Sauceda de la Borda        22°49’15.73"N, 102°31’28.99"W

Zacatecas, Mexico

Find: October, 2016

Classification: Iron meteorite (IAB-MG)

History: Found October, 2016, by Mr. Alejandro Cortes while he was clearing brush along a dirt road about 2 km southwest of Sauceda de la Borda. About 1/4 of the stone was sticking out of the ground. Purchased by Michael Farmer from the finder in December 2016.

Physical characteristics: A relatively unweathered and beautifully regmaglypted 41 kg stone, measuring approximately 37 × 19 × 18 cm. A 17 × 12 cm cut and etched slice shows a well-developed Widmanstätten pattern and an abundance of graphite/troilite nodules, one 8 × 2 cm. A well-developed heat-affected zone is present around the whole of the slice. Layered fusion crust and beads of ablated material are present within some of the regmaglypts.

Petrography: (L. Garvie, ASU) A 13 × 10 cm slice demonstrates a variable structure typical of many group IAB octahedrites, with half of the section rich in decomposed cohenite and the other half showing a well-developed coarse octahedral structure with few inclusions - the two structural types are separated by an elongated graphite/troilite nodule. Etched sections display a coarse Widmanstätten pattern with short but bulky kamacite lamellae, typically 2 mm wide. Kamacite shows two generations of Neumann bands: the first are strongly developed, whereas the second are partially annealed and decorated with sub-μm sized precipitates. Taenite/plessite occupies ~4 areal%, most commonly as ribbons and wedge-shaped blocks separating kamacite lamellae. Plessite shows dominantly pearlitic, comb, and net structures. Decomposed cohenite is present in abundance as smoothly rounded elongated grains (to 4 cm), within and following kamacite lamellae, enveloping schreibersite grains, and around graphite/troilite nodules. In a polished mount, all of the cohenite is decomposed to graphite and ferrite. Schreibersite occurs in three forms: as large grains (to 8 mm) with enveloping decomposed cohenite and surrounded by swathing kamacite; as grain boundary veinlets; and as rhabdites and sub-micron precipitates. The troilite/graphite nodules are entirely mantled by a structurally complex rim (<5 mm thick) of troilite, graphite, schreibersite, and decomposed cohenite. Deep emerald green crystals, possibly krinovite or kosmochlor, occur in several of the graphite nodules.

Geochemistry: (J.T. Wasson, UCLA), INAA, Ni = 69.9 and Co = 4.76 (both mg/g), Ga = 77.4, As = 15.3, Ir = 2.14, and Au = 1.59 (all μg/g).

Classification: Found only a few km from the iron Zacatecas (1792), though Sauceda de la Borda is structurally and chemically distinct. It is a member of the IAB main group and there are no other Mexican irons that are chemically similar.

Specimens: 1.2 kg at ASU. The rest with MFarmer.