Malotas (b)        28°56’S, 63°14’W

Santiago del Estero, Argentina

Find, doubtful fall: 22 Jun 1931

Classification: HED achondrite (Eucrite)

History: On the morning of 22 June 1931, a meteorite fall was reported in the province of Santiago del Estero, Argentina. Samples were given to Professor Juan Olsacher (Cordoba University) who decided to travel to the place of the fall and collect some pieces. Many of the studied pieces were H5 ordinary chondrites, named Malotas. However, during a visit to the small meteorite collection at MMG-Cordoba in 2015, Maria Eugenia Varela discovered two very different looking stones labeled as "Malotas, 1931" in the showcase. Recognizing that one of the stones was not an ordinary chondrite, Varela, in agreement with the director of the museum, made a detailed study of these stones. One of the samples was a piece of the ordinary chondrite, Malotas; the second one turned out to be a eucrite. The director of MMG-Cordoba provided Varela with a copy of Prof. Olsacher’s paper, entitled: The meteorite of Salavina, 1931, in which he clearly indicates that the fall consisted of two different types of rocks (translated Spanish text, see attached pdf): "This meteorite, which represents a case of ’stone rain’ is made up of fragments that correspond to two different types based on their composition and structure: one is chondritic, composed by olivine and pyroxene with abundant metallic minerals; the second one is feldspar-rich with scarce presence of the previous components and with an ophitic texture (1). (1) This feldspar-rich meteorite is mainly composed of anorthite with few olivine and troilite grains. As we do not have enough bibliography concerning feldspar-rich meteorites I have left the description for a complementary publication, to the one presented now in which only the chondrite is studied."

Physical characteristics: Single 62.4 g stone covered by fresh shiny fusion crust.

Petrography: The meteorite is a monomict breccia, mainly composed of pyroxene (pigeonite, often with augite lamellae) and plagioclase with coarse- and fine-grained domains. Accessory phases include: troilite, silica-polymorphs, ilmenite, chromite, F-apatite, merrillite, and zircon. There are only few shock features, with some minerals showing faint undulatory extinction. One of the thin sections (sample belonging to the NHMV) show a small melt vein. The state of shock is minimal. The rock is unaltered.

Geochemistry: Mineral composition and geochemistry: (J. Roszjar; NHMV) Electron Microprobe examination of two polished mounts show very similar plagioclase and pyroxene composition for the coarse-and fine-grained areas. Low-Ca pyroxene Fs_62.5±0.8Wo_2.23±0.9 , n= 96; Ca-rich Pyroxenes Fs_26.9±0.9Wo_43.9±1, n= 96. The composition of the rock-forming plagioclase is An_82.1±3.4, n=110 with minor, secondary anorthitic plagioclase with An_95.7±2.7, n= 50. Spinel composition is chromite (Chr_0.74Her_0.18), n=124. Oxygen Isotopes (R. Tanaka, OkaU): A cleaned sample was analyzed by laser fluorination. Replicate analyses are, respectively, δ¹⁷O of 1.785 and 1.775 permil; δ¹⁸O of 3.779 and 3.787 permil; Δ¹⁷O of -0.208 and -0.222 permil.

Classification: Basaltic eucrite, type 4 (based on the chemical composition of spinels and characteristic of pyroxenes that are also diagnostic for this thermal metamorphic classification). Because this meteorite is fresh and the ordinary chondrite Malotas is weathered, this eucrite may actually be the meteorite that fell on June 22, 1931, and the ordinary chondrite may be an unrelated find. This hypothesis remains untested. Henceforth, Malotas (a) will be a synonym for the Malotas H5 chondrite.

Specimens: Type specimen of 20 g in deposit at the MLP and three thin sections are on deposit at ICATE. One thin section and another sample mount are in deposit at NHMV. The main mass is at the MMG-Cordoba.