Watson 012        30°34’S, 131°30’E

South Australia, Australia

Found: 12 May 2009

Classification: Ordinary chondrite (H7)

History: The first piece was found by A. Tomkins during May 2009. A further 11 pieces were collected from the Nullarbor Plain in 2009 by A. Tomkins and A. Langendam. The following year, on 14 April 2010, another 9 pieces were found by A. Tomkins, using a metal detector. The total number of pieces is 21.

Physical characteristics: The total mass collected weighs 103.1 g, with the largest piece measuring 3.3 cm x 2.6 cm x 1.4 cm. The exterior is a dull brown with no obvious chondrules or fusion crust. The cut surface is a rusty brown with the occasional fleck of metal and no obvious chondrules. The 21 pieces were all found within ~25 m of each other (some buried), they are all broken pieces with the same externally weathered appearance, they have the same magnetic response, and they respond similarly to a metal detector as a function of size. We have noted that other weathered meteorites on the Nullarbor show a similar breakup and dispersal pattern, which widens as a function of weathering duration.

Petrography: (Kim Lai N. Bell, Monash). A fine- to coarse-grained, granular texture distinguishes this meteorite from most chondrites. Grain sizes have a heterogeneous distribution such that fine grains are associated with domains of extensively interconnected intergranular plagioclase, and coarse grains occur away from these zones. Grain size ranges from <0.1-0.7 mm, with an average of ~0.3-0.4 mm. Mineralogy is similar to an ordinary chondrite and includes olivine (45%), pyroxene (25%), plagioclase (9%), Fe-Ni metal (8%) and troilite (10%). Olivine and pyroxene dominantly display sharp extinctions, with occasional very weakly undulose extinction being observed, plus some irregular fracturing in larger grains. Plagioclase (50 μm to 1 mm) has a distinctly unusual texture, occurring in extensively interconnected interstitial domains and containing numerous spherical inclusions of olivine and pyroxene. Nine indistinct chondrules were identified in 4 separate thin sections. These chondrules include textural types, BO, PO, POP and PP, with diameters ranging from 0.5 mm to 1 mm (average ~0.8 mm). Metal and sulfides are highly oxidized with up to 90% of these minerals being replaced.

Geochemistry: EMPA (wt%) Olivine: SiO₂ = 38.63, TiO₂ = 0.00, Al₂O₃ = 0.00, FeO = 16.04, MnO = 0.49, MgO = 44.90, CaO = 0.02, Na₂O = 0.02, K₂O = 0.01, (Fa_16.70±0.11, n = 8). Low-Ca pyroxene: SiO₂ = 54.95 TiO₂ = 0.18, Al₂O₃ = 0.41, FeO = 10.17, MnO = 0.53, MgO = 30.92, CaO = 1.87, Na₂O = 0.07, K₂O = 0.01, (Fs_15.11±0.22, n = 4). Plagioclase: SiO₂ = 65.08, TiO₂ = 0.05, Al₂O₃ = 22.32, FeO = 0.90, MnO = 0.01, MgO = 0.03, CaO = 3.21, Na₂O = 8.95, K₂O = 0.95 (An_16.18±2.22, n = 7). Kamacite: Ni = 6.89, Co = 0.35. Troilite: Fe = 62.68, Ni = 0.11, Cr = 0.11. Oxygen isotopes: δ¹⁷O = +3.44, δ¹⁸O = + 5.50, Δ¹⁷O = 0.58.

Classification: Ordinary chondrite (H7, S1/2, W3)

Specimens: All pieces and four thin sections held by A. Tomkins at Monash.