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

Che'ng Wan, Subrata Ghose
Department of Geological Sciences
University of Washington
Seattle, Washington 98195

George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technoogy
Pasadena, CA 91125

Abstract

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) Å, beta = 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 1.472 Å respectively. Cu²⁺ shows the usual Jahn-Teller distortion with four short Cu-O(W) bonds (av. 1.985 Å) and two long Cu-O bonds (av. 2.264 Å. The pleochroismin guilditeis similar to that of other materials containing hydroxobridged Fe³⁺ chains .Intense color and the highest refractive index occur when the vibration direction ofthe 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 brpad abspr[topm bamds at 1285 m, amd 780 nm have been assigned to Cu²⁺. Additional bands, including a sharp band at 1953 nm, are due to (OH) ions and water molecules.

Optical absorption spectrum of 0.150 mm thick guildite crystals.

last updated: 9-Apr-2023