Choteau

Montana, United States

Purchased: 2011

Classification: Pallasite (ungrouped)

History: Purchased in 2011 by Debbie Cilz at an estate sale in Choteau, Montana, and presumed to have been found locally by the deceased owner.

Physical characteristics: A single dense, brownish mass weighing 8474 g. Interior slices exhibit separated, angular clasts of olivine (~40 vol.%) within metal.

Petrography: (A. Irving and S. Kuehner, UWS) Composed predominantly of metal (kamacite with Ni-poor taenite) and large angular grains of olivine with accessory iron sulfide (pyrrhotite), schreibersite, merrillite, chromite and orthopyroxene. One large (4.5 mm) grain of exsolved calcic pyroxene was found (now composed of augite with orthopyroxene exsolution lamellae). Chromite and orthopyroxene occur in symplectitic intergrowths, and there also are small patches composed of chromite+iron sulfide+merrillite exhibiting triple grain junctions.

Geochemistry: Olivine (Fa_9.2-10.1; FeO/MnO = 27-35), orthopyroxene (Fs_11.2Wo_0.4; FeO/MnO = 23), host clinopyroxene (Fs_6.6Wo_44.3; FeO/MnO = 12), orthopyroxene exsolution lamella (Fs_15.3Wo_1.7; FeO/MnO = 18). Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed olivine by laser fluorination gave, respectively δ¹⁷O = 0.152, 0.067; δ¹⁸O = 2.502, 2.347; Δ¹⁷O = -1.169, -1.172 (all per mil).

Classification: Pallasite, ungrouped. The oxygen isotopic composition is unlike those for any other pallasites, and falls on the broad trend for acapulcoites and lodranites.

Specimens: A 20.2 g polished slice is on deposit at UWB. 94.2 g at ASU, 158 g at UNM, 80 g at TCU, 81.9 g at SI. The main mass is held by Boudreaux; additional material is held by Mr. R. Garcia and Mr. R. Cucchiara.