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Willamette

Iron, ungrouped, recrystallized octahedrite
standby for willamette photo
Found 1902
45° 22′ N., 122° 35′ W. This historic iron meteorite, weighing 14,900 kg, was discovered by a Welsh immigrant named Ellis Hughes while prospecting for minerals. The location was 3 km northwest of Willamette, Clackamas County, Oregon, which is presently a part of West Linn. The iron mass was probably deposited at its find location by the largest floods ever scientifically documented, the Missoula Floods, which occurred at the end of the last Ice Age about 14,000 years ago. Willamette is a heavily weathered meteorite, possibly losing a fourth of its original weight through tens of thousands of years of terrestrial corrosion in the rainy Oregon valley. Over 8.5 kg of oxide–shale was present inside the pit, and nickel that was leached from the meteorite was found in the surrounding ground. The apex has lost 1–2 cm from its surface, and any fusion crust, regmaglypts, or flow structures that might have been present on the meteorite have since been removed.

Through long-term electrochemical action of dilute sulfuric acid, produced from the dissolution of troilite, an array of pits, holes, and deep cavities has been sculpted, while the material persisting between these cavities has taken the form of tapered, hourglass-shaped pedestals (see bottom photo). When found, and probably upon entry, the bell-shaped mass was oriented with its blunt apex pointing into the ground, and the corroded, cavity-riddled, rear side directly level with the ground. Consequently, the cavities and pedestals are mostly aligned perpendicular to the rear side of the meteorite and the ground.

Because the meteorite was found situated on land belonging to the Oregon Iron and Steel Company, Mr. Hughes tried to purchase the portion of land on which it was located. However, after failing to acquire the necessary funds for the purchase, he and his 15-year-old son (as the story is told) prepared to secretly relocate the iron mass nearly 1.2 km to his own property. He first constructed a strong cart made of logs, with wheels fashioned out of tree stumps. Using a block and tackle, he utilizing his horse as a winch by walking it around a capstan from which a wire rope was attached to the meteorite. Through his mechanism he slowly levered the huge iron mass high enough to overturn it onto the cart. He then used this rudimentary winching procedure over 40 times to pull the cart over soft ground and obstructions and onto his own property, which amazingly took only three months. Thereafter, Mr. Hughes charged visitors 25¢ admission to view his meteorite, until the day an attorney for the Oregon Iron and Steel Company noticed the cleared path leading back to their land.

Mr. Hughes soon found himself a defendant in Clackamas County Circuit Court, Case #7587. He based his defense on the fact that he rightfully found an abandoned Indian relic, previously revered by the Clackamas tribe as their Tomonowos, variously translated as ‘Heavenly Visitor’ or ‘Visitor From the Moon’. In testimony given by an Elder Wasco Indian, he said he had been told by the Clackamas Chief Wochimo that before battles young warriors would dip their arrows into the sacred water which had collected in the basins of the meteorite. Others testified that Native doctors of the Clackamas Tribe used the meteorite ceremonially in healing and purifying rituals. Other testimony revealed that songs and dances based on the ‘Heavenly Visitor’ have been passed down from Tribal ancestors. Nevertheless, the jury found in favor of the Iron and Steel Company, and this verdict was upheld in the Oregon State Supreme Court with the ruling:
‘Meteorites, though not embedded in the earth, are real estate, and consequently belong to the owner of the land on which they are found.’ Following a short period on display at the Lewis and Clark Centennial Exposition in Portland, the meteorite was sold to a New York heiress, Elizabeth E. Dodge II, for $20,600, who then presented it to the American Museum of Natural History in New York in 1906. In 1935, the Hayden Planetarium was built around the Willamette and Cape York masses. In the late 1990’s, amid publicity surrounding the construction of the Museum’s Rose Center of Earth and Space, the Clackamas Tribe, now the Confederated Tribes of the Grand Ronde Community of Oregon, filed a claim under the Native American Graves Protection and Repatriation Act to have the meteorite returned to their lands in Oregon. A federal lawsuit was filed by the AMNH against the Grand Ronde, and the fate of the meteorite was finally settled out-of-court. An agreement was made recognizing the Tribes spiritual relationship with the meteorite, and ensuring an annual private visit to the meteorite by the Grand Ronde for religious, historical, and cultural purposes.

