P L. Smedley, D.G. Kinniburgh/ Applied Geochemistry 17(2002 )517-568 Table 2 Major As minerals occurring in nature Mineral Occurrence Native arsenic Hydrothermal veins ein deposits and norites Realgar Ass ein deposits, often associated with orpiment, clays and limestones, also deposits n hot Orpiment Hydrothermal veins, hot springs Cobaltite High-temperature deposits, metar The most abundant As mineral ly in mineral veins Cu, Fe),S13 hydrothermal veins Hydrothermal veins Secondary mineral formed by oxidation of realgar, arsenopyrite and other As minerals FeAsO42H,0 Ni, Co)3(AsO4)2.8H20 Secondary mineral Mg:(AsO4h.8H20 Secondary mineral, smelter wastes Pharmacosiderite Fe3(AsO4)2(OH)3. 5H2o Oxidation product of arsenopyrite and other As minerals minerals, with arsenopyrite cores zoning out to arsenian acid mine drainage, and for the presence of As problems pyrite and realgar-orpiment rims. Oxides and sulphates around coal mines and areas of intensive coal burning. are formed at the latest stages of ore mineralisation High As concentrations are also found in many oxide (Arehart et al., 1993) minerals and hydrous metal oxides, either as part of the mineral structure or as sorbed species. Concentrations 3. 1.2. Rock-forming minerals in Fe oxides can also reach weight percent values Though not a major component, As is also often (Table 3), particularly where they form as the oxidation present in varying concentrations in other common products of primary Fe sulphide minerals, which have rock-forming minerals. As the chemistry of As follo an abundant supply of As. Adsorption of arsenate to closely that of s, the greatest concentrations of the ele- hydrous Fe oxides is particularly strong and sorbed ment tend to occur in sulphide minerals, of which pyrite loadings can be appreciable even at very low As con is the most abundant Concentrations in pyrite, chalco- centrations in solution(Goldberg, 1986: Manning and pyrite, galena and marcasite can be very variable, even Goldberg, 1996: Hiemstra and van Riemsdijk, 1996) within a given grain, but in some cases exceed 10 wt. Adsorption to hydrous Al and Mn oxides may also be (Table 3). Arsenic is present in the crystal structure of important if these oxides are present in quantity (e.g. many sulphide minerals as a substitute for s. Besides Peterson and Carpenter, 1983; Brannon and Patrick, being an important component of ore bodies, pyrite is 1987). Arsenic may also be sorbed to the edges of clays also formed in low-temperature sedimentary environ and on the surface of calcite ( Goldberg and Glaubig ments under reducing conditions. Such authigenic pyrite 1988), a common mineral in many sediments. However, plays a very important role in present-day geochemical these loadings are much smaller on a weight basis than cycles. It is present in the sediments of many rivers, for the Fe oxides. Adsorption reactions are responsible commonly forms preferentially in zones of intense As found in most natural wat oxic) concentrations of lakes and the oceans as well as of many aquifers. Pyrite for the relatively low(and non reduction such as around buried plant roots or other Arsenic concentrations in phosphate minerals are nuclei of decomposing organic matter. It is sometimes variable but can also reach high values, for example up to present in a characteristic form as framboidal pyrite. 1000 mg kg-I in apatite(Table 3). However, phosphate During the formation of this pyrite, it is likely that some minerals are much less abundant than oxide minerals of the soluble As will also be incorporated. Pyrite is not and so make a correspondingly small contribution to stable in aerobic systems and oxidises to Fe oxides with the As concentration in most sediments. Arsenic can the release of large amounts of So4, acidity and asso- also substitute for Si+, Al+, Fe+ and Ti ciated trace constituents, including As. The presence of mineral structures and is therefore present in many pyrite as a minor constituent in sulfide-rich coals is ulti other rock-forming minerals, albeit at much lower con ately responsible for the production of 'acid rain and centrations. Most common silicate minerals containminerals, with arsenopyrite cores zoning out to arsenian pyrite and realgar-orpiment rims. Oxides and sulphates are formed at the latest stages of ore mineralisation (Arehart et al., 1993). 