A COMPARISON OF IRON EXTRACTION METHODS FOR THE DETERMINATION OF DEGREE OF PYRITISATION AND THE RECOGNITION OF IRON-LIMITED PYRITE FORMATION

Measurements of degree of pyritisation require an estimate of sediment iron which is capable of reaction with dissolved sulphide to form pyr ite, either directly or indirectly via iron monosulphide precursors. T hree dissolution techniques (buffered dithionite, cold 1 M HCl, boilin g 12 M HCl) were examined for their capacity to extract iron from a va riety of iron minerals, and iron-bearing sediments, as a function of d ifferent extraction times and different grain sizes. All the iron oxid es studied are quantitatively extracted by dithionite and boiling HCl (but not by cold HCl). Both HCl techniques extract more iron from sili cates than does dithionite but probably about the same amounts as are potentially capable of sulphidation. Modern sediment studies indicate that most sedimentary pyrite is formed rapidly from iron oxides, with smaller amounts formed more slowly from iron silicates (if sufficient geologic time is available). It is therefore recommended that the degr ee of pyritisation be defined with respect to the dithionite-extractab le (mainly iron oxide) pool and/or the boiling HCl-extractable pool (w hich includes some silicate iron) for the recognition of iron-limited pyritisation.