Crystallite size of haematite from thermal oxidation of pyrite and marcasite - effects of grain size and iron disulphide polymorph

A study relating the properties of haematite (alpha-Fe2O3) produced by thermal oxidation of pyrite and marcasite (both FeS2) in air for 1 h at temperatures between 200 and 650 degreesC, to the iron disulphide polymorph and grain size of the sample is reported. Comparisons feature crystallite sizes of haematite from pyrite and marcasite oxidation, and the 125-250 and 710-1000 mum fractions. The crystallite sizes of the haematite were obtained from analysis of the broadening of X-ray diffraction lines. Haematite could be detected at temperatures above 450-475 degreesC for both pyrite and marcasite. The results show that the initial haematite crystallites forming are equidimensional, having dimensions of approximately 15-20 nm following oxidation at around 500 C. At higher temperatures the crystallites develop a platy habit by sintering, and at 650 degreesC mean crystallite dimensions of 25-35 nm along c-axis and 45-60 nm along a-axis were found. Model surface areas calculated from XRD fall within the range 80-20 m(2)/g. Oxidation of the 710-1000 mum size fractions produced significantly larger crystallites than the 125-250 mum fractions. A comparison of the polymorphs shows that haematite crystals from marcasite were smaller than those forming from pyrite. The oxidation followed a reaction route favouring higher oxidation states of iron. (C) 2003 Elsevier Ltd. All rights reserved.