentials (depending on the reaction stoichiometry), is a controlling factor in arsenic cycling between abiotic and biotic compartments in the environment. The structural configuration of As 3+ has three pyramidal bonds and a lone electron pair occupying the fourth arm of a tetrahedron Because arsenic is typically present in natural waters at low concentrations (micromolar to nanomolar), its adsorption and desorption behavior on mineral surfaces plays an important role in regulating its aqueous concentration in groundwaters and surface waters. Both arsenite and arsenate have a strong, pHdependent sorption affinity for iron hydroxide and oxyhydroxide minerals such as ferrihydrite and goethite. Surface coatings of these iron oxides on detrital minerals are thought to be important sorbents of arsenic in groundwater aquifers and have been exploited as remediation technologies for arsenic removal Because the solid forms of arsenic sulfide are fairly insoluble and precipitate rapidly (see below), aqueous As 3+ sulfide species (thioarsenites) are not well known. The exact stoichiometry of aqueous thioarsenite species is controversial, but the most important species are probably As(OH)(SH) 2°a nd its two deprotonated forms, and As(OH) 2 (SH)°and its deprotonated form, although other species with only sulfur (-S) and thiol (-SH) ligands have been proposed (Nordstrom 78 Summary of important inorganic, organic, and biological forms of arsenic in the environment. For inorganic aqueous species, stepwise acid dissociation constants (pKa n = -log (Ka n ), where n is the number of protons dissociated and K a is the equilibrium constant) are given. The position of lone-pair electrons are shown for inorganic As 3+ . A wide variety of organoarsenic compounds and lipids have been identified; several of the major environmental species are shown here. FIGURE 1 and Archer 2003; Arsine gas (AsH 3 ), which is formally the most reduced form of arsenic (formal valence is -3, analogous to ammonia, NH 3 ), is notable for its extreme toxicity. It is a volatile gas that is only slightly soluble in water. Because of its volatility, its importance in the environment as a microbiological end product may be overlooked (Cullen and Reimer 1989; MINERALOGY Arsenic Oxides The affinity of arsenic to bond with other elements and species means that it is rarely found as a native element, with the occasional exception in hydrothermal ores. The simple As 3+ oxides, arsenolite and claudetite (both As 2 O 3 ), are polymorphs with similar thermodynamic stability, with claudetite thought to be slightly more stable at standard conditions Arsenate minerals comprise a large class with extensive substitution and solid solution, and new minerals continue to be identified. Mineralogically, arsenates are usually considered a subclass of the phosphate mineral group (in the Dana classification) because of the similarity in size and charge of the phosphate and arsenate anionic unit ( 79 E L E M E N T S APRIL 2006 Examples of molecular structures of arsenic minerals. Atom colors: red -As; yellow -S; white -O; light blueFe; dark blue -Ca; pink -H. (A) Atomic and polyhedral views of arsenolite (As 3+ oxide); (B) two arsenate minerals, scorodite and austinite, showing linkages of arsenate tetrahedra (red) between divalent cation octahedra (Fe 3+ -yellow; Zn 2+ -gray); (C) atomic and polyhedral views of arsenopyrite, with octahedrally coordinated Fe 2+ (blue) and As-S bonds; (D) atomic and polyhedral views of löllingite, with octahedrally coordinated Fe 2+ (blue) and As-As bonds; (E) two common arsenic sulfide minerals, realgar and orpiment; molecular cages in realgar have As-As and As-S dimers, while the chain-like structure in orpiment has only As-S bonds. depending on the degree of desiccation. They are also commonly found as weathering products of arsenic-bearing ore deposits, where sulfidic ore minerals are often coated with surface layers of oxidized and hydrated arsenate minerals Arsenic Sulfides Arsenopyrite, orpiment, and realgar are the most common arsenic sulfide minerals (TABLE 1), occurring primarily in hydrothermal and magmatic ore deposits. As a pigment and dye, orpiment was sought in ancient times for its golden color (from Latin auri pigmentum). Realgar (from Arabic rahj al ghar, "powder of the mine") was a common red pigment for paints and dyes in paintings and manuscripts. It is unstable in air, however, and alters to pararealgar, a yelloworange powder, which is seen now in old, unrestored paintings or manuscripts that have a yellow or orange hue over the original red (Clark 1999). Other transition metals, such as Co, Ni, and Cu, also combine with arsenic and sulfur to form a variety of minor sulfides and sulfosalts, often with extensive solid solution. More recently, spectroscopic methods have verified the formation of orpiment-and realgartype structures in low-temperature, sulfate-reducing environments, probably related to microbiological sulfur and arsenic reduction Compared to the structure and bonding properties found in oxide minerals, arsenic sulfides are quite different from the oxides and even differ substantially within the group of sulfides. Arsenic and sulfur bonds in these structures are strongly covalent, with different arrangements of As-S and As-As dimeric units creating a common structural basis. The structure of arsenopyrite is characterized by iron atoms octahedrally coordinated by three arsenic and three sulfur atoms and linked through both edges and corners Realgar and its polymorphs have discrete [As 4 -S 4 ] molecular cage-like units with linked As-As and S-S dimers Metal Arsenides A large number of metal arsenides are known and can be considered as alloys of metals, generally with the form MeAs n , where n = 1, 2, or 3 and Me is a metallic element. Electron counting would indicate a negative formal oxidation state for arsenic in these compounds. However, bonding is strongly covalent in these semiconducting solids, and they are better described by band gap formalism (Partik and Lutz 1999). The well-known industrial semiconductor GeAs is a typical example. In nature, the most common arsenides form with Fe, Co, Ni, and Cu (TABLE 1), but are typically rare, occurring in hydrothermal and magmatic ore deposits in association with a variety of sulfide minerals. They often form solid solutions with each other and with sulfide minerals in structural arrangements of common sulfides (e.g. pyrite, pyrrhotite, marcasite, galena). ORGANIC AND BIOLOGICAL COMPOUNDS Similar to nitrogen and phosphorus, arsenic has rich organic chemistry. Its ability to bind to a variety of organic ligands with different coordination geometries and its greater redox potential than phosphorus help to explain why mechanisms of arsenic toxicity in living organisms are still incompletely understood despite much investigation 80 E L E M E N T S APRIL 2006 developed as agricultural pesticides; mono-and disodium methanoarsonate and dimethylarsenic acid have been used worldwide as herbicides from World War II to today One of the most common classes of natural organoarsenic compounds are methylated forms of As 3+ and As 5+ , such as the mono-and dimethyl oxoacids in their simplest forms, which can be generated by replacing a hydroxyl (-OH) ligand by a methyl group (-CH 3 ) in inorganic arsenate or arsenite structures Many organoarsenic compounds have arsenic substitution for nitrogen in an organic moiety, for example arsenobetaine and arsenocholine, which are major forms of arsenic found in marine animal