Iron, IAB complex, sLH subgroup
standby for tazewell photo
Found 1853
36° 26′ N., 83° 45′ W. A mass of about 60 pounds was plowed up 10 miles west of Tazewell, Tennessee on land owned by Mr. William Rogers. Atmospheric ablation during entry left an irregularly sculptured shape with large regmaglypts, deep fusion-crusted pits, large holes, and long protuberances.

Tazewell contains inclusions of schreibersite, haxonite, and troilite. Upon etching, it displays a Thomson (Widmanstätten) structure of finest octahedrite (Off) texture due to its high Ni content of 17 wt%. Tazewell was previously a member of the IIID iron group, but following a taxonomic revision by Wasson and Kallemeyn (2002), it is now included within the IAB iron-meteorite complex. On a Ni–Au diagram, Tazewell forms a low-Au, high-Ni subgroup (sLH) of the main group. In another study of the IAB subgroups, employing precise Mo, W, and Os isotope data along with HSE and other literature data, Worsham et al. (2017) ascertained that the IAB complex irons represent at least three distinct parent bodies and at least three impact-generated metal–silicate segregation events (see schematic diagram below). They contend that the sLM and sLH subgroups likely formed in two different impact events on a common parent body, and in the same reservoir of the protoplanetary disk as the MG and sLL parent body. standby for iab iron formation diagram
Diagram credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, p. 164 (2017)
‘Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex’
Dey et al. (2019) employed 17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its associated phases originated on a common parent body (i.e., an endogenous mixture of core and mantle vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Three IAB irons were employed in the study, and it was demonstrated on a coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, values for the silicate component plot in a distinct region on an O–Cr coupled diagram (see diagram below). From these results they ascertained that the the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-related parent body and silicate from an unrelated and otherwise unsampled parent body. Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, to produce the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages. 17O vs. ε54Cr for Irons and Pallasites
standby for o-cr isotope diagram
click on photo for a magnified view

Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)
The FeNi-chloride named ‘Lawrencite’ was first identified in 1877 in a sample of Tazewell. This mineral absorbs moisture from the air and liquefies, a property known as deliquescence. The reaction with water and oxygen produces iron hydroxide and then hydrochloric acid, which can lead to the eventual disintegration of some meteorites. Tazewell is now the type locality for this mineral.

To learn more about the relationships within the IAB complex, and among other iron chemical groups, see the Appendix, Part III. The Tazewell specimen shown above is a 2.9 g etched partial slice with a small remnant of fusion crust. A large slice of Tazewell can be seen on the collection page of Mike Farmer.

Leave a Reply