508 MINERALS/Arsenates Table Examples of hydrous and complex arsenates Mineral Formula Crystal system Point group Adamite Arsentsumebite Bayldonite Carminite Clinoclase Cuproadamite Kankite Legrandite Liroconite Olivenite Paradamite Pharmacolite Pharmacosiderite Picropharmacolite Symplesite Talmessite Tilasite Tsumcorite Walpurgite Zn2AsO4(OH) Pb2Cu(AsO4)(SO4)OH Cu3Pb(AsO4)2 H2O PbFe2(AsO4)2(OH)2 Cu3AsO4(OH)3 (Cu,Zn)2(AsO4)(OH) Fe(AsO4)3 5H2O Zn2AsO4(OH) H2O Cu2Al(AsO4)(OH)4 4H2O Cu2AsO4(OH) Zn2AsO4(OH) Ca(AsO3OH) 2H2O KFe4(AsO4)3(OH)4 7H2O H2Ca4Mg(AsO4)4 11H2O Fe3(AsO4)2 8H2O Ca2Mg(AsO4) 2H2O CaMg(AsO4)F Pb(Zn,Fe)2(AsO4)2 H2O (BiO)4UO2(AsO4)2 H2O Orthorhombic Monoclinic Monoclinic Orthorhombic Monoclinic Orthorhombic Monoclinic Monoclinic Monoclinic Orthorhombic Triclinic Monoclinic Cubic Triclinic Triclinic Triclinic Monoclinic Monoclinic Trigonal 2/m 2/m 2/m 2/m 2/m 2/m 2/m 2/m 2/m 2/m 2/m 2/m 2/m 2/m 2/m bar bar 3m bar bar 2/m Figure Photomicrograph of synthetic beudantite (PbFe3AsO4SO4(OH)6) crystals, exhibiting tabular cluster crystal shape Image courtesy of AML Smith Solubility and Alteration The solubilities of the arsenates are dependent on their degree of crystallinity and water content Many have high log Ksp values under standard conditions, and are thus quite soluble (e.g., ferrarisite, guerinite, Table 3) Others, such as annabergite and scorodite, are less soluble and, as a result, tend to control the distribution of arsenic in soils and other environmental media Scorodite is one of the most common arsenates in soils, but is only stable under acid (pH 4) oxidizing conditions Under more neutral pH or reducing conditions, scorodite dissolves incongruently to form Fe oxides and soluble As species; amorphous scorodite is thought to be more soluble than crystalline forms (Table 3) Similarly, the arsenate beudantite (PbFe3AsO4SO4(OH)6) dissolves incongruently in acid and