3.1.2. Rock-forming minerals Though not a major component, As is also often present in varying concentrations in other common rock-forming minerals. As the chemistry of As follows closely that of S, the greatest concentrations of the ele￾ment tend to occur in sulphide minerals, of which pyrite is the most abundant. Concentrations in pyrite, chalco￾pyrite, galena and marcasite can be very variable, even within a given grain, but in some cases exceed 10 wt.% (Table 3). Arsenic is present in the crystal structure of many sulphide minerals as a substitute for S. Besides being an important component of ore bodies, pyrite is also formed in low-temperature sedimentary environ￾ments under reducing conditions. Such authigenic pyrite plays a very important role in present-day geochemical cycles. It is present in the sediments of many rivers, lakes and the oceans as well as of many aquifers. Pyrite commonly forms preferentially in zones of intense reduction such as around buried plant roots or other nuclei of decomposing organic matter. It is sometimes present in a characteristic form as framboidal pyrite. During the formation of this pyrite, it is likely that some of the soluble As will also be incorporated. Pyrite is not stable in aerobic systems and oxidises to Fe oxides with the release of large amounts of SO4, acidity and asso￾ciated trace constituents, including As. The presence of pyrite as a minor constituent in sulfide-rich coals is ulti￾mately responsible for the production of ‘acid rain’ and acid mine drainage, and for the presence of As problems around coal mines and areas of intensive coal burning. High As concentrations are also found in many oxide minerals and hydrous metal oxides, either as part of the mineral structure or as sorbed species. Concentrations in Fe oxides can also reach weight percent values (Table 3), particularly where they form as the oxidation products of primary Fe sulphide minerals, which have an abundant supply of As. Adsorption of arsenate to hydrous Fe oxides is particularly strong and sorbed loadings can be appreciable even at very low As con￾centrations in solution (Goldberg, 1986; Manning and Goldberg, 1996; Hiemstra and van Riemsdijk, 1996). Adsorption to hydrous Al and Mn oxides may also be important if these oxides are present in quantity (e.g. Peterson and Carpenter, 1983; Brannon and Patrick, 1987). Arsenic may also be sorbed to the edges of clays and on the surface of calcite (Goldberg and Glaubig, 1988), a common mineral in many sediments. However, these loadings are much smaller on a weight basis than for the Fe oxides. Adsorption reactions are responsible for the relatively low (and non-toxic) concentrations of As found in most natural waters. Arsenic concentrations in phosphate minerals are variable but can also reach high values, for example up to 1000 mg kg
1 in apatite (Table 3). However, phosphate minerals are much less abundant than oxide minerals and so make a correspondingly small contribution to the As concentration in most sediments. Arsenic can also substitute for Si4+, Al3+, Fe3+ and Ti4+ in many mineral structures and is therefore present in many other rock-forming minerals, albeit at much lower con￾centrations. Most common silicate minerals contain Table 2 Major As minerals occurring in nature Mineral Composition Occurrence Native arsenic As Hydrothermal veins Niccolite NiAs Vein deposits and norites Realgar AsS Vein deposits, often associated with orpiment, clays and limestones, also deposits from hot springs Orpiment As2S3 Hydrothermal veins, hot springs, volcanic sublimation products Cobaltite CoAsS High-temperature deposits, metamorphic rocks Arsenopyrite FeAsS The most abundant As mineral, dominantly in mineral veins Tennantite (Cu,Fe)12As4S13 Hydrothermal veins Enargite Cu3AsS4 Hydrothermal veins Arsenolite As2O3 Secondary mineral formed by oxidation of arsenopyrite, native arsenic and other As minerals Claudetite As2O3 Secondary mineral formed by oxidation of realgar, arsenopyrite and other As minerals Scorodite FeAsO4.2H2O Secondary mineral Annabergite (Ni,Co)3(AsO4)2.8H2O Secondary mineral Hoernesite Mg3(AsO4)2.8H2O Secondary mineral, smelter wastes Haematolite (Mn,Mg)4Al(AsO4)(OH)8 Conichalcite CaCu(AsO4)(OH) Secondary mineral Pharmacosiderite Fe3(AsO4)2(OH)3.5H2O Oxidation product of arsenopyrite and other As minerals P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 529