Hi everyone,

This is now going off on a tangent perhaps, but following up on Eric’s remark, I am wondering where we are with the Fe³⁺ in biotite problem. There seems to be some consensus that all biotite contains substantial Fe³⁺, even under reducing conditions, based on quite a number of published ferrous/ferric Fe determinations in biotites from a variety of rocks which appear to confirm that. There are two comments I want to make, just to play the devil’s advocate (and to provoke debate):

(1) I remember that many years ago Moessbauer analyses had been done on drillcore samples from the KTB deep drilling project showing variable Fe²⁺/Fe³⁺ ratios in biotite, but Fe³⁺ was not detected in all biotite samples. If that is real, any Fe³⁺ measured may well be secondary. I believe all these rock samples were graphite-bearing and compositionally similar.

(2) Biotite is notoriously sensitive to secondary oxidation. Whether it “looks fresh” in the microscope is not a particularly strong argument to exclude any secondary alteration. It just means there is no obvious visual evidence. So, how can we ever be sure that the Fe³⁺ contents actually measured, let alone those introduced via correction procedures, are even roughly correct, in terms of primary biotite composition? Most commonly, we pick our samples from surface outcrops, and yet assume that minerals such as biotite remained isolated from the influence of subsurface oxidizing fluids (before exposure) and weathering agents. Isn’t that wishful thinking in order to keep our lives simple?

Are there any experiments to shed some light on this?

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