ST. GENEVIEVE COUNTY


Iron, IIIF, octahedrite
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Found during Fall of 1888
37° 58' N., 90° 19' W.

A single spheroidal mass of 244 kg was found by Mr. Z. Murphy, a surveyor, ~1 mile west of the hamlet of Punjaub, which is near present-day Lawrenceton, Missouri. The St. Genevieve mass was bought by Ward several years later. Because of the long terrestrial age, several mm has been removed from the surface and terrestrial oxides are evident.

Group characteristics of the IIIF magmatic irons include very low-Co, high-Cr, and low-P contents. Other meteorites constituting group IIIF include Moonby, Clark County, Nelson County, and Oakley. In their paper ‘Chemical classification of iron meteorites–VIII’, Wasson and Scott (1975) determined that Oakley was the fifth member of a common grouping, which was thereby established as group IIIF. Since then the number of irons in this group has increased to nine. However, it should be noted that fractional crystallization modeling conducted by Zhang et al. (2021) indicated four of the IIIF members—Binya, Cerro del Inca, Moonbi, and St. Genevieve County—do not fit their model's Ga–As and Ge–As trends; therefore, they tentatively suggest that these four irons may represent a separate parent body from that of the other group members.

Based on O, Cr, Ti, and Ni stable-isotopic data, Warren (2011) recognized the existence of two distinct taxonomic super-groups: those which accreted inside the orbit of Jupiter where thermal processing occurred under reducing conditions, termed 'non-carbonaceous' (NC), and those which accreted outside of its orbit where thermal processing occurred under oxidizing conditions, termed 'carbonaceous' (CC); the difference in redox conditions is attributed to differences in the ice, dust, and gas abundances. In a study of W, Mo, and Ru isotopes in iron meteorites, Kruijer et al. (2017) recognized that both of these reservoirs were coeval and remained spatially separated within the protoplanetary disk for a prolonged period (~ 3.6–4.8 m.y., inferred from timing of CR and CB parent body accretion, respectively) due to the rapid growth of proto-Jupiter (~30 M core at 2.9–3.2 AU within the first 0.6 m.y.; Desch et al., 2018). Importantly, the pyroxene pallasites NWA 1911 and Zinder were determined to contain metal with a composition that is chemically identical to that of group IIIF irons (Boesenberg et al., 2017; Humayun et al., 2018). However, the IIIF irons formed in the carbonaceous region of the Solar System beyond Jupiter, whereas the negative ε54Cr and δ26Mg values of Zinder indicate that it formed in the non-carbonaceous, inner region of the Solar System (Wimpenny et al., 2019). See the Protoplanetary Disk page for further details about these two regions.

The specimen of St. Genevieve shown above is a 55.4 g etched partial slice exhibiting a fine to medium Thomson (Widmanstätten) structure. The photo below shows the entire mass as photographed by Ward.

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