Crystal structure refinements have been performed on four grunerite crystals in order to compare oxidation effects in the grunerite structure with those reported for calcic and alkali amphiboles. The crystals examined are natural grunerite (NAT) and samples that were air heated at 700 °C, 1 atm for durations of 30 min (30M) and 2 h (2HR-A, 2HR-B).

The initial stages of oxidation in grunerite consist of both dehydrogenation and, unlike calcic and alkali amphiboles, an “oxygenation” process that involves structural decomposition. SEM photographs of the oxidized grunerite samples reveal that decomposition products formed both as irregular subparallel ridges roughly perpendicular to the axis of elongation of the grunerite crystals and as small subhedral prismatic crystals along heating-induced cleavage traces parallel to the axis of elongation. The decomposition products are greatly enriched in Fe and depleted in Mg and Si relative to the grunerite, and the oxidized samples are slightly magnetic, suggesting that they are, at least in part, magnetite.

Site-refinement data suggest that Fe is preferentially lost from M4 with oxidation, resulting in about 7% vacant M4 sites in the crystals heated for 2 h. There also appear to be smaller amounts of Fe loss from M3 and M2 but not M1.

These results agree with earlier investigations that indicate the grunerite structure is less able to accommodate the oxy-amphibole component than are the calcic and alkali amphibole structures.

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