Although Willamette shows compositional similarities to members of both the IIIAB and IIIE groups, Rubin et al. (2015) observed significant differences in inclusions from these iron groups; therefore, they suggest the reclassification of Willamette as an ungrouped iron. The meteorite exhibits a recrystallized texture with only remnant evidence of a medium Thomson (Widmanstätten) structure, the result of a significant impact-heating event. Following an extended period of annealing, another lesser shock event produced Neumann bands and further annealing of kamacite at depth. Troilite nodules exhibit features of crushing and limited shock-melting, while monocrystalline schreibersite occurs throughout the meteorite. Structural evidence indicates the meteorite experienced a molten stage on its parent body. Based on cooling rate models, the diameter of the parent body is estimated to have been ~100 km, taking ~50 m.y. for the core to solidify.

The 2.81 g partial slice of Willamette pictured above was sectioned from the 96.47 g mass shown below, previously purchased at the 2002 Macovich Auction in Tucson, Arizona. This 96.47 g mass was acquired for the Macovich Collection of Meteorites through a trade with the American Museum of Natural History in New York. standby for willamette photo
Photo of the 96.47 g section of Willamette from which the above specimen was sectioned
Photo courtesy of The Macovich Collection of Meteorites.

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The heavily corroded rear side of the Willamette meteorite

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Historical image of the Willamette iron meteorite in transport to the Hughes property


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Taza

Iron, ungrouped plessitic octahedrite
standby for taza photo
Found 2000
no coordinates recorded Many individual masses from this rare shower-producing meteorite have been recovered in Taza, Morocco, having a total weight of 75.3 kg. This meteorite has been assigned number 859 in the NWA series. Many of the Taza masses have well-oriented shapes.

Taza is chemically anomalous and unrelated to any established iron chemical group. However, Taza has an elemental composition very similar to the ungrouped, plessitic iron, Butler, and the two can be clearly grouped together (see table below). They have an extremely high Ge concentration of 2000–2300 ppm (Wasson, 2011), four times higher than any other iron meteorite. Similarly, their Au and As contents are high. A high Ni content of ~16% promotes kamacite to form discontinuous, pointed spindles, rimmed by taenite, with widths measuring ~0.15 mm. Tetrataenite (~50% Ni) forms a narrow border on some kamacite spindles. In the dense, fine-grained plessite matrix, this spindle pattern is repeated on a scale ten times finer to form a µm-sized Thomson (Widmanstätten) structure. This uncommon plessitic microstructure is transitional between the octahedrites and the ataxites.

It has been tentatively declared that Taza and Butler are probably nonmagmatic irons (Wasson, 2011). The specimen of Taza shown above is a 13.42 g etched partial slice that displays kamacite spindles in a Thomson (Widmanstätten) pattern.


Comparison of Volatile Element Abundances for Taza and Butler
Co(%) Ni(%) Cu(ppm) Ga(ppm) Ge(ppm) As(ppm) W(ppm) Ir(ppm) Pt(ppm) Au(ppm)
Taza 1.29 16.34 291 87.2 2290 54 6.7 2.42 38 6.48
Butler 1.21 15.67 151 87.1 1970 48 5.1 1.8 34.9 6.77

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Sombrerete

Iron, ungrouped
(possibly CR chondrite related)
standby for sombrerete photo
Found 1958
23° 38′ N., 103° 40′ W. A single mass of ~10 kg was found in Sombrerete, Zacatecas, Mexico. Sombrerete was initially considered to be an anomalous iron related to the small non-magmatic IIE group, some members of which contain similar globular silicate inclusions, and it was routinely included in studies of this group. However, the silicate inclusions in Sombrerete have O-isotopic compositions that plot far away from those of IIE irons, and furthermore, the Δ17O is significantly more negative than those for typical IAB irons; this suggests an origin for the sHL subgroup on a different asteroid (Wasson, 2011). Notably, the Δ17O of Sombrerete is nearly identical to that of the silicate-bearing NWA 468, the two showing only a small difference in mass fractionation on an oxygen three-isotope diagram. As demonstrated by its plot on a Δ17O vs. ε54Cr diagram, NWA 468 is now strongly considered to be an anomalous, metal-rich lodranite (Sanborn et al., 2014). It has been demonstrated that the acapulcoite–lodranite clan accreted in the non-carbonaceous reservoir, which remained separated from the carbonaceous reservoir in the early Solar System due to the rapid accretion of proto-Jupiter. Contrariwise, it has been demonstrated through Mo- and Os-isotopic analyses that Sombrerete formed in the carbonaceous reservoir closely allied with group IVB irons (Worsham et al., 2017). Oxygen Isotope Compositions of Silicate-bearing Irons
Diagram credit: A. Ruzicka, Chemie der Erde–Geochemistry, vol. 74, no. 1, p. 6 (Mar 2014)
‘Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies’
(https://doi.org/10.1016/j.chemer.2013.10.001)
standby for silicated iron o-isotopic diagram
click on image for a magnified view

