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, ismonoclinic, 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 andFourier syntheses and refined by the method of least squares to an R factor of 0.056 for 942reflections, measured on an automatic single-crystal diffractometer.The crystal structurecontains 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 chainare shared by S(1)O4 and S(2)O4tetrahedra.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-Obond lengths are 2.005 and 1.472 Å respectively. Cu2+ shows the usual Jahn-Teller distortionwith four short Cu-O(W)bonds (av. 1.985 Å) and two long Cu-Obonds (av. 2.264 Å.The pleochroismin guilditeis similar to that of other materials containing hydroxobridgedFe3+chains .Intense color and the highest refractive index occur when the vibration directionofthe 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.