Kaatialaite occurs admixed with arsenolite and other minor phases as a pale gray to yellow powdery weathering product on löllingite from the granite pegmatite at Kaatiala, Kuortane, W. Finland. Microchemical analysis yields a formula close to FeAs3O9 · 6H2O (or Fe(H2AsO4)3 · 3H2O) for the natural material. The X-ray powder pattern is identical with that of synthetic FeAs3O9 · 8H2O (or Fe(H2AsO4)3 · 5H2O), showing that the water content may be variable. Physical properties and X-ray data were determined by examination of synthetic material. The substance is monoclinic P21 or P21/m; the cell parameters refined from powder data are a = 15.363(5), b = 19.844(5), c = 4.736(2)Å, β = 91.77(3)°; Z = 4. The strongest lines in the X-ray powder pattern are [d in Å, (I/I1), (hkl)]: 9.94(50)(020), 8.33(100)(120), 7.68(70)(200), 6.08(40)(130,220), 3.410(40)(231), 3.153(45)(241). For the natural mineral the corresponding d-values and intensities are: 9.91 (s), 8.35 (vs), 7.72 (s), 6.09 (m), 3.408 (m), 3.159 (m). The synthetic material is optically biaxial positive, 2VZ = 15±2°; the refractive indices are α = 1.581 ≈ β (calc. 1.582), γ = 1.625 (both ± 0.002); extinction angle Z:c ≈ 3°. The measured and calculated densities are 2.64 ± 0.02 and 2.62 g/cm3 respectively. The crystal-chemical formula Fe(H2AsO4)3 · 3–5H2O is favored by the infrared study, whereas the formula FeAs3O9 · 6–8H2O seems to be more compatible with the detailed thermal investigations (DTA, TGA, and stepwise heating).

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