MINERALS/Arsenates 509 Table Solubilities of selected arsenates at 25 C, bar Log Ksp Mineral Dissociation reaction Annabergite TriCa arsenate Calcium arsenate hydrate Ferrarisite Guerinite Scorodite, amorphous Scorodite, crystalline Ni3 AsO4 ị2 8H2 O 3Ni ỵ 2AsO3 ỵ 8H2 O Ca3 AsO4 ị2 4H2 O 3Ca2ỵ ỵ 2AsO3 ỵ 4H2 O Ca4(OH)2(AsO4)2 4H2O 4Ca2ỵ ỵ 2H3AsO4 ỵ 6H2O 8Hỵ Ca5(HAsO4)2(AsO4)2 9H2O 5Ca2ỵ ỵ 4H3AsO4 ỵ 9H2O 10 Ca5(HAsO4)2(AsO4)2 9H2O 5Ca2ỵ ỵ 4H3AsO4 ỵ 9H2O 10 FeAsO4 2H2 O Fe3ỵ ỵ AsO3 ỵ 2H2 O FeAsO4 2H2 O Fe3ỵ ỵ AsO3 ỵ 2H2 O alkali solutions, yielding As(V) ions, a-FeOOH, Fe(OH)3, and PbSO4 One of the arsenates, meta-zeunerite, forms by dehydration and pseudomorphing of the parent arsenate zeunerite Occurrence Most of the arsenates are rare to very rare They are prized by mineral collectors because of their range of spectacular colours, lusters, and crystal habits The arsenate balydonite, found in Cornwall, England, has even been used to craft cabochons (convex gems and beads) and cabinets Many arsenates are found in the oxidation zone of sulphide ore deposits or in oxidized mine waste, where they occur as discrete precipitates or coatings on other mineral grains They are associated with sulphide minerals such as arsenopyrite, chalcopyrite, sphalerite, and galena, and with other arsenates and secondary ore minerals such as iron oxides, azurite, and malachite The most common of these arsenates include annabergite, beudantite, conichalcite, hoă rnesite, parasymplesite, rauenthalite, and, particularly, scorodite Although not common, two of the most famous of the oxidation zone arsenates are erythrite and annabergite, known by miners as ‘cobalt bloom’ and ‘nickel bloom’, respectively These are used as indicator minerals of Co and Ni sulphide ores, and as ores of Co and Ni themselves Several other arsenates are also minor ores of metals; examples are agardite (REE), beudantite (Pb), chalcopyllite (Cu), conichalcite (Cu), koă ttigite (Zn), mimetite (Pb), and walpurgite (U and Bi) These mine-related arsenates have been found at well-known mines in Devon and Cornwall, England; Mexico (Mapimi); Cobalt, Ontario, Canada; Sweden (La˚ngban, Warmland); Nambia (Tsumeb); Alsace, France; Arizona; Romania; Russia; and Zaire The arsenate nealite (Pb4Fe(AsO4)2Cl4) is also mine-related, but has been found in Greek and Roman slags in Lavrio, Greece, rather than outcrops, leading to some controversy over its classification as a mineral, since by definition, minerals should be natural, rather than man-made 2ỵ Hỵ Hỵ 28.38 21.257 40.2 27.75 28.55 22.89 25.89 The arsenates scorodite and beudantite have been reported frequently in sulphide tailings In uranium tailings, Ca–arsenates occur as coprecipitates with gypsum and as amorphous precipitates Ca3(AsO4) is the arsenates generally thought to control As solubility in aqueous systems where Ca2ỵ is present, but Ca4(OH)2(AsO4)2 4H2O and CaHAsO4 Á H2O have also been suggested to occur in tailings with high Ca/ As ratios and alkaline pH (10) conditions, and low Ca/As ratios (0.88–1.00) and slightly acid pH (5.76– 6.22) conditions, respectively Arsenic occurs most commonly in soil waters as the complexes H2 AsO4 (low pH) and HAsO24 (higher pH) The species H3 AsO04 and AsO34 occur in extremely acidic and alkaline conditions, respectively Sources of these complexes are many and varied, and include pesticides, mining, munitions waste, wood preservatives, and tannery wastes The arsenate complex is sorbed onto common soil minerals, such as Fe oxides and clays, or forms discrete arsenates, many of which are listed in Tables and The type of arsenate formed in soils depends on the availability of other cations (e.g., Ca, K, Mg, H, Pb) in the soils Many of the hydrous arsenates (Tables 1, 2) are found in soils (e.g., carminite, kankite, pharmacosiderite, talmessite, tilasite, and members of the vivianite group) Arsenates such as scorodite have also been reported to occur as crusts in hot spring deposits Due to its insolubility, scorodite is also used in metallurgy for the disposal of arsenic wastes from metallurgical effluents and flue dusts See Also Economic Geology Environmental Geochemistry Mineral Deposits and Their Genesis Minerals: Vanadates Further Reading Bothe JV and Brown PW (1999) As immobilization by calcium arsenate formation Environmental Science Technology 33: 3806 3811 Bothe JV and Brown PW (1999) The stabilities of calcium arsenates Journal of Hazardous Materials 69: 197 207