PERRYVILLE


Iron, IIC, plessitic octahedrite
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Found August 1906
37° 44' N., 89° 53' W.

An angular mass of 17.5 kg was found on the farm of Patrick Monaghan about one mile west of Perryville, Missouri. The mass was sticking out of the soil as if it had been previously disturbed by plowing. It has suffered from terrestrial weathering and lost an average of 1–2 mm from the surface, but still preserves sections of the heat-affected zone and retains evidence of ablation pits.

A Ni content of ~9.6% creates a µm-sized Thomson (Widmanstätten) structure of fine-grained plessite composed of kamacite spindles having widths of ~0.06 mm and lengths up to ~0.9 mm, most all of them containing nuclei of schreibersite. Perryville was formed at an early-stage (~10%) of fractional crystallization of an evolving liquid associated with core formation (Tornabene et al., 2019). The iron began as a single taenite crystal that formed a progressively finer structure as it cooled. This uncommon plessitic structure is transitional between ataxites and finest octahedrites.

The Re–Os age for the IIC group was determined by Tornabene et al. (2019) to be 4.538 (±0.140) b.y. Perryville is closely related to the other six irons constituting the IIC group: Ballinoo, Cratheús [1950], Darinskoe, Kumerina, Salt River, and Unter Mässing. The previously included iron Wiley has been eliminated on the basis of its HSE values and the relative enrichments in its 183W and 94/95/97Mo isotopes (Tornabene et al., 2018, 2019). The IIC irons are associated with the carbonaceous reservoir in the outer region of the Solar System (see diagram below).

Ru vs. Mo Isotopes
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Diagram credit: Tornabene et al., 50th LPSC, #1236 (2019)

To learn more about the relationship between this and other iron chemical groups, see Appendix III. The photo above shows a 11.5 g etched partial slice that displays kamacite spindles in a µm-sized Thomson (Widmanstätten) pattern. An ~1 mm shocked troilite nodule is visible on this slice. These 0.1–2 mm nodules generally occur every 25 cm², rarely attaining a size of up to 8 mm. The photo below is from the National Museum, showing the original mass.

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