Abbreviations: TF = terrestrial fractionation line, CCAM = carbonaceous chondrite anhydrous materials mixing line; silicated iron meteorites include IAB, IIICD, IIE fractionated (IIE fr.) and IIE unfractionated (IIE unfr.), IVA, and IIIAB Puente del Zacate (PdZ); ungrouped irons (Ungr.) include Guin (G), Enon (E), NWA 468 (468), Sombrerete (S), Tucson (T), Mbosi (Mb), Bocaiuva (B), and NWA 176 (176); other meteorites include H, L and LL chondrites, winonaites, mesosiderites (meso.), main-group pallasites (MG pall.) Eagle Station pallasites (ES pall.), and pyroxene pallasites (px pall.)

standby for carbonaceous vs. non-carbonaceous reservoirs diagram
Diagram credit: Scott et al., The Astrophysical Journal, vol. 854, #2 (2018)
‘Isotopic Dichotomy among Meteorites and Its Bearing on the Protoplanetary Disk’
(https://doi.org/10.3847/1538-4357/aaa5a5)
In a study of the IAB subgroups, employing precise Mo, W, and Os isotope data along with HSE and other literature data, Worsham et al. (2017) demonstrated on a coupled µ97Mo vs. µ189Os diagram that Sombrerete plots with the IVB magmatic irons. Notably, the ungrouped iron Chinga also plots with Sombrerete and the IVB irons, attesting to the formation of these meteorites in a common reservoir in a spatial and/or temporal aspect (see the Chinga page). Two distinct reservoirs existed in the early protoplanetary disk—carbonaceous chondrite (CC) and non-carbonaceous (NC). These reservoirs were segregated by the rapid accretion of proto-Jupiter and reflect differences in the contribution (i.e., susceptibility to thermal processing) of p-, r-, and s-process isotopes inherited as dust ejecta from explosive stellar nucleosynthesis (Poole et al., 2017; Bermingham et al., 2018). It was demonstrated that the IVB irons formed is the carbonaceous chondrite reservoir (see further details in the Appendix, Part III). Since major compositional differences exist among all of these meteorites, it is likely that they represent distinct parent bodies. See diagrams below, where Sombrerete is the crossed yellow triangle and &#181 notation denotes deviation from terrestrial standards in parts per million. CRE-corrected Mo vs. Os for IAB Complex Irons
standby for sombrerete mo-os diagram

CRE-corrected Mo Isotopic Compositions of Meteorite Groups
standby for chinga mo diagram
click on photo for a magnified view

Diagrams credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, pp. 157–166 (2017)
‘Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex’
(https://doi.org/10.1016/j.epsl.2017.02.044)
With reference to the many mineral and textural similarities in their silicates, it can be argued that Sombrerete followed a similar petrogenetic path as the silicate-bearing IIE irons, and possibly other silicated irons. A taxonomic revision of the IAB–IIICD iron group was proposed by Wasson and Kallemeyn (2002), which led to the tentative inclusion of Sombrerete into the newly defined IAB complex and resolution on a Ni–Au diagram as a member of the high-Au, low-Ni subgroup (sHL). The fact that a large concentration of sHL members has been found near Erfoud, Morocco, and that four of them might be paired (Hassi-Jekna and NWA-series members 3200, 4706, and 4710), provides evidence for the presumption that the IAB subgroups derive from separate impact-melt pools on a single unique asteroid.

