Fibroferrite: crystallographic, optical and synthesis experiments

Abstract

Fibroferrite [Fe(OH)SO4•5H2O] is a product of the AMD (acid mine drainage) process and forms by chemical weathering and oxidation of iron sulfides in hydrothermal and magmatic ore deposits. The present work consists of two distinct parts. The first part is a crystallographic and optical study of 30 samples of fibroferrite from natural occurences and mine waste. The second part covers the description of all the experimental attempts to synthesize fibroferrite. Powder-diffraction data were collected and the Rietveld refinement procedure was conducted for each of the 30 natural samples in order to refine the values of the lattice constants of each sample. The a and c values of the lattice constants of fibroferrite measured in this study were found to conform to the values reported in the most recent literature. Optical observations were conducted, using a spindle stage, on each of the 30 natural samples. The purpose of the optical analysis was the selection of crystals adequate for a single crystal X-ray diffraction study. Samples were selected for a study on a rotating anode source and further studies at a synchrotron facility. None of the selected crystals was suitable for a single crystal X-ray diffraction study and the unit cell could not be found. Energy dispersive X-ray spectroscopy chemical analyses using a scanning electron microscope were performed on the 10 natural samples supplied by the Royal Ontario Museum and the Canadian Museum of Nature. The energy dispersive X-ray spectroscopy chemical analyses did not allow a real understanding of the level of substitutions of other cations for ferric iron within the atomic structure of fibroferrite. Several experiments were performed in order to synthesize fibroferrite. Commercially available reagents were used. All the experimental attempts conducted failed in synthesizing fibroferrite. A too restricted Eh and pH range of the laboratory conditions at which the experiments were conducted and the formation of metastable phases could be evoked as reasons of this failure.