Effects of quench methods on Fe (super 3+) /Fe (super 2+) ratios; a Moessbauer and wet-chemical study

The effect of quenching method on Fe³⁺/Fe²⁺ ratios in three silicate glasses has been studied using natural rock compositions (the USGS rock standards basalt BCR-1 and rhyolite RGM and an andesite K-2B from Dennison volcano, Alaska). Samples were equilibrated at high temperatures and then quenched into an air jet, a 4% H₂-96% Ar jet, or a brine and ice bath. The glasses were analyzed by both Mössbauer spectroscopy and a microcolorimetric technique to compare the results from these commonly used methods for Fe³⁺-Fe²⁺ determinations. Because the glasses were low in Fe3+ content, both analytical methods determined Fe³⁺ estimates by difference, resulting in relatively large uncertainties (3% for Mössbauer spectroscopy and up to 8% for colorimetry). Within these uncertainties, estimates of Fe³⁺/Fe²⁺ ratios determined by Mössbauer spectroscopy and microcolorimetry are the same. None of the quenching methods used was observed to produce significantly different partitioning of Fe²⁺ among tetrahedral and octahedral structural sites in the basalt glasse. The highest cooling rate (brine and ice bath) yielded the largest tetrahedral Fe²⁺ values. No observable effects of quenching rate were measured with the other glass compositions investigated.