Sulphides with chemical compositions between idaite and bornite have been identified from several quartz veins crosscutting Ordovician schists in Vielsalm, Belgium. Idaite is an oxidation product of bornite, and occurs in the more oxidised parts of the quartz veins. The alteration sequence bornite → “anomalous bornite” → idaite is observed, associated with the growth of chalcopyrite lamellae parallel to the {100} and {111} directions of precursor bornite. Electron microprobe analyses indicate a chemical composition significantly enriched in Cu, when compared with the ideal composition of idaite, Cu3FeS4, as well as a progressive compositional evolution from bornite to idaite. The X-ray powder pattern is indexed in the space group I4̄2m, similar to that of stannite, and gives the unit-cell parameters a = 5.279(4) and c = 10.47(2) Å.

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