We received sediment cores from a depth of about 150m below ground, and conducted rudimentary XRD and XRF analyses on the cores. They showed a significant amount of Fe2O3 (which I assume means ferric oxides) content (~3%), and I wanted to know whether that is realistic or we should assume that most iron containing minerals were oxidized during transport or storage (they were frozen at -80 C) and they occurred as ferrous phases originally.
If not, I was wondering what form I should assume these to be in, if they really are ferric/oxides/hydroxides. The groundwater is mildly anaerobic eH of ~100-150 mV with a pH range of 7-7.5.
The other name of Fe2O3 is Hematite that could be solves in acidic environment, but no changes when you transport the sample no change will be occur in compound.
Steric hindrance by organic molecules or other reduced compounds, especially sulfur species, may inhibit reduction.
Neither of your analytical techniques would be suitable to tell you whether you have ferric or ferrous iron in your sediments. XRF determines only total Fe - to see if you have either ferrous or ferric iron you need to carry out XAS. XRD identifies a mineral phase - do you know wich mineral is present? Only with the identification of the mineral you can tackle that question. Because there are many possible ferric oxides, which all have different stabilities.
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