Guildite, a layer structure with a new type of ferric hydroxy-sulphate chain and its optical absorption spectra

Guildite, Cu²⁺Fe³⁺(SO₄)₂(OH) · 4H₂O, from the United Verde Mine, Jerome, Arizona, is monoclinic, space group P2₁/m, with cell dimensions a = 9.786(2), b = 7.134(1), c = 7.263(1)A, β = 105.28(1)°, Z = 2. The crystal structure has been determined by Patterson and Fourier syntheses and refined by the method of least squares to an R factor of 0.056 for 942 reflections, measured on an automatic single-crystal diffractometer. The crystal structure contains an octahedral-tetrahedral chain, which consists of zig-zag chains of [FeO₅(OH)] octahedra sharing the (OH) corner; two pairs of octahedral corners on either side of this chain are shared by S(1)O₄ and S(2)O₄ tetrahedra. These chains are crosslinked through corner-sharing trans-[CuO₂(H₂O)₄] octahedra into corrugated sheets parallel to the (001) plane. Adjacent sheets are hydrogen-bonded through water molecules. The average Fe³⁺-O and S-O bond lengths are 2.005 and 1.472A respectively. Cu²⁺ shows the usual Jahn-Teller distortion with four short Cu-O(W) bonds (av. 1.985A) and two long Cu-O bonds (av. 2.264A).

The pleochroism in guildite is similar to that of other materials containing hydroxobridged Fe³⁺ chains. Intense color and the highest refractive index occur when the vibration direction of the incident light is along the chain direction. An intense absorption band at 430 nm and a band at 874 nm are due to Fe³⁺, whereas broad absorption bands at 1285 nm and 780 nm have been assigned to Cu²⁺. Additional bands, including a sharp band at 1953 nm, are due to (OH) ions and water molecules.