INMAN


L/LL3.4
standby for inman photo
Found August 1966
38° 15' N., 98° 40' W.

A single stone weighing 7.25 kg was found by M. Neufeldt during plowing in McPherson County, Kansas. It was taken to the Kansas State University Geology Department for analysis and subsequently classified as an L3 chondrite with a shock stage of S2. The mass as found was moderately weathered but still had shallow regmaglypts over most of its surface. Fusion crust was thin and missing on some portions of the surface.

Inman does not strictly follow the ordinary chondrite metal–silicate trends for classification. It has a total iron content of 19.45 wt% placing it in the range of the LL group (18.5–21.5 wt%), although this could also be due to terrestrial leaching. The average fayalite content of 21 mole% places it at the extreme lower end of the L group (22.7–25.6 mole%), biased towards the H group range (16.9–20.4 mole%); nevertheless, the Fa in certain olivines approaches 33 mole%, which is in the LL group range (27.0–33.0 mole%). There is a broad range in ferrosilite content averaging near 15 mole%, slightly outside the range of the L group (18.7–21.8 mole%) and also biased towards the H group range (15.7–18.1 mole%). Other elemental ratios are comparable to the L group; an Fe/SiO ratio of 0.46, and a metallic nickel-iron content of 7.94 wt% (L group = 4.4–11.7 wt%).

Evidence is slowly building in favor of a separate L/LL chondrite parent body. In 1990, a brecciated chondrite fell in Glanerbrug, Netherlands, consisting of two distinct lithologies: a dark-gray portion with olivine plotting at the high end of the LL field, and a light-gray portion with olivine plotting at the high end of the L field. Further analysis finds that a kamacite vs. olivine plot of the light portion falls within the L/LL field. A similar plot of the dark portion ranges widely within the LL field. Siderophiles of the light portion appear to be LL like. It is suggested that the light portion is a xenolithic L/LL fragment incorporated into a dark LL chondrite host.

It was demonstrated by Szurgot (2016) that the mean atomic weight (Amean) of meteorites can be used to resolve the OC groups, including the intermediate groups L/LL and H/L. Amean values can also be predicted through various equations based on other parameters such as atomic Fe/Si ratio, grain density, and magnetic susceptibility, and these Amean values all consistently resolve these groups into the ordered sequence LL < L/LL < L < H/L < H. Inman has Amean values of 23.49 (chemical composition) and 23.08 (Fe/Si atomic ratio), with the former but not the latter value indicative of an L/LL classification. Furthermore, it was demonstrated that Amean values are lower for unequilibrated type 3 samples than for equilibrated samples within each OC group due to the presence of water; Amean values for petrologic types 4–6 are indistinguishable within each group.

standby for amean diagram
Diagram credit: M. Szurgot, 47th LPSC, #2180 (2016)
Amean based on chemical composition (Eq. 1), Fe/Si atomic ratio (Eq. 2), and grain density (Eq. 3)

Inman also has a distinctly different silicon isotopic composition compared to other meteorites. Contained within a SiC carrier is an abundance of interstellar noble gases including exotic Ne and Xe. It is calculated that Inman would have had to spend ~1.5 m.y. in a regolith setting, in addition to the 20 m.y. in space as inferred by its CRE age, to account for its fraction of anomalous noble gases. Two different forms of silicon nitride, or nierite, have also been identified in Inman.

A most surprising result was obtained from the use of Raman microspectrometry techniques. Studies of the carbon structural order as it relates to petrography infer that Inman has a petrologic type of 3.0/3.1 rather than the commonly accepted 3.4 type. In a similar way, TL sensitivity data for Inman are consistent with its being a petrologic type 3.3, and it has in fact been studied as an LL3.3 as well as an LL3.4. Much of the confusion over its petrologic type probably lies with the fact that Inman is terrestrially contaminated. However, this terrestrial alteration cannot explain its anomalously high 15N enrichment, which is thought to be likely caused by the presence of a pristine component.

Other meteorites assigned to this intermediate chondrite group include Seemore Downs (L/LL4), Bjurböle (L/LL4), Cynthiana (L/LL4), Knyahinya (L/LL5), Nyirábrany (L/LL4-5), Chelyabinsk (L/LL5; Szurgot, 2016), and Naryilco (L/LL6).The specimen of Inman shown above is a 1.4 g edge fragment removed during thin section preparation.