Although no other sHL members have undergone O-isotopic analyses, Sombrerete has a very different O-isotopic value than the other measured IAB irons (Δ17O = –1.39‰ vs. the typical ~ –0.5‰). Therefore, it may be more plausible that Sombrerete, and potentially the other sHL subgroup members, formed on a separate asteroid, perhaps related to the CR chondrite clan (Ruzicka et al., 2006; Ruzicka, 2014). In their study of the Mo isotope systematics among IAB complex irons, Dauphas et al. (2002) reported that the sHL subgroup iron Magnesia is distinct from the IAB main group. Further evidence supporting the hypothesis for separate parent bodies for Sombrerete and possibly the entire sHL subgroup was presented by Worsham and Walker (2015, 2016). They studied the Mo-isotopic compositions of representative meteorites from the IAB iron complex, and it was ascertained that Sombrerete is clearly resolved from members of the MG, sLL, and sLM subgroups of the IAB complex. They also recognized that the latter three subgroups share very similar W- and Mo-isotopic values and have Mo-isotopic values indistinguishable from that of the Earth. Notably, these subgroups represent the Earth’s closest genetic relatives. Furthermore, they found that a member of the sHH subgroup (ALHA80104) was also clearly resolved from other members of the IAB complex (MG, sLL, sLM) with respect to its Mo- and W-isotopic values and by its older metal–silicate segregation age as determined by Hf–W systematics, and therefore it was concluded that both the sHL and sHH subgroups might derive from distinct parent bodies located in separate nebular regions from other members of the IAB complex irons. Therefore, because the Δ17O value, HSE data, and Mo-isotopic composition determined for Sombrerete are significantly different from that of all other IAB complex irons, Worsham et al. (2017) suggest that it no longer be classified as a IAB iron.

Sombrerete contains 7.3 vol% highly fractionated, rounded silicates, 1–10 mm in size (mostly ~2 mm), located mainly along metal grain boundaries (Prinz et al., 1982). The silicates show evidence of rapid quenching from a flowing melt, exemplified by the presence of crystal alignment and skeletal crystals. These silicates are highly enriched in alkalis, with compositions ranging from trachybasalt (~48 wt% silica) to alkali-rich basaltic andesite (~55 wt% silica) to andesite (~60 wt% silica) to dacite (~65 wt% silica). The plagioclase in Sombrerete is unusually Ca-rich compared to that in most other silicated irons, a likely consequence of a fractional crystallization process (Ruzicka, 2014).

A number of different types of silicate inclusions have been distinguished by Ruzicka et al. (2006). Some inclusions are composed primarily of albitic glass, comprised of equal amounts of plagioclase and quartz with varying amounts of chlorapatite and very fine-grained orthopyroxene. Others are composed of glass containing significant amounts of the rare mineral yagiite, a mineral which otherwise has only been reported in the IIE iron Colomera. The occurrence of yagiite infers its crystallization from an immiscibly separated K-rich melt (Ruzicka, 2014). Still other types of glass inclusions, which may be Na-, K-, or Na–K-rich, are thought to be derived from immiscible melt fractions. These also contain a complex mixture of mineral constituents, including titanean kaersutite, ilmenite, plagioclase, chromite, merrillite, and tridymite. Some plagioclase present in inclusions of the latter type exhibits a porous texture (‘spongy’), produced through the crystallization of an immiscible, quartz-enriched melt. Other inclusions of this same type contain P-rich crescent-shaped regions, with orthopyroxene and plagioclase grains showing preferential alignment to these regions suggestive of flow. The metallic host phase is composed of a plessitic intergrowth of kamacite and taenite, along with troilite and schreibersite.

The globular silicate inclusions, considered by some to reflect metal–silicate liquid immiscibility (Prinz et al., 1983; Ruzicka and Hutson, 2003), are now presumed to reflect a filter-press fractionation mechanism (Ruzicka and Hutson, 2005; Ruzicka et al. 2006). Based on their studies, Ruzicka et al. (2006) hypothesize a two-stage formation scenario leading to the observed high fractionation of silicates: Initially, a CR-like chondritic protolith experienced low-degree (~4–8%) partial melting as a result of endogenous heating from the decay of short-lived radionuclides. A CR-like protolith is consistent with the measured O-isotopic compositions, as well as the P content of Sombrerete. This partial melting phase produced a phosphoran basaltic andesite. Thereafter, the partially molten metallic host acted as a filter to separate the emergent silicate crystals (primarily chlorapatite and orthopyroxene) from the residual silicate melt as it flowed in-between inclusions. This flow was likely generated by an impact event or a close gravitational interaction, which may have also resulted in the tidal disruption and re-accretion of the planetesimal, thereby separating the solid and molten phases and moving the molten metal–silicate mixture nearer the surface where it was rapidly cooled. A possible period of slower cooling may have followed re-accretion. The compositional variation observed among the silicate inclusions (trachybasalt to dacite) is the result of the variable loss of chlorapatite and orthopyroxene from the Si-poor, P-rich parental liquid. A similar chain of events may have occurred in other silicate-bearing, nonmagmatic irons such as the evolved members of the IIE group, with Colomera showing very close similarities to Sombrerete. The absolute I–Xe age for Sombrerete, calculated relative to Shallowater (4.5623 [±0.0004] b.y.), was determined to be 4.5619 (±0.0010) b.y. (Bogard and Garrison, 2009). This age is considered to reflect the differentiation of the silicate and its admixture with the metal phase during parent body disruption. Application of the Hf–W chronometer gave a metal segregation age of 2.1 (±0.9) m.y. after CAI formation (Worsham et al., 2014). This is ~2.4 m.y. earlier than the onset of metal segregation in the IAB main group and members of the sLL subgroup. On the other hand, Worsham and Walker (2016) reported W-isotopic values for Sombrerete that are consistent with those of IAB MG members. Notably, they also determined a W-isotopic composition for sLM subgroup member Persimmon Creek that corresponds to a younger metal –silicate segregation age than that of MG members, which provides support for an impact-melt-pool formation scenario on a common parent body.

An Ar–Ar age of 4.541 (±0.012) b.y. was established for Sombrerete, indicating closure occurred only 20 m.y. later than for the I–Xe system. However, since it was inferred that no resetting event had occurred since crystallization (Bogard et al., 2000), an Ar–Ar age correction of ~20 m.y. was applied based on improved 40K decay parameters calculated by Vogel and Renne et al. (2008); this correction brings the two chronometers into agreement. These chronometers are therefore most consistent with the scenario of rapid cooling after formation. Both of these ages are older than the corresponding radiometric ages of the IIE irons. The CRE age for Sombrerete was calculated to be 278 m.y. to 819 m.y. (based on 21Ne and 38Ar, respectively), which is also older than that of the IIE irons.

The photo shown above is a 49.72 g partial slice of Sombrerete, while the top photo below shows the crusted side. This specimen is from the 433 g section shown in the bottom photo below. The 433 g section was previously part of the J. Schwade Collection, originally obtained from M. Cilz.

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standby for sombrerete photo
Photo courtesy of Dr. J. Piatek


For additional information on collisional dynamics, read the PSRD article by G. Jeffrey Taylor—’Tagish Lake—Hit-and-Run as Planets Formed‘, Nov. 2006.


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NWA 176

Iron with silicate inclusions, ungrouped
(CO–CV clan related)
standby for northwest africa 176 photo
Found 1999
no coordinates recorded A rare Saharan iron meteorite weighing 2 kg was found near the border of Morocco and Algeria. The mass was obtained by G. Cintron of Island Meteorites, and a portion was sent for study to several institutions, including the Hawaii Institute of Geophysics & Planetology, the Institute of Geophysics and Planetary Physics in Los Angeles, the Enrico Fermi Institute in Chicago, the Universitat Bern in Switzerland, and Washington University in St. Louis. This meteorite contains sub-mm- to cm-sized, rounded, greenish-yellow, silicate inclusions dispersed throughout the metal host constituting ~44 vol%. Some of the silicates are aligned along shear planes that were probably created during an impact event.

Northwest Africa 176 is nearly identical to the meteorite Bocaiuva in elemental abundances, oxygen isotope composition, and petrographic features, and both were likely derived from the same parent body—an asteroid with a composition similar to the CO–CV clan. Although the O-isotopic composition for both NWA 176 (Δ17O = –5.2‰) and Bocaiuva is similar to that of the IAB iron complex, they are clearly resolved from that group both by their significantly lower Au and As values (Wasson, 2011) and Mo isotope systematics (Budde et al., 2016). This O-isotopic value is also similar to the pallasites of the Eagle Station grouplet (see top diagram below), but the Ge and Ga contents are higher in these silicated irons which suggests an origin from similar chondritic material in the same region of the protoplanetary disk. Notably, utilizing a coupled Δ17O vs. ε54Cr diagram (see bottom diagram below), Sanborn et al. (2018) have demonstrated that Eagle Station plots closer to the CK (or CO) chondrite group. It could be inferred that both the CV and CK planetesimals experienced a similar petrogenetic history in a similar isotopic reservoir of the nascent Solar System. standby for o-isotopic diagram
Diagram credit: Greenwood et al., Chemie der Erde, vol. 77, p. 21, (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)

Chromium vs. Oxygen Isotope Plot
standby for o-cr diagram
click on diagram for a magnified view

Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018) Northwest Africa 176 and Bocaiuva, along with the Eagle Station group pallasites, are resolved from the main-group pallasites and the major iron groups by their higher Ge/Ga ratios, higher Cu and Ir contents, and lower Au, As, and Sb contents. For those few irons that do have Ge/Ga ratios similar to NWA 176, in particular, the irons of group IIF and certain ungrouped irons such as the silicated iron Mbosi, their elemental abundance ratios rule against a genetic relationship with NWA 176 and its relatives. Nevertheless, it is likely they all originated from similar chondritic precursor material in the carbonaceous reservoir beyond Jupiter.

A multi-stage formation history is proposed for NWA 176 in which an initial impact generated enough heat (~1100°C) to form a melt pool. This was followed by gravitational differentiation that was sustained above ~500°C for a significant time. Differentiation resulted in a lower metallic layer with a cumulate silicate layer above. A subsequent impact event shattered the silicate layer and mobilized metal forcing it into existing cracks in the silicate layer, while initiating a rapid cooling phase at a rate of 1000°C/m.y. This cooling data would be consistent with a small body of only a few km in radius, or possibly reflects the breakup phase of a much larger object. Extended annealing at depth led to the rounding of the corners on silicate grains, a thermodynamic process acting to minimize the surface energy.

Several factors support this origin rather than the core–mantle interface origin commonly envisioned for the main-group pallasites. The nearly chondritic silicate composition with its relatively low proportion of olivine is more consistent with an impact-melt model than for a core–mantle interface origin. Moreover, the heat-generating radiogenic isotopes present in the later-forming carbonaceous chondrite parent bodies would be insufficient to produce melts that could form pallasitic compositions. Furthermore, the shear forces that produced the deformation features, as well as the loss of alkali volatiles, can be reconciled through impact mixing processes. Finally, the low Ir content of NWA 176 and its relatives is not consistent with fractional crystallization processes necessary for a core–mantle origin. One possible alternative origin proposed for this meteorite is that the precursor was a metal-rich, highly metamorphosed, chondritic asteroid. Further details about possible origins of NWA 176 can be found on the Allende page.

Northwest Africa 176 has an absolute I–Xe age, calculated relative to Shallowater (4.5623 [±0.0004] b.y.), of ~4.544 (±0.007) b.y., indicating a relatively late formation, probably through impact processes (Bogard and Garrison, 2009). The meteorite has a remarkably high Ar–Ar age of 4.524 (±0.013) b.y. The CRE age of 41 (±12) m.y. calculated for NWA 176 is low compared to other iron meteorites.

A portion of the information above was gleaned from the paper ‘Bocaiuva—A Silicate-Inclusion Bearing Iron Meteorite Related To The Eagle-Station Pallasites’ (Malvin, Wasson, Clayton, Mayeda, & Curvello), Meteoritics, vol. 20, #2, Part 1 (1985). The specimen of NWA 176 pictured above is an 8.07 g partial slice displaying rounded silicate inclusions. It’s fresh dark fusion crust attests to a recent arrival.